Wood, White Wood, Canary Wood, Poplar, Blue Poplar, White Poplar,
Hickory Poplar). A medium- to large-sized tree, does not form forests,
but is quite common, especially in the Ohio basin. Wood usually light,
but varies in weight, it is soft, tough, but not strong, of fine
texture, and yellowish color. The wood shrinks considerably, but
seasons without much injury, and works and stands extremely well.
Heartwood light yellow or greenish brown, the sapwood is thin, nearly
white, and decays rapidly. The heartwood is fairly durable when
exposed to the weather or in contact with the soil. It bends readily
when steamed, and takes stain and paint well. The mature forest-grown
tree has a long, straight, cylindrical bole, clear of branches for at
least two thirds of its length, surmounted by a short, open, irregular
crown. When growing in the open, the tree maintains a straight stem,
but the crown extends almost to the ground, and is of conical shape.
Yellow poplar, or tulip wood, ordinarily grows to a height of from 100
to 125 feet, with a diameter of from 3 to 6 feet, and a clear length
of about 70 feet. Trees have been found 190 feet high and ten feet in
diameter. Used in cooperage, for siding, for panelling and finishing
lumber in houses, car- and shipbuilding, for sideboards, panels of
wagons and carriages, for aeroplanes, for automobiles, also in the
manufacture of furniture farm implements, machinery, for pump logs,
and almost every kind of common woodenware, boxes shelving, drawers,
etc., etc. Also in the manufacture of toys, culinary woodenware, and
backing for veneer. It is in great demand throughout the vehicle and
implement trade, and also makes a fair grade of wood pulp. In fact the
tulip tree is one of the most useful of woods throughout the
woodworking industry of this country. Occurs from New England to
Missouri and southward to Florida.
TUPELO (See Gum)
WAAHOO
=105. Waahoo= (_Evonymus atropurpureus_). (Burning Bush, Spindle Tree).
A small-sized tree. Wood white, tinged with orange; heavy, hard,
tough, and close-grained, works well and stands well. Used principally
for arrows and spindles. Widely distributed. Usually a shrub six to
ten feet high, becoming a tree only in southern Arkansas and Oklahoma.
WALNUT
=106. Black Walnut= (_Juglans nigra_) (Walnut). A large, beautiful, and
quickly-growing tree, about 60 feet and upwards in height. Wood heavy,
hard, strong, of coarse texture, very durable in contact with the
soil. The narrow sapwood whitish, the heartwood dark, rich, chocolate
brown, sometimes almost black; aged trees of fine quality bring fancy
prices. The wood shrinks moderately in seasoning, works well and
stands well, and takes a fine polish. It is quite handsome, and has
been for a long time the favorite wood for cabinet and furniture
making. It is used for gun-stocks, fixtures, interior decoration,
veneer, panelling, stair newells, and all classes of work demanding a
high priced grade of wood. Black walnut is a large tree with stout
trunk, of rapid growth, and was formerly quite abundant throughout
the Alleghany region. Occurs from New England to Texas, and from
Michigan to Florida. Not common.
WHITE WALNUT (See Butternut)
WHITE WOOD (See Tulip and also Basswood)
WHITE WILLOW
=107. White Willow= (_Salix alba_ var. _vitellina_) (Willow, Yellow
Willow, Blue Willow). The wood is very soft, light, flexible, and
fairly strong, is fairly durable in contact with the soil, works well
and stands well when seasoned. Medium-sized tree, characterized by a
short, thick trunk, and a large, rather irregular crown composed of
many branches. The size of the tree at maturity varies with the
locality. In the region where it occurs naturally, a height of 70 to
80 feet, and a diameter of three to four feet are often attained. When
planted in the Middle West, a height of from 50 to 60 feet, and a
diameter of one and one-half to two feet are all that may be expected.
When closely planted on moist soil, the tree forms a tall, slender
stem, well cleared branches. Is widely naturalized in the United
States. It is used in cooperage, for woodenware, for cricket and
baseball bats, for basket work, etc. Charcoal made from the wood is
used in the manufacture of gunpowder. It has been generally used for
fence posts on the Northwestern plains, because of scarcity of better
material. Well seasoned posts will last from four to seven years.
Widely distributed throughout the United States.
=108. Black Willow= (_Salix nigra_). Small-sized tree. Heartwood light
reddish brown, sapwood nearly white. Wood soft, light, not strong,
close-grained, and very flexible. Used in basket making, etc. Ranges
from New York to Rocky Mountains and southward to Mexico.
=109. Shining Willow= (_Salix lucida_). A small-sized tree. Wood in its
quality and uses similiar to the preceding. Ranges from Newfoundland
to Rocky Mountains and southward to Pennsylvania and Nebraska.
=110. Perch Willow= (_Salix amygdaloides_) (Almond-leaf Willow). Small
to medium-sized tree. Heartwood light brown, sapwood lighter color.
Wood light, soft, flexible, not strong, close-grained. Uses similiar
to the preceding. Follows the water courses and ranges across the
continent; less abundant in New England than elsewhere. Common in the
West.
=111. Long-Leaf Willow= (_Salix fluviatilis_) (Sand Bar Willow). A
small-sized tree. Ranges from the Arctic Circle to Northern Mexico.
=112. Bebb Willow= (_Salix bebbiana_ var. _rostrata_). A small-sized
tree. More abundant in British America than in the United States,
where it ranges southward to Pennsylvania and westward to Minnesota.
=113. Glaucous Willow= (_Salix discolor_) (Pussy Willow). A small-sized
tree. Common along the banks of streams, and ranges from Nova Scotia
to Manitoba, and south to Delaware; west to Indiana and northwestern
Missouri.
=114. Crack Willow= (_Salix fragilis_). A medium to large-sized tree.
Wood is very soft, light, very flexible and fairly strong, is fairly
durable in contact with the soil, works well and stands well. Used
principally for basket making, hoops, etc., and to produce charcoal
for gunpowder. Very common, and widely distributed in the United
States.
=115. Weeping Willow= (_Salix babylonica_). Medium- to large-sized tree.
Wood similiar to _Salix nigra_, but not so valuable. Mostly an
ornamental tree. Originally came from China. Widely planted in the
United States.
YELLOW WOOD
=116. Yellow Wood= (_Cladrastis lutea_) (Virgilia). A small to
medium-sized tree. Wood yellow to pale brown, heavy, hard,
close-grained and strong. Not used to much extent in manufacturing.
Not common. Found principally on the limestone cliffs of Kentucky,
Tennessee, and North Carolina.
SECTION IV
GRAIN, COLOR, ODOR, WEIGHT, AND FIGURE IN WOOD
DIFFERENT GRAINS OF WOOD
The terms "fine-grained," "coarse-grained," "straight-grained," and
"cross-grained" are frequently applied in the trade. In common usage,
wood is coarse-grained if its annual rings are wide; fine-grained if
they are narrow. In the finer wood industries a fine-grained wood is
capable of high polish, while a coarse-grained wood is not, so that in
this latter case the distinction depends chiefly on hardness, and in
the former on an accidental case of slow or rapid growth. Generally if
the direction of the wood fibres is parallel to the axis of the stem
or limb in which they occur, the wood is straight-grained; but in many
cases the course of the fibres is spiral or twisted around the tree
(as shown in Fig. 15), and sometimes commonly in the butts of gum and
cypress, the fibres of several layers are oblique in one direction,
and those of the next series of layers are oblique in the opposite
direction. (As shown in Fig. 16 the wood is cross or twisted grain.)
Wavy-grain in a tangential plane as seen on the radial section is
illustrated in Fig. 17, which represents an extreme case observed in
beech. This same form also occurs on the radial plane, causing the
tangential section to appear wavy or in transverse folds.
When wavy grain is fine (_i.e._, the folds or ridges small but
numerous) it gives rise to the "curly" structure frequently seen in
maple. Ordinarily, neither wavy, spiral, nor alternate grain is
visible on the cross-section; its existence often escapes the eye even
on smooth, longitudinal faces in the sawed material, so that the only
guide to their discovery lies in splitting the wood in two, in the two
normal plains.
[Illustration: Fig. 15. Spiral Grain. Season checks, after
removal of bark, indicate the direction of the fibres or
grain of the wood.]
[Illustration: Fig. 16. Alternating Spiral Grain in Cypress.
Side and end view of same piece. When the bark was at _o_,
the grain of this piece was straight. From that time, each
year it grew more oblique in one direction, reaching a climax
at _a_, and then turned back in the opposite direction. These
alternations were repeated periodically, the bark sharing in
these changes.]
Generally the surface of the wood under the bark, and therefore also
that of any layer in the interior, is not uniform and smooth, but is
channelled and pitted by numerous depressions, which differ greatly in
size and form. Usually, any one depression or elevation is restricted
to one or few annual layers (_i.e._, seen only in one or few rings)
and is then lost, being compensated (the surface at the particular
spot evened up) by growth. In some woods, however, any depression or
elevation once attained grows from year to year and reaches a maximum
size, which is maintained for many years, sometimes throughout life.
In maple, where this tendency to preserve any particular contour is
very great, the depressions and elevations are usually small
(commonly less than one-eighth inch) but very numerous.
On tangent boards of such wood, the sections, pits, and prominences
appear as circlets, and give rise to the beautiful "bird's eye" or
"landscape" structure. Similiar structures in the burls of black ash,
maple, etc., are frequently due to the presence of dormant buds, which
cause the surface of all the layers through which they pass to be
covered by small conical elevations, whose cross-sections on the sawed
board appear as irregular circlets or islets, each with a dark speck,
the section of the pith or "trace" of the dormant bud in the center.
[Illustration: Fig. 17. Wavy Grain in Beech (_after
Nordlinger_).]
In the wood of many broad-leaved trees the wood fibres are much longer
when full grown than when they are first formed in the cambium or
growing zone. This causes the tips of each fibre to crowd in between
the fibres above and below, and leads to an irregular interlacement of
these fibres, which adds to the toughness, but reduces the
cleavability of the wood. At the juncture of the limb and stem the
fibres on the upper and lower sides of the limb behave differently.
On the lower side they run from the stem into the limb, forming an
uninterrupted strand or tissue and a perfect union. On the upper side
the fibres bend aside, are not continuous into the limb, and hence the
connection is not perfect (see Fig. 18). Owing to this arrangement of
the fibres, the cleft made in splitting never runs into the knot if
started on the side above the limb, but is apt to enter the knot if
started below, a fact well understood in woodcraft. When limbs die,
decay, and break off, the remaining stubs are surrounded, and may
finally be covered by the growth of the trunk and thus give rise to
the annoying "dead" or "loose" knots.
[Illustration: Fig. 18. Section of Wood showing Position of
the Grain at Base of a Limb. P, pith of both stem and limb;
1-7, seven yearly layers of wood; _a_, _b_, knot or basal
part of a limb which lived for four years, then died and
broke off near the stem, leaving the part to the left of _a_,
_b_, a "sound" knot, the part to the right a "dead" knot,
which would soon be entirely covered by the growing stem.]
COLOR AND ODOR OF WOOD
Color, like structure, lends beauty to the wood, aids in its
identification, and is of great value in the determination of its
quality. If we consider only the heartwood, the black color of the
persimmon, the dark brown of the walnut, the light brown of the white
oaks, the reddish brown of the red oaks, the yellowish white of the
tulip and poplars, the brownish red of the redwood and cedars, the
yellow of the papaw and sumac, are all reliable marks of distinction
and color. Together with luster and weight, they are only too often
the only features depended upon in practice. Newly formed wood, like
that of the outer few rings, has but little color. The sapwood
generally is light, and the wood of trees which form no heartwood
changes but little, except when stained by forerunners of disease.
The different tints of colors, whether the brown of oak, the orange
brown of pine, the blackish tint of walnut, or the reddish cast of
cedar, are due to pigments, while the deeper shade of the summer-wood
bands in pine, cedar, oak, or walnut is due to the fact that the wood
being denser, more of the colored wood substance occurs on a given
space, _i.e._, there is more colored matter per square inch. Wood is
translucent, a thin disk of pine permitting light to pass through
quite freely. This translucency affects the luster and brightness of
lumber.
When lumber is attacked by fungi, it becomes more opaque, loses its
brightness, and in practice is designated "dead," in distinction to
"live" or bright timber. Exposure to air darkens all wood; direct
sunlight and occasional moistening hasten this change, and cause it to
penetrate deeper. Prolonged immersion has the same effect, pine wood
becoming a dark gray, while oak changes to a blackish brown.
Odor, like color, depends on chemical compounds, forming no part of
the wood substance itself. Exposure to weather reduces and often
changes the odor, but a piece of long-leaf pine, cedar, or camphor
wood exhales apparently as much odor as ever when a new surface is
exposed. Heartwood is more odoriferous than sapwood. Many kinds of
wood are distinguished by strong and peculiar odors. This is
especially the case with camphor, cedar, pine, oak, and mahogany, and
the list would comprise every kind of wood in use were our sense of
smell developed in keeping with its importance.
Decomposition is usually accompanied by pronounced odors. Decaying
poplar emits a disagreeable odor, while red oak often becomes
fragrant, its smell resembling that of heliotrope.
WEIGHT OF WOOD
A small cross-section of wood (as in Fig. 19) dropped into water
sinks, showing that the substance of which wood fibre or wood is built
up is heavier than water. By immersing the wood successively in
heavier liquids, until we find a liquid in which it does not sink, and
comparing the weight of the same with water, we find that wood
substance is about 1.6 times as heavy as water, and that this is as
true of poplar as of oak or pine.
[Illustration: Fig. 19. Cross-section of a Group of Wood
Fibres (Highly Magnified.)]
Separating a single cell (as shown in Fig. 20, _a_), drying and then
dropping it into water, it floats. The air-filled cell cavity or
interior reduces its weight, and, like an empty corked bottle, it
weighs less than the water. Soon, however, water soaks into the cell,
when it fills up and sinks. Many such cells grown together, as in a
block of wood, when all or most of them are filled with water, will
float as long as the majority of them are empty or only partially
filled. This is why a green, sappy pine pole soon sinks in "driving"
(floating down stream). Its cells are largely filled before it is
thrown in, and but little additional water suffices to make its weight
greater than that of the water. In a good-sized white pine log,
composed chiefly of empty cells (heartwood), the water requires a very
long time to fill up the cells (five years would not suffice to fill
them all), and therefore the log may float for many months. When the
wall of the wood fibre is very thick (five eighths or more of the
volume, as in Fig. 20, _b_), the fibre sinks whether empty or filled.
This applies to most of the fibres of the dark summer-wood bands in
pines, and to the compact fibres of oak or hickory, and many,
especially tropical woods, have such thick-walled cells and so little
empty or air space that they never float.
[Illustration: Fig. 20. Isolated Fibres of Wood.]
Here, then, are the two main factors of weight in wood; the amount of
cell wall or wood substance constant for any given piece, and the
amount of water contained in the wood, variable even in the standing
tree, and only in part eliminated in drying.
The weight of the green wood of any species varies chiefly as a second
factor, and is entirely misleading, if the relative weight of
different kinds is sought. Thus some green sticks of the otherwise
lighter cypress and gum sink more readily than fresh oak.
The weight of sapwood or the sappy, peripheral part of our common
lumber woods is always great, whether cut in winter or summer. It
rarely falls much below forty-five pounds, and commonly exceeds
fifty-five pounds to the cubic foot, even in our lighter wooded
species. It follows that the green wood of a sapling is heavier than
that of an old tree, the fresh wood from a disk of the upper part of a
tree is often heavier than that of the lower part, and the wood near
the bark heavier than that nearer the pith; and also that the
advantage of drying the wood before shipping is most important in
sappy and light kinds.
When kiln-dried, the misleading moisture factor of weight is uniformly
reduced, and a fair comparison possible. For the sake of convenience
in comparison, the weight of wood is expressed either as the weight
per cubic foot, or, what is still more convenient, as specific weight
or density. If an old long-leaf pine is cut up (as shown in Fig. 21)
the wood of disk No. 1 is heavier than that of disk No. 2, the latter
heavier than that of disk No. 3, and the wood of the top disk is found
to be only about three fourths as heavy as that of disk No. 1.
Similiarly, if disk No. 2 is cut up, as in the figure, the specific
weight of the different parts is:
_a_, about 0.52
_b_, about 0.64
_c_, about 0.67
_d_, _e_, _f_, about 0.65
showing that in this disk at least the wood formed during the many
years' growth, represented in piece _a_, is much lighter than that of
former years. It also shows that the best wood is the middle part,
with its large proportion of dark summer bands.
[Illustration: Fig. 21. Orientation of Wood Samples.]
Cutting up all disks in the same way, it will be found that the piece
_a_ of the first disk is heavier than the piece _a_ of the fifth, and
that piece _c_ of the first disk excels the piece _c_ of all the other
disks. This shows that the wood grown during the same number of years
is lighter in the upper parts of the stem; and if the disks are
smoothed on the radial surfaces and set up one on top of the other in
their regular order, for the sake of comparison, this decrease in
weight will be seen to be accompanied by a decrease in the amount of
summer-wood. The color effect of the upper disks is conspicuously
lighter. If our old pine had been cut one hundred and fifty years ago,
before the outer, lighter wood was laid on, it is evident that the
weight of the wood of any one disk would have been found to increase
from the center outward, and no subsequent decrease could have been
observed.
In a thrifty young pine, then, the wood is heavier from the center
outward, and lighter from below upward; only the wood laid on in old
age falls in weight below the average. The number of brownish bands of
summer-wood are a direct indication of these differences. If an old
oak is cut up in the same manner, the butt cut is also found heaviest
and the top lightest, but, unlike the disk of pine, the disk of oak
has its firmest wood at the center, and each successive piece from the
center outward is lighter than its neighbor.
Examining the pieces, this difference is not as readily explained by
the appearance of each piece as in the case of pine wood.
Nevertheless, one conspicuous point appears at once. The pores, so
very distinct in oak, are very minute in the wood near the center, and
thus the wood is far less porous.
Studying different trees, it is found that in the pines, wood with
narrow rings is just as heavy as and often heavier than the wood with
wider rings; but if the rings are unusually narrow in any part of the
disk, the wood has a lighter color; that is, there is less summer-wood
and therefore less weight.
In oak, ash, or elm trees of thrifty growth, the rings, fairly wide
(not less than one-twelfth inch), always form the heaviest wood, while
any piece with very narrow rings is light. On the other hand, the
weight of a piece of hard maple or birch is quite independent of the
width of its rings.
The bases of limbs (knots) are usually heavy, very heavy in conifers,
and also the wood which surrounds them, but generally the wood of the
limbs is lighter than that of the stem, and the wood of the roots is
the lightest.
In general, it may be said that none of the native woods in common use
in this country are when dry as heavy as water, _i.e._, sixty-two
pounds to the cubic foot. Few exceed fifty pounds, while most of them
fall below forty pounds, and much of the pine and other coniferous
wood weigh less than thirty pounds per cubic foot. The weight of the
wood is in itself an important quality. Weight assists in
distinguishing maple from poplar. Lightness coupled with great
strength and stiffness recommends wood for a thousand different uses.
To a large extent weight predicates the strength of the wood, at least
in the same species, so that a heavy piece of oak will exceed in
strength a light piece of the same species, and in pine it appears
probable that, weight for weight, the strength of the wood of various
pines is nearly equal.
WEIGHT OF KILN-DRIED WOOD OF DIFFERENT SPECIES
-----------------------------------------+----------------------------
| Approximate
|----------+-----------------
| | Weight of
| |---------+-------
Species | Specific | 1 | 1,000
| Weight | Cubic | Feet
| | Foot | Lumber
-----------------------------------------+----------+---------+-------
(_a_) Very Heavy Woods: | | |
Hickory, Oak, Persimmon, Osage Orange, | | |
Black Locust, Hackberry, Blue Beech, | | |
best of Elm and Ash |0.70-0.80 | 42-48 | 3,700
(_b_) Heavy Woods | | |
Ash, Elm, Cherry, Birch, Maple, Beech, | | |
Walnut, Sour Gum, Coffee Tree, Honey | | |
Locust, best of Southern Pine and | | |
Tamarack |0.60-0.70 | 36-42 | 3,200
(_c_) Woods of Medium Weight: | | |
Southern Pine, Pitch Pine, Tamarack, | | |
Douglas Spruce, Western Hemlock, | | |
Sweet Gum, Soft Maple, Sycamore, | | |
Sassafras, Mulberry, light grades of | | |
Birch and Cherry |0.50-0.60 | 30-36 | 2,700
(_d_) Light Woods: | | |
Norway and Bull Pine, Red Cedar, | | |
Cypress, Hemlock, the Heavier Spruces | | |
and Firs, Redwood, Basswood, Chestnut, | | |
Butternut, Tulip, Catalpa, Buckeye, | | |
heavier grades of Poplar |0.40-0.50 | 24-30 | 2,200
(_e_) Very Light Woods: | | |
White Pine, Spruce, Fir, White Cedar, | | |
Poplar |0.30-0.40 | 18-24 | 1,800
-----------------------------------------+----------+---------+-------
"FIGURE" IN WOOD
Many theories have been propounded as to the cause of "figure" in
timber; while it is true that all timber possesses "figure" in some
degree, which is more noticeable if it be cut in certain ways, yet
there are some woods in which it is more conspicuous than in others,
and which for cabinet or furniture work are much appreciated, as it
adds to the value of the work produced.
The characteristic "figure" of oak is due to the broad and deep
medullary rays so conspicuous in this timber, and the same applies to
honeysuckle. Figure due to the same cause is found in sycamore and
beech, but is not so pronounced. The beautiful figure in "bird's eye
maple" is supposed to be due to the boring action of insects in the
early growth of the tree, causing pits or grooves, which in time
become filled up by being overlain by fresh layers of wood growth;
these peculiar and unique markings are found only in the older and
inner portion of the tree.
Pitch pine has sometimes a very beautiful "figure," but it generally
does not go deep into the timber; walnut has quite a variety of
"figures," and so has the elm. It is in mahogany, however, that we
find the greatest variety of "figure," and as this timber is only used
for furniture and fancy work, a good "figure" greatly enhances its
value, as firmly figured logs bring fancy prices.
Mahogany, unlike the oak, never draws its "figure" from its small and
almost unnoticeable medullary rays, but from the twisted condition of
its fibres; the natural growth of mahogany produces a straight wood;
what is called "figured" is unnatural and exceptional, and thus adds
to its value as an ornamental wood. These peculiarities are rarely
found in the earlier portion of the tree that is near the center,
being in this respect quite different from maple; they appear when the
tree is more fully developed, and consist of bundles of woody fibres
which, instead of being laid in straight lines, behave in an erratic
manner and are deposited in a twisted form; sometimes it may be caused
by the intersection of branches, or possibly by the crackling of the
bark pressing on the wood, and thus moving it out of its natural
straight course, causing a wavy line which in time becomes
accentuated.
It will have been observed by most people that the outer portion of a
tree is often indented by the bark, and the outer rings often follow a
sinuous course which corresponds to this indention, but in most trees,
after a few years, this is evened up and the annual rings assume their
nearly circular form; it is supposed by some that in the case of
mahogany this is not the case, and that the indentations are even
accentuated.
The best figured logs of timber are secured from trees which grow in
firm rocky soil; those growing on low-lying or swampy ground are
seldom figured. To the practical woodworker the figure in mahogany
causes some difficulty in planing the wood to a smooth surface; some
portions plane smooth, others are the "wrong way of the grain."
Figure in wood is effected by the way light is thrown upon it, showing
light if seen from one direction, and dark if viewed from another, as
may easily be observed by holding a piece of figured mahogany under
artificial light and looking at it from opposite directions. The
characteristic markings on mahogany are "mottle," which is also found
in sycamore, and is conspicuous on the backs of fiddles and violins,
and is not in itself valuable; it runs the transverse way of the
fibres and is probably the effect of the wind upon the tree in its
early stages of growth. "Roe," which is said to be caused by the
contortion of the woody fibres, and takes a wavy line parallel to
them, is also found in the hollow of bent stems and in the root
structure, and when combined with "mottle" is very valuable. "Dapple"
is an exaggerated form of mottle. "Thunder shake," "wind shake," or
"tornado shake" is a rupture of the fibres across the grain, which in
mahogany does not always break them; the tree swaying in the wind only
strains its fibres, and thus produces mottle in the wood.
SECTION V
ENEMIES OF WOOD
From the writer's personal investigations of this subject in different
sections of the country, the damage to forest products of various
kinds from this cause seems to be far more extensive than is generally
recognized. Allowing a loss of five per cent on the total value of the
forest products of the country, which the writer believes to be a
conservative estimate, it would amount to something over $30,000,000
annually. This loss differs from that resulting from insect damage to
natural forest resources, in that it represents more directly a loss
of money invested in material and labor. In dealing with the insects
mentioned, as with forest insects in general, the methods which yield
the best results are those which relate directly to preventing attack,
as well as those which are unattractive or unfavorable. The insects
have two objects in their attack: one is to obtain food, the other is
to prepare for the development of their broods. Different species of
insects have special periods during the season of activity (March to
November), when the adults are on the wing in search of suitable
material in which to deposit their eggs. Some species, which fly in
April, will be attracted to the trunks of recently felled pine trees
or to piles of pine sawlogs from trees felled the previous winter.
They are not attracted to any other kind of timber, because they can
live only in the bark or wood of pine, and only in that which is in
the proper condition to favor the hatching of their eggs and the
normal development of their young. As they fly only in April, they
cannot injure the logs of trees felled during the remainder of the
year.
There are also oak insects, which attack nothing but oak; hickory,
cypress, and spruce insects, etc., which have different habits and
different periods of flight, and require special conditions of the
bark and wood for depositing their eggs or for subsequent development
of their broods. Some of these insects have but one generation in a
year, others have two or more, while some require more than one year
for the complete development and transformation. Some species deposit
their eggs in the bark or wood of trees soon after they are felled or
before any perceptible change from the normal living tissue has taken
place; other species are attracted only to dead bark and dead wood of
trees which have been felled or girdled for several months; others are
attracted to dry and seasoned wood; while another class will attack
nothing but very old, dry bark or wood of special kinds and under
special conditions. Thus it will be seen how important it is for the
practical man to have knowledge of such of the foregoing facts as
apply to his immediate interest in the manufacture or utilization of a
given forest product, in order that he may with the least trouble and
expense adjust his business methods to meet the requirements for
preventing losses.
The work of different kinds of insects, as represented by special
injuries to forest products, is the first thing to attract attention,
and the distinctive character of this work is easily observed, while
the insect responsible for it is seldom seen, or it is so difficult to
determine by the general observer from descriptions or illustrations
that the species is rarely recognized. Fortunately, the character of
the work is often sufficient in itself to identify the cause and
suggest a remedy, and in this section primary consideration is given
to this phase of the subject.
Ambrosia or Timber Beetles
[Illustration: Fig. 22. Work of Ambrosia Beetles in Tulip or
Yellow Poplar Wood. _a_, work of _Xyleborus affinis_ and
_Xyleborus inermis_; _b_, _Xyleborus obesus_ and work; _c_,
bark; _d_, sapwood; _e_, heartwood.]
[Illustration: Fig. 23. Work of Ambrosia Beetles in Oak. _a_,
_Monarthrum mali_ and work; _b_, _Platypus compositus_ and
work; _c_, bark; _d_, sapwood; _e_, heartwood; _f_, character
of work in wood from injured log.]
The characteristic work of this class of wood-boring beetles is shown
in Figs. 22 and 23. The injury consists of pinhole and stained-wood
defects in the sapwood and heartwood of recently felled or girdled
trees, sawlogs, pulpwood, stave and shingle bolts, green or
unseasoned lumber, and staves and heads of barrels containing
alcoholic liquids. The holes and galleries are made by the adult
parent beetles, to serve as entrances and temporary houses or
nurseries for the development of their broods of young, which feed on
a fungus growing on the walls of the galleries.
The growth of this ambrosia-like fungus is induced and controlled by
the parent beetles, and the young are dependent upon it for food. The
wood must be in exactly the proper condition for the growth of the
fungus in order to attract the beetles and induce them to excavate
their galleries; it must have a certain degree of moisture and other
favorable qualities, which usually prevail during the period involved
in the change from living, or normal, to dead or dry wood; such a
condition is found in recently felled trees, sawlogs, or like crude
products.
There are two general types or classes of these galleries: one in
which the broods develop together in the main burrows (see Fig. 22),
the other in which the individuals develop in short, separate side
chambers, extending at right angles from the primary galleries (see
Fig. 23). The galleries of the latter type are usually accompanied by
a distinct staining of the wood, while those of the former are not.
The beetles responsible for this work are cylindrical in form,
apparently with a head (the prothorax) half as long as the remainder
of the body (see Figs. 22, _a_, and 23, _a_).
North American species vary in size from less than one-tenth to
slightly more than two-tenths of an inch, while some of the
subtropical and tropical species attain a much larger size. The
diameter of the holes made by each species corresponds closely to that
of the body, and varies from about one-twentieth to one-sixteenth of
an inch for the tropical species.
Round-headed Borers
[Illustration: Fig. 24. Work of Round-headed and Flat-headed
Borers in Pine. _a_, work of round-headed borer, "sawyer,"
_Monohammus spiculatus_, natural size _b_, _Ergates
spiculatus_; _c_, work of flat-headed borer, _Buprestis_,
larva and adult; _d_, bark; _e_, sapwood; _f_, heartwood.]
The character of the work of this class of wood- and bark-boring grubs
is shown in Fig. 24. The injuries consist of irregular flattened or
nearly round wormhole defects in the wood, which sometimes result in
the destruction of valuable parts of the wood or bark material. The
sapwood and heartwood of recently felled trees, sawlogs, poles posts,
mine props, pulpwood and cordwood, also lumber or square timber, with
bark on the edges, and construction timber in new and old buildings,
are injured by wormhole defects, while the valuable parts of stored
oak and hemlock tanbark and certain kinds of wood are converted into
worm-dust. These injuries are caused by the young or larvae of
long-horned beetles. Those which infest the wood hatch from eggs
deposited in the outer bark of logs and like material, and the minute
grubs hatching therefrom bore into the inner bark, through which they
extend their irregular burrows, for the purpose of obtaining food from
the sap and other nutritive material found in the plant tissue. They
continue to extend and enlarge their burrows as they increase in size,
until they are nearly or quite full grown. They then enter the wood
and continue their excavations deep into the sapwood or heartwood
until they attain their normal size. They then excavate pupa cells in
which to transform into adults, which emerge from the wood through
exit holes in the surface. This class of borers is represented by a
large number of species. The adults, however, are seldom seen by the
general observer unless cut out of the wood before they have emerged.
Flat-headed Borers
The work of the flat-headed borers (Fig. 24) is only distinguished
from that of the preceding by the broad, shallow burrows, and the much
more oblong form of the exit holes. In general, the injuries are
similiar, and effect the same class of products, but they are of much
less importance. The adult forms are flattened, metallic-colored
beetles, and represent many species, of various sizes.
Timber Worms
[Illustration: Fig. 25. Work of Timber Worms in Oak. _a_,
work of oak timber worm, _Eupsalis minuta_; _b_, barked
surface; _c_, bark; _d_, sapwood timber worm, _Hylocoetus
lugubris_, and work; _e_, sapwood.]
The character of the work done by this class is shown in Fig. 25. The
injury consists of pinhole defects in the sapwood and heartwood of
felled trees, sawlogs and like material which have been left in the
woods or in piles in the open for several months during the warmer
seasons. Stave and shingle bolts and closely piled oak lumber and
square timbers also suffer from injury of this kind. These injuries
are made by elongate, slender worms or larvae, which hatch from eggs
deposited by the adult beetles in the outer bark, or, where there is
no bark, just beneath the surface of the wood. At first the young
larvae bore almost invisible holes for a long distance through the
sapwood and heartwood, but as they increase in size the same holes are
enlarged and extended until the larvae have attained their full
growth. They then transform to adults, and emerge through the enlarged
entrance burrows. The work of these timber worms is distinguished from
that of the timber beetles by the greater variation in the size of
holes in the same piece of wood, also by the fact that they are not
branched from a single entrance or gallery, as are those made by the
beetles.
[Illustration: Fig. 26. Work of Powder Post Beetle,
_Sinoxylon basilare_, in Hickory Poles, showing Transverse
Egg Galleries excavated by the Adult, _a_, entrance; _b_,
gallery; _c_, adult.]
[Illustration: Fig. 27. Work of Powder
Post Beetle, _Sinoxylon basilare_, in Hickory Pole. _a_,
character of work by larvae; _b_, exit holes made by
emerging broods.]
Powder Post Borers
The character of the work of this class of insects is shown in Figs.
26, 27, and 28. The injury consists of closely placed burrows, packed
with borings, or a completely destroyed or powdered condition of the
wood of seasoned products, such as lumber, crude and finished handle
and wagon stock, cooperage and wooden truss hoops, furniture, and
inside finish woodwork, in old buildings, as well as in many other
crude or finished and utilized woods. This is the work of both the
adults and young stages of some species, or of the larval stage alone
of others. In the former, the adult beetles deposit their eggs in
burrows or galleries excavated for the purpose, as in Figs. 26 and 27,
while in the latter (Fig. 28) the eggs are on or beneath the surface
of the wood. The grubs complete the destruction by boring through the
solid wood in all directions and packing their burrows with the
powdered wood. When they are full grown they transform to the adult,
and emerge from the injured material through holes in the surface.
Some of the species continue to work in the same wood until many
generations have developed and emerged or until every particle of wood
tissue has been destroyed and the available nutritive substance
extracted.
[Illustration: Fig. 28. Work of Powder Post Beetles, _Lyctus
striatus_, in Hickory Handles and Spokes. _a_, larva; _b_,
pupa; _c_, adult; _d_, exit holes; _e_, entrance of larvae
(vents for borings are exits of parasites); _f_, work of
larvae; _g_, wood, completely destroyed; _h_, sapwood; _i_,
heartwood.]
Conditions Favorable for Insect
Injury--Crude Products--Round Timber with Bark on
Newly felled trees, sawlogs, stave and heading bolts, telegraph poles,
posts, and the like material, cut in the fall and winter, and left on
the ground or in close piles during a few weeks or months in the
spring or summer, causing them to heat and sweat, are especially
liable to injury by ambrosia beetles (Figs. 22 and 23), round and
flat-headed borers (Fig. 24), and timber worms (Fig. 25), as are also
trees felled in the warm season, and left for a time before working up
into lumber.
The proper degree of moisture found in freshly cut living or dying
wood, and the period when the insects are flying, are the conditions
most favorable for attack. This period of danger varies with the time
of the year the timber is felled and with the different kinds of
trees. Those felled in late fall and winter will generally remain
attractive to ambrosia beetles, and to the adults of round- and
flat-headed borers during March, April, and May. Those felled in April
to September may be attacked in a few days after they are felled, and
the period of danger may not extend over more than a few weeks.
Certain kinds of trees felled during certain months and seasons are
never attacked, because the danger period prevails only when the
insects are flying; on the other hand, if the same kinds of trees are
felled at a different time, the conditions may be most attractive when
the insects are active, and they will be thickly infested and ruined.
The presence of bark is absolutely necessary for infestation by most
of the wood-boring grubs, since the eggs and young stages must occupy
the outer and inner portions before they can enter the wood. Some
ambrosia and timber worms will, however, attack barked logs,
especially those in close piles, and others shaded and protected from
rapid drying.
The sapwood of pine, spruce, fir, cedar, cypress, and the like
softwoods is especially liable to injury by ambrosia beetles, while
the heartwood is sometimes ruined by a class of round-headed borers,
known as "sawyers." Yellow poplar, oak, chestnut, gum, hickory, and
most other hardwoods are as a rule attacked by species of ambrosia
beetles, sawyers, and timber worms, different from those infesting the
pines, there being but very few species which attack both.
Mahogany and other rare and valuable woods imported from the tropics
to this country in the form of round logs, with or without bark on,
are commonly damaged more or less seriously by ambrosia beetles and
timber worms.
It would appear from the writer's investigations of logs received at
the mills in this country, that the principal damage is done during a
limited period--from the time the trees are felled until they are
placed in fresh or salt water for transportation to the shipping
points. If, however, the logs are loaded on a vessel direct from the
shore, or if not left in the water long enough to kill the insects,
the latter will continue their destructive work during transportation
to other countries and after they arrive, and until cold weather
ensues or the logs are converted into lumber.
It was also found that a thorough soaking in sea-water, while it
usually killed the insects at the time, did not prevent subsequent
attacks by both foreign and native ambrosia beetles; also, that the
removal of the bark from such logs previous to immersion did not
render them entirely immune. Those with the bark off were attacked
more than those with it on, owing to a greater amount of saline
moisture retained by the bark.
How to Prevent Injury
From the foregoing it will be seen that some requisites for preventing
these insect injuries to round timber are:
1. To provide for as little delay as possible between the
felling of the tree and its manufacture into rough products.
This is especially necessary with trees felled from April to
September, in the region north of the Gulf States, and from
March to November in the latter, while the late fall and
winter cutting should all be worked up by March or April.
2. If the round timber must be left in the woods or on the
skidways during the danger period, every precaution should
be taken to facilitate rapid drying of the inner bark, by
keeping the logs off the ground in the sun, or in loose
piles; or else the opposite extreme should be adopted and
the logs kept in water.
3. The immediate removal of all the bark from poles, posts,
and other material which will not be seriously damaged by
checking or season checks.
4. To determine and utilize the proper months or seasons to
girdle or fell different kinds of trees: Bald cypress in the
swamps of the South are "girdled" in order that they may
die, and in a few weeks or months dry out and become light
enough to float. This method has been extensively adopted in
sections where it is the only practicable one by which the
timber can be transported to the sawmills. It is found,
however, that some of these "girdled" trees are especially
attractive to several species of ambrosia beetles (Figs. 22
and 23), round-headed borers (Fig. 24) and timber worms
(Fig. 25), which cause serious injury to the sapwood or
heartwood, while other trees "girdled" at a different time
or season are not injured. This suggested to the writer the
importance of experiments to determine the proper time to
"girdle" trees to avoid losses, and they are now being
conducted on an extensive scale by the United States Forest
Service, in co-operation with prominent cypress operators in
different sections of the cypress-growing region.
Saplings
Saplings, including hickory and other round hoop-poles and similiar
products, are subject to serious injuries and destruction by round-
and flat-headed borers (Fig. 24), and certain species of powder post
borers (Figs. 26 and 27) before the bark and wood are dead or dry, and
also by other powder post borers (Fig. 28) after they are dried and
seasoned. The conditions favoring attack by the former class are those
resulting from leaving the poles in piles or bundles in or near the
forest for a few weeks during the season of insect activity, and by
the latter from leaving them stored in one place for several months.
Stave, Heading and Shingle Bolts
These are attacked by ambrosia beetles (Figs. 22 and 23), and the oak
timber worm (Fig. 25, _a_), which, as has been frequently reported,
cause serious losses. The conditions favoring attack by these insects
are similiar to those mentioned under "Round Timber." The insects may
enter the wood before the bolts are cut from the log or afterward,
especially if the bolts are left in moist, shady places in the woods,
in close piles during the danger period. If cut during the warm
season, the bark should be removed and the bolts converted into the
smallest practicable size and piled in such manner as to facilitate
rapid drying.
Unseasoned Products in the Rough
Freshly sawn hardwood, placed in close piles during warm, damp weather
in July and September, presents especially favorable conditions for
injury by ambrosia beetles (Figs. 22, _a_, and 23, _a_). This is due
to the continued moist condition of such material.
Heavy two-inch or three-inch stuff is also liable to attack even in
loose piles with lumber or cross sticks. An example of the latter was
found in a valuable lot of mahogany lumber of first grade, the value
of which was reduced two thirds by injury from a native ambrosia
beetle. Numerous complaints have been received from different sections
of the country of this class of injury to oak, poplar, gum, and other
hardwoods. In all cases it is the moist condition and retarded drying
of the lumber which induces attack; therefore, any method which will
provide for the rapid drying of the wood before or after piling will
tend to prevent losses.
It is important that heavy lumber should, as far as possible, be cut
in the winter months and piled so that it will be well dried out
before the middle of March. Square timber, stave and heading bolts,
with the bark on, often suffer from injuries by flat- or round-headed
borers, hatching from eggs deposited in the bark of the logs before
they are sawed and piled. One example of serious damage and loss was
reported in which white pine staves for paint buckets and other small
wooden vessels, which had been sawed from small logs, and the bark
left on the edges, were attacked by a round-headed borer, the adults
having deposited their eggs in the bark after the stock was sawn and
piled. The character of the injury is shown in Fig. 29. Another
example was reported from a manufacturer in the South, where the
pieces of lumber which had strips of bark on one side were seriously
damaged by the same kind of borer, the eggs having been deposited in
the logs before sawing or in the bark after the lumber was piled. If
the eggs are deposited in the logs, and the borers have entered the
inner bark or the wood before sawing, they may continue their work
regardless of methods of piling, but if such lumber is cut from new
logs and placed in the pile while green, with the bark surface up, it
will be much less liable to attack than if piled with the bark edges
down. This liability of lumber with bark edges or sides to be attacked
by insects suggests the importance of the removal of the bark, to
prevent damage, or, if this is not practicable, the lumber with the
bark on the sides should be piled in open, loose piles with the bark
up, while that with the bark on the edges should be placed on the
outer edges of the piles, exposed to the light and air.
[Illustration: Fig. 29. Work of Round-headed Borers,
_Callidium antennatum_, in White Pine Bucket Staves from New
Hampshire. _a_, where egg was deposited in bark; _b_, larval
mine; _c_, pupal cell; _d_, exit in bark; _e_, adult.]
In the Southern States it is difficult to keep green timber in the
woods or in piles for any length of time, because of the rapidity
which wood-destroying fungi attack it. This is particularly true
during the summer season, when the humidity is greatest. There is
really no easily-applied, general specific for these summer troubles
in the handling of wood, but there are some suggestions that are worth
while that it may be well to mention. One of these, and the most
important, is to remove all the bark from the timber that has been
cut, just as soon as possible after felling. And, in this, emphasis
should be laid on the ALL, as a piece of bark no larger than a man's
little finger will furnish an entering place for insects, and once
they get in, it is a difficult matter to get rid of them, for they
seldom stop boring until they ruin the stick. And again, after the
timber has been felled and the bark removed, it is well to get it to
the mill pond or cut up into merchantable sizes and on to the pile as
soon as possible. What is wanted is to get the timber up off the
ground, to a place where it can get plenty of air, to enable the sap
to dry up before it sours; and, besides, large units of wood are more
likely to crack open on the ends from the heat than they would if cut
up into the smaller units for merchandizing.
A moist condition of lumber and square timber, such as results from
close or solid piles, with the bottom layers on the ground or on
foundations of old decaying logs or near decaying stumps and logs,
offers especially favorable conditions for the attack of white ants.
Seasoned Products in the Rough
Seasoned or dry timber in stacks or storage is liable to injury by
powder post borers (Fig. 28). The conditions favoring attack are: (1)
The presence of a large proportion of sapwood, as in hickory, ash, and
similiar woods; (2) material which is two or more years old, or that
which has been kept in one place for a long time; (3) access to old
infested material. Therefore, such stock should be frequently examined
for evidence of the presence of these insects. This is always
indicated by fine, flour-like powder on or beneath the piles, or
otherwise associated with such material. All infested material should
be at once removed and the infested parts destroyed by burning.
Dry Cooperage Stock and Wooden Truss Hoops
These are especially liable to attack and serious injury by powder
post borers (Fig. 28), under the same or similiar conditions as the
preceding.
Staves and Heads of Barrels containing Alcoholic Liquids
These are liable to attack by ambrosia beetles (Figs. 22, _a_, and 23,
_a_), which are attracted by the moist condition and possibly by the
peculiar odor of the wood, resembling that of dying sapwood of trees
and logs, which is their normal breeding place.
There are many examples on record of serious losses of liquors from
leakage caused by the beetles boring through the staves and heads of
the barrels and casks in cellars and storerooms.
The condition, in addition to the moisture of the wood, which is
favorable for the presence of the beetles, is proximity to their
breeding places, such as the trunks and stumps of recently felled or
dying oak, maple, and other hardwood or deciduous trees; lumber yards,
sawmills, freshly-cut cordwood, from living or dead trees, and forests
of hardwood timber. Under such conditions the beetles occur in great
numbers, and if the storerooms and cellars in which the barrels are
kept stored are damp, poorly ventilated, and readily accessible to
them, serious injury is almost certain to follow.
SECTION VI
WATER IN WOOD
DISTRIBUTION OF WATER IN WOOD
Local Distribution of Water in Wood
As seasoning means essentially the more or less rapid evaporation of
water from wood, it will be necessary to discuss at the very outset
where water is found in wood, and its local seasonal distribution in a
tree.
Water may occur in wood in three conditions: (1) It forms the greater
part (over 90 per cent) of the protoplasmic contents of the living
cells; (2) it saturates the walls of all cells; and (3) it entirely or
at least partly fills the cavities of the lifeless cells, fibres, and
vessels.
In the sapwood of pine it occurs in all three forms; in the heartwood
only in the second form, it merely saturates the walls.
Of 100 pounds of water associated with 100 pounds of dry wood
substance taken from 200 pounds of fresh sapwood of white pine, about
35 pounds are needed to saturate the cell walls, less than 5 pounds
are contained in the living cells, and the remaining 60 pounds partly
fill the cavities of the wood fibres. This latter forms the sap as
ordinarily understood.
The wood next to the bark contains the most water. In the species
which do not form heartwood, the decrease toward the pith is gradual,
but where heartwood is formed the change from a more moist to a drier
condition is usually quite abrupt at the sapwood limit.
In long-leaf pine, the wood of the outer one inch of a disk may
contain 50 per cent of water, that of the next, or the second inch,
only 35 per cent, and that of the heartwood, only 20 per cent. In
such a tree the amount of water in any one section varies with the
amount of sapwood, and is greater for the upper than the lower cuts,
greater for the limbs than the stems, and greatest of all in the
roots.
Different trees, even of the same kind and from the same place, differ
as to the amount of water they contain. A thrifty tree contains more
water than a stunted one, and a young tree more than on old one, while
the wood of all trees varies in its moisture relations with the season
of the year.
Seasonal Distribution of Water in Wood
It is generally supposed that trees contain less water in winter than
in summer. This is evidenced by the popular saying that "the sap is
down in the winter." This is probably not always the case; some trees
contain as much water in winter as in summer, if not more. Trees
normally contain the greatest amount of water during that period when
the roots are active and the leaves are not yet out. This activity
commonly begins in January, February, and March, the exact time
varying with the kind of timber and the local atmospheric conditions.
And it has been found that green wood becomes lighter or contains less
water in late spring or early summer, when transpiration through the
foliage is most rapid. The amount of water at any one season, however,
is doubtless much influenced by the amount of moisture in the soil.
The fact that the bark peels easily in the spring depends on the
presence of incomplete, soft tissue found between wood and bark during
this season, and has little to do with the total amount of water
contained in the wood of the stem.
Even in the living tree a flow of sap from a cut occurs only in
certain kinds of trees and under special circumstances. From boards,
felled timber, etc., the water does not flow out, as is sometimes
believed, but must be evaporated. The seeming exceptions to this rule
are mostly referable to two causes; clefts or "shakes" will allow
water contained in them to flow out, and water is forced out of sound
wood, if very sappy, whenever the wood is warmed, just as water flows
from green wood when put in a stove.
Composition of Sap
The term "sap" is an ambiguous expression. The sap in the tree
descends through the bark, and except in early spring is not present
in the wood of the tree except in the medullary rays and living
tissues in the "sapwood."
What flows through the "sapwood" is chiefly water brought from the
soil. It is not pure water, but contains many substances in solution,
such as mineral salts, and in certain species--maple, birch, etc., it
also contains at certain times a small percentage of sugar and other
organic matter.
The water rises from the roots through the sapwood to the leaves,
where it is converted into true "sap" which descends through the bark
and feeds the living tissues between the bark and the wood, which
tissues make the annual growth of the trunk. The wood itself contains
very little true sap and the heartwood none.
The wood contains, however, mineral substances, organic acids,
volatile oils and gums, as resin, cedar oil, etc.
All the conifers--pines, cedars, junipers, cypresses, sequoias, yews,
and spruces--contain resin. The sap of deciduous trees--those which
shed their leaves at stated seasons--is lacking in this element, and
its constituents vary greatly in the different species. But there is
one element common to all trees, and for that matter to almost all
plant growth, and that is albumen.
Both resin and albumen, as they exist in the sap of woods, are soluble
in water; and both harden with heat, much the same as the white of an
egg, which is almost pure albumen.
These organic substances are the dissolved reserve food, stored during
the winter in the pith rays, etc., of the wood and bark; generally but
a mere trace of them is to be found. From this it appears that the
solids contained in the sap, such as albumen, gum, sugar, etc.,
cannot exercise the influence on the strength of the wood which is so
commonly claimed for them.
Effects of Moisture on Wood
The question of the effect of moisture upon the strength and stiffness
of wood offers a wide scope for study, and authorities consulted
differ in conclusions. Two authorities give the tensile strength in
pounds per square inch for white oak as 10,000 and 19,500,
respectively; for spruce, 8,000 to 19,500, and other species in
similiar startling contrasts.
Wood, we are told, is composed of organic products. The chief material
is cellulose, and this in its natural state in the living plant or
green wood contains from 25 to 35 per cent of its weight in moisture.
The moisture renders the cellulose substance pliable. What the
physical action of the water is upon the molecular structure of
organic material, to render it softer and more pliable, is largely a
matter of conjecture.
The strength of a timber depends not only upon its relative freedom
from imperfections, such as knots, crookedness of grain, decay,
wormholes or ring-shakes, but also upon its density; upon the rate at
which it grew, and upon the arrangement of the various elements which
compose it.
The factors effecting the strength of wood are therefore of two
classes: (1) Those inherent in the wood itself and which may cause
differences to exist between two pieces from the same species of wood
or even between the two ends of a piece, and (2) those which are
foreign to the wood itself, such as moisture, oils, and heat.
Though the effect of moisture is generally temporary, it is far more
important than is generally realized. So great, indeed, is the effect
of moisture that under some conditions it outweighs all the other
causes which effect strength, with the exception, perhaps of decided
imperfections in the wood itself.
The Fibre Saturation Point in Wood
Water exists in green wood in two forms: (1) As liquid water contained
in the cavities of the cells or pores, and (2) as "imbibed" water
intimately absorbed in the substance of which the wood is composed.
The removal of the free water from the cells or pores will evidently
have no effect upon the physical properties or shrinkage of the wood,
but as soon as any of the "imbibed" moisture is removed from the cell
walls, shrinkage begins to take place and other changes occur. The
strength also begins to increase at this time.
The point where the cell walls or wood substance becomes saturated is
called the "fibre saturation point," and is a very significant point
in the drying of wood.
It is easy to remove the free water from woods which will stand a high
temperature, as it is only necessary to heat the wood slightly above
the boiling point in a closed vessel, which will allow the escape of
the steam as it is formed, but will not allow dry air to come in
contact with the wood, so that the surface will not become dried below
its saturation point. This can be accomplished with most of the
softwoods, but not as a rule with the hardwoods, as they are injured
by the temperature necessary.
The chief difficulties are encountered in evaporating the "imbibed"
moisture and also where the free water has to be removed through its
gradual transfusion instead of boiling. As soon as the imbibed
moisture begins to be extracted from any portion, shrinkage takes
place and stresses are set up in the wood which tend to cause
checking.
The fibre saturation point lies between moisture conditions of 25 and
30 per cent of the dry weight of the wood, depending on the species.
Certain species of eucalyptus, and probably other woods, however,
appear to be exceptional in this respect, in that shrinkage begins to
take place at a moisture condition of 80 to 90 per cent of the dry
weight.
SECTION VII
WHAT SEASONING IS
Seasoning is ordinarily understood to mean drying. When exposed to the
sun and air, the water in green wood rapidly evaporates. The rate of
evaporation will depend on: (1) the kind of wood; (2) the shape and
thickness of the timber; and (3) the conditions under which the wood
is placed or piled.
Pieces of wood completely surrounded by air, exposed to the wind and
the sun, and protected by a roof from rain and snow, will dry out very
rapidly, while wood piled or packed close together so as to exclude
the air, or left in the shade and exposed to rain and snow, will dry
out very slowly and will also be subject to mould and decay.
But seasoning implies other changes besides the evaporation of water.
Although we have as yet only a vague conception as to the exact nature
of the difference between seasoned and unseasoned wood, it is very
probable that one of these consists in changes in the albuminous
substances in the wood fibres, and possibly also in the tannins,
resins, and other incrusting substances. Whether the change in these
substances is merely a drying-out, or whether it consists in a partial
decomposition is at yet undetermined. That the change during the
seasoning process is a profound one there can be no doubt, because
experience has shown again and again that seasoned wood fibre is very
much more permeable, both for liquids and gases than the living,
unseasoned fibre.
One can picture the albuminous substances as forming a coating which
dries out and possibly disintegrates when the wood dries. The
drying-out may result in considerable shrinkage, which may make the
wood fibre more porous. It is also possible that there are oxidizing
influences at work within these substances which result in their
disintegration. Whatever the exact nature of the change may be, one
can say without hesitation that exposure to the wind and air brings
about changes in the wood, which are of such a nature that the wood
becomes drier and more permeable.
When seasoned by exposure to live steam, similiar changes may take
place; the water leaves the wood in the form of steam, while the
organic compounds in the walls probably coagulate or disintegrate
under the high temperature.
The most effective seasoning is without doubt that obtained by the
uniform, slow drying which takes place in properly constructed piles
outdoors, under exposure to the winds and the sun and under cover from
the rain and snow, and is what has been termed "air-seasoning." By
air-seasoning oak and similiar hardwoods, nature performs certain
functions that cannot be duplicated by any artificial means. Because
of this, woods of this class cannot be successfully kiln-dried green
from the saw.
In drying wood, the free water within the cells passes through the
cell walls until the cells are empty, while the cell walls remain
saturated. When all the free water has been removed, the cell walls
begin to yield up their moisture. Heat raises the absorptive power of
the fibres and so aids the passage of water from the interior of the
cells. A confusion in the word "sap" is to be found in many
discussions of kiln-drying; in some instances it means water, in other
cases it is applied to the organic substances held in a water solution
in the cell cavities. The term is best confined to the organic
substances from the living cell. These substances, for the most part
of the nature of sugar, have a strong attraction for water and water
vapor, and so retard drying and absorb moisture into dried wood. High
temperatures, especially those produced by live steam, appear to
destroy these organic compounds and therefore both to retard and to
limit the reabsorption of moisture when the wood is subsequently
exposed to the atmosphere.
Air-dried wood, under ordinary atmospheric temperatures, retains from
10 to 20 per cent of moisture, whereas kiln-dried wood may have no
more than 5 per cent as it comes from the kiln. The exact figures for
a given species depend in the first case upon the weather conditions,
and in the second case upon the temperature in the kiln and the time
during which the wood is exposed to it. When wood that has been
kiln-dried is allowed to stand in the open, it apparently ceases to
reabsorb moisture from the air before its moisture content equals that
of wood which has merely been air-dried in the same place, and under
the same conditions, in other words kiln-dried wood will not absorb as
much moisture as air-dried wood under the same conditions.
Difference between Seasoned and Unseasoned Wood
Although it has been known for a long time that there is a marked
difference in the length of life of seasoned and of unseasoned wood,
the consumers of wood have shown very little interest in its
seasoning, except for the purpose of doing away with the evils which
result from checking, warping, and shrinking. For this purpose both
kiln-drying and air-seasoning are largely in use.
The drying of material is a subject which is extremely important to
most industries, and in no industry is it of more importance than in
the lumber trade. Timber drying means not only the extracting of so
much water, but goes very deeply into the quality of the wood, its
workability and its cell strength, etc.
Kiln-drying, which dries the wood at a uniformly rapid rate by
artificially heating it in inclosed rooms, has become a part of almost
every woodworking industry, as without it the construction of the
finished product would often be impossible. Nevertheless much
unseasoned or imperfectly seasoned wood is used, as is evidenced by
the frequent shrinkage and warping of the finished articles. This is
explained to a certain extent by the fact that the manufacturer is
often so hard pressed for his product that he is forced to send out an
inferior article, which the consumer is willing to accept in that
condition rather than to wait several weeks or months for an article
made up of thoroughly seasoned material, and also that dry kilns are
at present constructed and operated largely without thoroughgoing
system.
Forms of kilns and mode of operation have commonly been copied by one
woodworking plant after the example of some neighboring establishment.
In this way it has been brought about that the present practices have
many shortcomings. The most progressive operators, however, have
experimented freely in the effort to secure special results desirable
for their peculiar products. Despite the diversity of practice, it is
possible to find among the larger and more enterprising operators a
measure of agreement, as to both methods and results, and from this to
outline the essentials of a correct theory. As a result, properly
seasoned wood commands a high price, and in some cases cannot be
obtained at all.
Wood seasoned out of doors, which by many is supposed to be much
superior to kiln-dried material, is becoming very scarce, as the
demand for any kind of wood is so great that it is thought not to pay
to hold it for the time necessary to season it properly. How long this
state of affairs is going to last it is difficult to say, but it is
believed that a reaction will come when the consumer learns that in
the long run it does not pay to use poorly seasoned material. Such a
condition has now arisen in connection with another phase of the
seasoning of wood; it is a commonly accepted fact that dry wood will
not decay nearly so fast as wet or green wood; nevertheless, the
immense superiority of seasoned over unseasoned wood for all purposes
where resistance to decay is necessary has not been sufficiently
recognized. In the times when wood of all kinds was both plentiful and
cheap, it mattered little in most cases how long it lasted or resisted
decay. Wood used for furniture, flooring, car construction, cooperage,
etc., usually got some chance to dry out before or after it was placed
in use. The wood which was exposed to decaying influences was
generally selected from those woods which, whatever their other
qualities might be, would resist decay longest.
To-day conditions have changed, so that wood can no longer be used to
the same extent as in former years. Inferior woods with less lasting
qualities have been pressed into service. Although haphazard methods
of cutting and subsequent use are still much in vogue, there are many
signs that both lumbermen and consumers are awakening to the fact that
such carelessness and wasteful methods of handling wood will no longer
do, and must give way to more exact and economical methods. The reason
why many manufacturers and consumers of wood are still using the older
methods is perhaps because of long custom, and because they have not
yet learned that, though the saving to be obtained by the application
of good methods has at all times been appreciable, now, when wood is
more valuable, a much greater saving is possible. The increased cost
of applying economical methods is really very slight, and is many
times exceeded by the value of the increased service which can be
secured through its use.
Manner of Evaporation of Water
The evaporation of water from wood takes place largely through the
ends, _i.e._, in the direction of the longitudinal axis of the wood
fibres. The evaporation from the other surfaces takes place very
slowly out of doors, and with greater rapidity in a dry kiln. The rate
of evaporation differs both with the kind of timber and its shape;
that is, thin material will dry more rapidly than heavier stock.
Sapwood dries faster than heartwood, and pine more rapidly than oak or
other hardwoods.
Tests made show little difference in the rate of evaporation in sawn
and hewn stock, the results, however, not being conclusive. Air-drying
out of doors takes from two months to a year, the time depending on
the kind of timber, its thickness, and the climatic conditions. After
wood has reached an air-dry condition it absorbs water in small
quantities after a rain or during damp weather, much of which is
immediately lost again when a few warm, dry days follow. In this way
wood exposed to the weather will continue to absorb water and lose it
for indefinite periods.
When soaked in water, seasoned woods absorb water rapidly. This at
first enters into the wood through the cell walls; when these are
soaked, the water will fill the cell lumen, so that if constantly
submerged the wood may become completely filled with water.
The following figures show the gain in weight by absorption of several
coniferous woods, air-dry at the start, expressed in per cent of the
kiln-dry weight:
ABSORPTION OF WATER BY DRY WOOD
---------------------------------------------------------------
| White Pine | Red Cedar | Hemlock | Tamarack
---------------------------------------------------------------
Air-dried | 108 | 109 | 111 | 108
Kiln-dried | 100 | 100 | 100 | 100
In water 1 day | 135 | 120 | 133 | 129
In water 2 days | 147 | 126 | 144 | 136
In water 3 days | 154 | 132 | 149 | 142
In water 4 days | 162 | 137 | 154 | 147
In water 5 days | 165 | 140 | 158 | 150
In water 7 days | 176 | 143 | 164 | 156
In water 9 days | 179 | 147 | 168 | 157
In water 11 days | 184 | 149 | 173 | 159
In water 14 days | 187 | 150 | 176 | 159
In water 17 days | 192 | 152 | 176 | 161
In water 25 days | 198 | 155 | 180 | 161
In water 30 days | 207 | 158 | 183 | 166
---------------------------------------------------------------
Rapidity of Evaporation
The rapidity with which water is evaporated, that is, the rate of
drying, depends on the size and shape of the piece and on the
structure of the wood. An inch board dries more than four times as
fast as a four-inch plank, and more than twenty times as fast as a
ten-inch timber. White pine dries faster than oak. A very moist piece
of pine or oak will, during one hour, lose more than four times as
much water per square inch from the cross-section, but only one half
as much from the tangential as from the radial section. In a long
timber, where the ends or cross-sections form but a small part of the
drying surface, this difference is not so evident. Nevertheless, the
ends dry and shrink first, and being opposed in this shrinkage by the
more moist adjoining parts, they check, the cracks largely
disappearing as seasoning progresses.
High temperatures are very effective in evaporating the water from
wood, no matter how humid the air, and a fresh piece of sapwood may
lose weight in boiling water, and can be dried to quite an extent in
hot steam.
In drying chemicals or fabrics, all that is required is to provide
heat enough to vaporize the moisture and circulation enough to carry
off the vapor thus secured, and the quickest and most economical means
to these ends may be used. While on the other hand, in drying wood,
whether in the form of standard stock or the finished product, the
application of the requisite heat and circulation must be carefully
regulated throughout the entire process, or warping and checking are
almost certain to result. Moreover, wood of different shapes and
thicknesses is very differently effected by the same treatment.
Finally, the tissues composing the wood, which vary in form and
physical properties, and which cross each other in regular directions,
exert their own peculiar influences upon its behavior during drying.
With our native woods, for instance, summer-wood and spring-wood show
distinct tendencies in drying, and the same is true in a less degree
of heartwood, as contrasted with sapwood. Or, again, pronounced
medullary rays further complicate the drying problem.
Physical Properties that influence Drying
The principal properties which render the drying of wood peculiarly
difficult are: (1) The irregular shrinkage; (2) the different ways in
which water is contained; (3) the manner in which moisture transfuses
through the wood from the center to the surface; (4) the plasticity of
the wood substance while moist and hot; (5) the changes which take
place in the hygroscopic and chemical nature of the surface; and (6)
the difference produced in the total shrinkage by different rates of
drying.
The shrinkage is unequal in different directions and in different
portions of the same piece. It is greatest in the circumferential
direction of the tree, being generally twice as great in this
direction as in the radial direction. In the longitudinal direction,
for most woods, it is almost negligible, being from 20 to over 100
times as great circumferentially as longitudinally.
There is a great variation in different species in this respect.
Consequently, it follows from necessity that large internal strains
are set up when the wood shrinks, and were it not for its plasticity
it would rupture. There is an enormous difference in the total amount
of shrinkage of different species of wood, varying from a shrinkage of
only 7 per cent in volume, based on the green dimensions, in the case
of some of the cedars to nearly 50 per cent in the case of some
species of eucalyptus.
When the free water in the capillary spaces of the wood fibre is
evaporated it follows the laws of evaporation from capillary spaces,
except that the passages are not all free passages, and much of the
water has to pass out by a process of transfusion through the moist
cell walls. These cell walls in the green wood completely surround the
cell cavities so that there are no openings large enough to offer a
passage to water or air.
The well-known "pits" in the cell walls extend through the secondary
thickening only, and not through the primary walls. This statement
applies to the tracheids and parenchyma cells in the conifer
(gymnosperms), and to the tracheids, parenchyma cells, and the wood
fibres in the broad-leaved trees (angiosperms); the vessels in the
latter, however, form open passages except when clogged by ingrowth
called tyloses, and the resin canals in the former sometimes form
occasional openings.
By heating the wood above the boiling point, corresponding to the
external pressure, the free water passes through the cell walls more
readily.
To remove the moisture from the wood substance requires heat in
addition to the latent heat of evaporation, because the molecules of
moisture are so intimately associated with the molecules, minute
particles composing the wood, that energy is required to separate them
therefrom.
Carefully conducted experiments show this to be from 16.6 to 19.6
calories per grain of dry wood in the case of beech, long-leaf pine,
and sugar maple.
The difficulty imposed in drying, however, is not so much the
additional heat required as it is in the rate at which the water
transfuses through the solid wood.
SECTION VIII
ADVANTAGES IN SEASONING
Three most important advantages of seasoning have already been made
apparent:
1. Seasoned timber lasts much longer than unseasoned. Since
the decay of timber is due to the attacks of wood-destroying
fungi, and since the most important condition of the growth
of these fungi is water, anything which lessens the amount
of water in wood aids in its preservation.
2. In the case of treated timber, seasoning before treatment
greatly increases the effectiveness of the ordinary methods
of treatment, and seasoning after treatment prevents the
rapid leaching out of the salts introduced to preserve the
timber.
3. The saving in freight where timber is shipped from one
place to another. Few persons realize how much water green
wood contains, or how much it will lose in a comparatively
short time. Experiments along this line with lodge-pole
pine, white oak, and chestnut gave results which were a
surprise to the companies owning the timber.
Freight charges vary considerably in different parts of the country;
but a decrease of 35 to 40 per cent in weight is important enough to
deserve everywhere serious consideration from those in charge of
timber operations.
When timber is shipped long distances over several roads, as is coming
to be more and more the case, the saving in freight will make a
material difference in the cost of lumber operations, irrespective of
any other advantages of seasoning.
Prevention of Checking and Splitting
Under present methods much timber is rendered unfit for use by
improper seasoning. Green timber, particularly when cut during
January, February, and March, when the roots are most active, contains
a large amount of water. When exposed to the sun and wind or to high
temperatures in a drying room, the water will evaporate more rapidly
from the outer than from the inner parts of the piece, and more
rapidly from the ends than from the sides. As the water evaporates,
the wood shrinks, and when the shrinkage is not fairly uniform the
wood cracks and splits.
When wet wood is piled in the sun, evaporation goes on with such
unevenness that the timbers split and crack in some cases so badly as
to become useless for the purpose for which it was intended. Such
uneven drying can be prevented by careful piling, keeping the logs
immersed in a log pond until wanted, or by piling or storing under an
open shed so that the sun cannot get at them.
Experiments have also demonstrated that injury to stock in the way of
checking and splitting always develops immediately after the stock is
taken into the dry kiln, and is due to the degree of humidity being
too low.
The receiving end of the kiln should always be kept moist, where the
stock has not been steamed before being put into the kiln, as when the
air is too dry it tends to dry the outside of the stock first--which
is termed "case-hardening"--and in so doing shrinks and closes up the
pores. As the material is moved down the kiln (as in the case of
"progressive kilns"), it absorbs a continually increasing amount of
heat, which tends to drive off the moisture still present in the
center of the piece, the pores on the outside having been closed up,
there is no exit for the vapor or steam that is being rapidly formed
in the center of the piece. It must find its way out in some manner,
and in doing so sets up strains, which result either in checking or
splitting. If the humidity had been kept higher, the outside of the
piece would not have dried so quickly, and the pores would have
remained open for the exit of the moisture from the interior of the
piece, and this trouble would have been avoided. (See also article
following.)
Shrinkage of Wood
Since in all our woods, cells with thick walls and cells with thin
walls are more or less intermixed, and especially as the spring-wood
and summer-wood nearly always differ from each other in this respect,
strains and tendencies to warp are always active when wood dries out,
because the summer-wood shrinks more than the spring-wood, and heavier
wood in general shrinks more than light wood of the same kind.
If a thin piece of wood after drying is placed upon a moist surface,
the cells on the under side of the piece take up moisture and swell
before the upper cells receive any moisture. This causes the under
side of the piece to become longer than the upper side, and as a
consequence warping occurs. Soon, however, the moisture penetrates to
all the cells and the piece straightens out. But while a thin board of
pine curves laterally it remains quite straight lengthwise, since in
this direction both shrinkage and swelling are small. If one side of a
green board is exposed to the sun, warping is produced by the removal
of water and consequent shrinkage of the side exposed; this may be
eliminated by the frequent turning of the topmost pieces of the piles
in order that they may be dried evenly.
As already stated, wood loses water faster from the ends than from the
longitudinal faces. Hence the ends shrink at a different rate from the
interior parts. The faster the drying at the surface, the greater is
the difference in the moisture of the different parts, and hence the
greater the strains and consequently also the greater amount of
checking. This becomes very evident when freshly cut wood is placed in
the sun, and still more when put into a hot, dry kiln. While most of
these smaller checks are only temporary, closing up again, some large
radial checks remain and even grow larger as drying progresses. Their
cause is a different one and will presently be explained. The
temporary checks not only appear at the ends, but are developed on
the sides also, only to a much smaller degree. They become especially
annoying on the surface of thick planks of hardwoods, and also on
peeled logs when exposed to the sun.
So far we have considered the wood as if made up only of parallel
fibres all placed longitudinally in the log. This, however, is not the
case. A large part of the wood is formed by the medullary or pith
rays. In pine over 15,000 of these occur on a square inch of a
tangential section, and even in oak the very large rays, which are
readily visible to the eye, represent scarcely a hundredth part of the
number which a microscope reveals, as the cells of these rays have
their length at right angles to the direction of the wood fibres.
If a large pith ray of white oak is whittled out and allowed to dry,
it is found to shrink greatly in its width, while, as we have stated,
the fibres to which the ray is firmly grown in the wood do not shrink
in the same direction. Therefore, in the wood, as the cells of the
pith ray dry they pull on the longitudinal fibres and try to shorten
them, and, being opposed by the rigidity of the fibres, the pith ray
is greatly strained. But this is not the only strain it has to bear.
Since the fibres shrink as much again as the pith ray, in this its
longitudinal direction, the fibres tend to shorten the ray, and the
latter in opposing this prevents the former from shrinking as much as
they otherwise would.
Thus the structure is subjected to two severe strains at right angles
to each other, and herein lies the greatest difficulty of wood
seasoning, for whenever the wood dries rapidly these fibres have not
the chance to "give" or accommodate themselves, and hence fibres and
pith rays separate and checking results, which, whether visible or
not, are detrimental in the use of the wood.
The contraction of the pith rays parallel to the length of the board
is probably one of the causes of the small amount of longitudinal
shrinkage which has been observed in boards. This smaller shrinkage of
the pith rays along the radius of the log (the length of the pith
ray), opposing the shrinkage of the fibres in this direction, becomes
one of the causes of the second great trouble in wood seasoning,
namely, the difference in the shrinkage along the radius and that
along the rings or tangent. This greater tangential shrinkage appears
to be due in part to the causes just mentioned, but also to the fact
that the greatly shrinking bands of summer-wood are interrupted along
the radius by as many bands of porous spring-wood, while they are
continuous in the tangential direction. In this direction, therefore,
each such band tends to shrink, as if the entire piece were composed
of summer-wood, and since the summer-wood represents the greater part
of the wood substance, this greater tendency to tangential shrinkage
prevails.
The effect of this greater tangential shrinkage effects every phase of
woodworking. It leads to permanent checks and causes the log or piece
to split open on drying. Sawed in two, the flat sides of the log
become convex; sawed into timber, it checks along the median line of
the four faces, and if converted into boards, the latter checks
considerably from the end through the center, all owing to the greater
tangential shrinkage of the wood.
Briefly, then, shrinkage of wood is due to the fact that the cell
walls grow thinner on drying. The thicker cell walls and therefore the
heavier wood shrinks most, while the water in the cell cavities does
not influence the volume of the wood.
Owing to the great difference of cells in shape, size, and thickness
of walls, and still more in their arrangement, shrinkage is not
uniform in any kind of wood. This irregularity produces strains, which
grow with the difference between adjoining cells and are greatest at
the pith rays. These strains cause warping and checking, but exist
even where no outward signs are visible. They are greater if the wood
is dried rapidly than if dried slowly, but can never be entirely
avoided.
Temporary checks are caused by the more rapid drying of the outer
parts of any stick; permanent checks are due to the greater shrinkage,
tangentially, along the rings than along the radius. This, too, is the
cause of most of the ordinary phenomena of shrinkage, such as the
difference in behavior of the entire and quartered logs, "bastard"
(tangent) and rift (radial) boards, etc., and explains many of the
phenomena erroneously attributed to the influence of bark, or of the
greater shrinkage of outer and inner parts of any log.
Once dry, wood may be swelled again to its original size by soaking in
water, boiling, or steaming. Soaked pieces on drying shrink again as
before; boiled and steamed pieces do the same, but to a slightly less
degree. Neither hygroscopicity, _i.e._, the capacity of taking up
water, nor shrinkage of wood can be overcome by drying at temperatures
below 200 degrees Fahrenheit. Higher temperatures, however, reduce
these qualities, but nothing short of a coaling heat robs wood of the
capacity to shrink and swell.
Rapidly dried in a kiln, the wood of oak and other hardwoods
"case-harden," that is, the outer part dries and shrinks before the
interior has a chance to do the same, and thus forms a firm shell or
case of shrunken, commonly checked wood around the interior. This
shell does not prevent the interior from drying, but when this drying
occurs the interior is commonly checked along the medullary rays,
commonly called "honeycombing" or "hollow-horning." In practice this
occurrence can be prevented by steaming or sweating the wood in the
kiln, and still better by drying the wood in the open air or in a shed
before placing in the kiln. Since only the first shrinkage is apt to
check the wood, any kind of lumber which has once been air-dried
(three to six months for one-inch stuff) may be subjected to kiln heat
without any danger from this source.
Kept in a bent or warped condition during the first shrinkage, the
wood retains the shape to which it has been bent and firmly opposes
any attempt at subsequent straightening.
Sapwood, as a rule, shrinks more than heartwood of the same weight,
but very heavy heartwood may shrink more than lighter sapwood. The
amount of water in wood is no criterion of its shrinkage, since in wet
wood most of the water is held in the cavities, where it has no effect
on the volume.
The wood of pine, spruce, cypress, etc., with its very regular
structure, dries and shrinks evenly, and suffers much less in
seasoning than the wood of broad-leaved (hardwood) trees. Among the
latter, oak is the most difficult to dry without injury.
Desiccating the air with certain chemicals will cause the wood to dry,
but wood thus dried at 80 degrees Fahrenheit will still lose water in
the kiln. Wood dried at 120 degrees Fahrenheit loses water still if
dried at 200 degrees Fahrenheit, and this again will lose more water
if the temperature be raised, so that _absolutely dry wood_ cannot be
obtained, and chemical destruction sets in before all the water is
driven off.
On removal from the kiln, the dry wood at once takes up moisture from
the air, even in the driest weather. At first the absorption is quite
rapid; at the end of a week a short piece of pine, 1-1/2 inches thick,
has regained two thirds of, and, in a few months, all the moisture
which it had when air-dry, 8 to 10 per cent, and also its former
dimensions. In thin boards all parts soon attain the same degree of
dryness. In heavy timbers the interior remains more moist for many
months, and even years, than the exterior parts. Finally an
equilibrium is reached, and then only the outer parts change with the
weather.
With kiln-dried woods all parts are equally dry, and when exposed, the
moisture coming from the air must pass through the outer parts, and
thus the order is reversed. Ordinary timber requires months before it
is at its best. Kiln-dried timber, if properly handled, is prime at
once.
Dry wood if soaked in water soon regains its original volume, and in
the heartwood portion it may even surpass it; that is to say, swell to
a larger dimension than it had when green. With the soaking it
continues to increase in weight, the cell cavities filling with water,
and if left many months all pieces sink. Yet after a year's immersion
a piece of oak 2 by 2 inches and only 6 inches long still contains
air; _i.e._, it has not taken up all the water it can. By rafting or
prolonged immersion, wood loses some of its weight, soluble materials
being leached out, but it is not impaired either as fuel or as
building material. Immersion, and still more boiling and steaming,
reduce the hygroscopicity of wood and therefore also the troublesome
"working," or shrinking and swelling.
Exposure in dry air to a temperature of 300 degrees Fahrenheit for a
short time reduces but does not destroy the hygroscopicity, and with
it the tendency to shrink and swell. A piece of red oak which has been
subjected to a temperature of over 300 degrees Fahrenheit still swells
in hot water and shrinks in a dry kiln.
Expansion of Wood
It must not be forgotten that timber, in common with every other
material, expands as well as contracts. If we extract the moisture
from a piece of wood and so cause it to shrink, it may be swelled to
its original volume by soaking it in water, but owing to the
protection given to most timber in dwelling-houses it is not much
affected by wet or damp weather. The shrinkage is more apparent, more
lasting, and of more consequence to the architect, builder, or owner
than the slight expansion which takes place, as, although the amount
of moisture contained in wood varies with the climate conditions, the
consequence of dampness or moisture on good timber used in houses only
makes itself apparent by the occasional jamming of a door or window in
wet or damp weather.
Considerable expansion, however, takes place in the wood-paving of
streets, and when this form of paving was in its infancy much trouble
occurred owing to all allowances not having been made for this
contingency, the trouble being doubtless increased owing to the blocks
not being properly seasoned; curbing was lifted or pushed out of line
and gully grids were broken by this action. As a rule in street paving
a space of one or two inches wide is now left next to the curb, which
is filled with sand or some soft material, so that the blocks may
expand longitudinally without injuring the contour or affecting the
curbs. But even with this arrangement it is not at all unusual for an
inch or more to have to be cut off paving blocks parallel to the
channels some time after the paving has been laid, owing to the
expansion of the wood exceeding the amounts allowed.
Considerable variation occurs in the expansion of wood blocks, and it
is noticeable in the hardwoods as well as in the softwoods, and is
often greater in the former than in the latter.
Expansion takes place in the direction of the length of the blocks as
they are laid across the street, and causes no trouble in the other
direction, the reason being that the lengthway of a block of wood is
across the grain, of the timber, and it expands or contracts as a
plank does. On one occasion, in a roadway forty feet wide, expansion
occurred until it amounted to four inches on each side, or eight
inches in all. This continual expansion and contraction is doubtless
the cause of a considerable amount of wood street-paving bulging and
becoming filled with ridges and depressions.
Elimination of Stain and Mildew
A great many manufacturers, and particularly those located in the
Southern States, experience a great amount of difficulty in their
timber becoming stained and mildewed. This is particularly true with
gum wood, as it will frequently stain and mould in twenty-four hours,
and they have experienced so much of this trouble that they have, in a
great many instances, discontinued cutting it during the summer
season.
If this matter were given proper attention they should be able to
eliminate a great deal of this difficulty, as no doubt they will find
after investigation that the mould has been caused by the stock being
improperly piled to the weather.
Freshly sawn wood, placed in close piles during warm, damp weather in
the months of July and August, presents especially favorable
conditions for mould and stain. In all cases it is the moist condition
and retarded drying of the wood which causes this. Therefore, any
method which will provide for the rapid drying of the wood before or
after piling will tend to prevent the difficulty, and the best method
for eliminating mould is (1) to provide for as little delay as
possible between the felling of the tree, and its manufacture into
rough products before the sap has had an opportunity of becoming sour.
This is especially necessary with trees felled from April to
September, in the region north of the Gulf States, and from March to
November in the latter, while the late fall and winter cutting should
all be worked up by March or April. (2) The material should be piled
to the weather immediately after being sawn or cut, and every
precaution should be taken in piling to facilitate rapid drying, by
keeping the piles or ricks up off the ground. (3) All weeds (and
emphasis should be placed on the ALL) and other vegetation should be
kept well clear of the piles, in order that the air may have a clear
and unobstructed passage through and around the piles, and (4) the
piles should be so constructed that each stick or piece will have as
much air space about it as it is possible to give to it.
If the above instructions are properly carried out, there will be
little or no difficulty experienced with mould appearing on the
lumber.
SECTION IX
DIFFICULTIES OF DRYING WOOD
Seasoning and kiln-drying is so important a process in the manufacture
of woods that a need is keenly felt for fuller information regarding
it, based upon scientific study of the behavior of various species at
different mechanical temperatures and under different mechanical
drying processes. The special precautions necessary to prevent loss of
strength or distortion of shape render the drying of wood especially
difficult.
All wood when undergoing a seasoning process, either natural (by air)
or mechanical (by steam or heat in a dry kiln), checks or splits more
or less. This is due to the uneven drying-out of the wood and the
consequent strains exerted in opposite directions by the wood fibres
in shrinking. This shrinkage, it has been proven, takes place both
end-wise and across the grain of the wood. The old tradition that wood
does not shrink end-wise has long since been shattered, and it has
long been demonstrated that there is an end-wise shrinkage.
In some woods it is very light, while in others it is easily
perceptible. It is claimed that the average end shrinkage, taking all
the woods, is only about 1-1/2 per cent. This, however, probably has
relation to the average shrinkage on ordinary lumber as it is used and
cut and dried. Now if we depart from this and take veneer, or basket
stock, or even stave bolts where they are boiled, causing swelling
both end-wise and across the grain or in dimension, after they are
thoroughly dried, there is considerably more evidence of end
shrinkage. In other words, a slack barrel stave of elm, say, 28 or 30
inches in length, after being boiled might shrink as much in
thoroughly drying-out as compared to its length when freshly cut, as a
12-foot elm board.
It is in cutting veneer that this end shrinkage becomes most readily
apparent. In trimming with scoring knives it is done to exact measure,
and where stock is cut to fit some specific place there has been
observed a shrinkage on some of the softer woods, like cottonwood,
amounting to fully 1/8 of an inch in 36 inches. And at times where
drying has been thorough the writer has noted a shrinkage of 1/8 of an
inch on an ordinary elm cabbage-crate strip 36 inches long, sawed from
the log without boiling.
There are really no fixed rules of measurement or allowance, however,
because the same piece of wood may vary under different conditions,
and, again, the grain may cross a little or wind around the tree, and
this of itself has a decided effect on the amount of what is termed
"end shrinkage."
There is more checking in the wood of the broad-leaf (hardwood) trees
than in that of the coniferous (softwood) trees, more in sapwood than
in heartwood, and more in summer-wood than in spring-wood.
Inasmuch as under normal conditions of weather, water evaporates less
rapidly during the early seasoning of winter, wood that is cut in the
autumn and early winter is considered less subject to checking than
that which is cut in spring and summer.
Rapid seasoning, except after wood has been thoroughly soaked or
steamed, almost invariably results in more or less serious checking.
All hardwoods which check or warp badly during the seasoning should be
reduced to the smallest practicable size before drying to avoid the
injuries involved in this process, and wood once seasoned _should
never again be exposed to the weather_, since all injuries due to
seasoning are thereby aggravated.
Seasoning increases the strength of wood in every respect, and it is
therefore of great importance to protect the wood against moisture.
Changes rendering Drying difficult
An important property rendering drying of wood peculiarly difficult is
the changes which occur in the hygroscopic properties of the surface
of a stick, and the rate at which it will allow moisture to pass
through it. If wood is dried rapidly the surface soon reaches a
condition where the transfusion is greatly hindered and sometimes
appears almost to cease. The nature of this action is not well
understood and it differs greatly in different species. Bald cypress
(_Taxodium distichum_) is an example in which this property is
particularly troublesome. The difficulty can be overcome by regulating
the humidity during the drying operation. It is one of the factors
entering into production of what is called "case-hardening" of wood,
where the surface of the piece becomes hardened in a stretched or
expanded condition, and subsequent shrinkage of the interior causes
"honeycombing," "hollow-horning," or internal checking. The outer
surface of the wood appears to undergo a chemical change in the nature
of hydrolization or oxidization, which alters the rate of absorption
and evaporation in the air.
As the total amount of shrinkage varies with the rate at which the
wood is dried, it follows that the outer surface of a rapidly dried
board shrinks less than the interior. This sets up an internal stress,
which, if the board be afterward resawed into two thinner boards by
slicing it through the middle, causes the two halves to cup with their
convex surfaces outward. This effect may occur even though the
moisture distribution in the board has reached a uniform condition,
and the board is thoroughly dry before it is resawed. It is distinct
from the well-known "case-hardening" effect spoken of above, which is
caused by unequal moisture conditions.
The manner in which the water passes from the interior of a piece of
wood to its surface has not as yet been fully determined, although it
is one of the most important factors which influence drying. This must
involve a transfusion of moisture through the cell walls, since, as
already mentioned, except for the open vessels in the hardwoods, free
resin ducts in the softwoods, and possibly the intercellular spaces,
the cells of green wood are enclosed by membranes and the water must
pass through the walls or the membranes of the pits. Heat appears to
increase this transfusion, but experimental data are lacking.
It is evident that to dry wood properly a great many factors must be
taken into consideration aside from the mere evaporation of moisture.
Losses Due to Improper Kiln-drying
In some cases there is practically no loss in drying, but more often
it ranges from 1 to 3 per cent, and 7 to 10 per cent in refractory
woods such as gum. In exceptional instances the losses are as high as
33 per cent.
In air-drying there is little or no control over the process; it may
take place too rapidly on some days and too slowly on others, and it
may be very non-uniform.
Hardwoods in large sizes almost invariably check.
By proper kiln-drying these unfavorable circumstances may be
eliminated. However, air-drying is unquestionably to be preferred to
bad kiln-drying, and when there is any doubt in the case it is
generally safer to trust to air-drying.
If the fundamental principles are all taken care of, green lumber can
be better dried in the dry kiln.
Properties of Wood that affect Drying
It is clear, from the previous discussion of the structure of wood,
that this property is of first importance among those influencing the
seasoning of wood. The free water way usually be extracted quite
readily from porous hardwoods. The presence of tyloses in white oak
makes even this a difficult problem. On the other hand, its more
complex structure usually renders the hygroscopic moisture quite
difficult to extract.
The lack of an open, porous structure renders the transfusion of
moisture through some woods very slow, while the reverse may be true
of other species. The point of interest is that all the different
variations in structure affect the drying rates of woods. The
structure of the gums suggests relatively easy seasoning.
Shrinkage is a very important factor affecting the drying of woods.
Generally speaking, the greater the shrinkage the more difficult it is
to dry wood. Wood shrinks about twice as much tangentially as
radially, thus introducing very serious stresses which may cause loss
in woods whose total shrinkage is large. It has been found that the
amount of shrinkage depends, to some extent, on the rate and
temperature at which woods season. Rapid drying at high or low
temperature results in slight shrinkage, while slow drying, especially
at high temperature, increases the shrinkage.
As some woods must be dried in one way and others in other ways, to
obtain the best general results, this effect may be for the best in
one case and the reverse in others. As an example one might cite the
case of Southern white oak. This species must be dried very slowly at
low temperatures in order to avoid the many evils to which it is heir.
It is interesting to note that this method tends to increase the
shrinkage, so that one might logically expect such treatment merely to
aggravate the evils. Such is not the case, however, as too fast drying
results in other defects much worse than that of excessive shrinkage.
Thus we see that the shrinkage of any given species of wood depends to
a great extent on the method of drying. Just how much the shrinkage of
gum is affected by the temperature and drying rate is not known at
present. There is no doubt that the method of seasoning affects the
shrinkage of the gums, however. It is just possible that these woods
may shrink longitudinally more than is normal, thus furnishing another
cause for their peculiar action under certain circumstances. It has
been found that the properties of wood which affect the seasoning of
the gums are, in the order of their importance: (1) The indeterminate
and erratic grain; (2) the uneven shrinkage with the resultant
opposing stresses; (3) the plasticity under high temperature while
moist; and (4) the slight apparent lack of cohesion between the
fibres. The first, second, and fourth properties are clearly
detrimental, while the third may possibly be an advantage in reducing
checking and "case-hardening."
The grain of the wood is a prominent factor also affecting the
problem. It is this factor, coupled with uneven shrinkage, which is
probably responsible, to a large extent, for the action of the gums in
drying. The grain may be said to be more or less indeterminate. It is
usually spiral, and the spiral may reverse from year to year of the
tree's growth. When a board in which this condition exists begins to
shrink, the result is the development of opposing stresses, the effect
of which is sometimes disastrous. The shrinkage around the knots seems
to be particularly uneven, so that checking at the knots is quite
common.
Some woods, such as Western red cedar, redwood, and eucalyptus, become
very plastic when hot and moist. The result of drying-out the free
water at high temperature may be to collapse the cells. The gums are
known to be quite soft and plastic, if they are moist, at high
temperature, but they do not collapse so far as we have been able to
determine.
The cells of certain species of wood appear to lack cohesion,
especially at the junction between the annual rings. As a result,
checks and ring shakes are very common in Western larch and hemlock.
The parenchyma cells of the medullary rays in oak do not cohere
strongly and often check open, especially when steamed too severely.
Unsolved Problems in Kiln-drying
1. Physical data of the properties of wood in relation to
heat are meagre.
2. Figures on the specific heat of wood are not readily
available, though upon this rests not only the exact
operation of heating coils for kilns, but the theory of
kiln-drying as a whole.
3. Great divergence is shown in the results of experiments
in the conductivity of wood. It remains to be seen whether
the known variation of conductivity with moisture content
will reduce these results to uniformity.
4. The maximum or highest temperature to which the different
species of wood may be exposed without serious loss of
strength has not yet been determined.
5. The optimum or absolute correct temperature for drying
the different species of wood is as yet entirely unsettled.
6. The inter-relation between wood and water is as
imperfectly known to dry-kiln operators as that between wood
and heat.
7. What moisture conditions obtain in a stick of air-dried
wood?
8. How is the moisture distinguished?
9. What is its form?
10. What is the meaning of the peculiar surface conditions
which even in air-dried wood appear to indicate incipient
"case-hardening"?
11. The manner in which the water passes from the interior
of a piece of wood to its surface has not as yet been fully
determined.
These questions can be answered thus far only by speculation or, at
best, on the basis of incomplete data.
Until these problems are solved, kiln-drying must necessarily remain
without the guidance of complete scientific theory.
A correct understanding of the principles of drying is rare, and
opinions in regard to the subject are very diverse. The same lack of
knowledge exists in regard to dry kilns. The physical properties of
the wood which complicate the drying operation and render it distinct
from that of merely evaporating free water from some substance like a
piece of cloth must be studied experimentally. It cannot well be
worked out theoretically.
SECTION X
HOW WOOD IS SEASONED
Methods of Drying
The choice of a method of drying depends largely upon the object in
view. The principal objects may be grouped under three main heads, as
follows:
1. To reduce shipping weight.
2. To reduce the quantity necessary to carry in stock.
3. To prepare the wood for its ultimate use and improve its
qualities.
When wood will stand the temperature without excessive checking or
undue shrinkage or loss in strength, the first object is most readily
attained by heating the wood above the boiling point in a closed
chamber, with a large circulation of air or vapor, so arranged that
the excess steam produced will escape. This process manifestly does
not apply to many of the hardwoods, but is applicable to many of the
softwoods. It is used especially in the northwestern part of the
United States, where Douglas fir boards one inch thick are dried in
from 40 to 65 hours, and sometimes in as short a time as 24 hours. In
the latter case superheated steam at 300 degrees Fahrenheit was forced
into the chamber but, of course, the lumber could not be heated
thereby much above the boiling point so long as it contained any free
water.
This lumber, however, contained but 34 per cent moisture to start
with, and the most rapid rate was 1.6 per cent loss per hour.
The heat of evaporation may be supplied either by superheated steam or
by steam pipes within the kiln itself.
The quantity of wood it is necessary to carry in stock is naturally
reduced when either of the other two objects is attained and,
therefore, need not necessarily be discussed.
In drying to prepare for use and to improve quality, careful and
scientific drying is called for. This applies more particularly to the
hardwoods, although it may be required for softwoods also.
Drying at Atmospheric Pressure
Present practice of kiln-drying varies tremendously and there is no
uniformity or standard method.
Temperatures vary anywhere from 65 to 165 degrees Fahrenheit, or even
higher, and inch boards three to six months on the sticks are being
dried in from four days to three weeks, and three-inch material in
from two to five months.
All methods in use at atmospheric pressure may be classified under the
following headings. The kilns may be either progressive or
compartment, and preliminary steaming may or may not be used with any
one of these methods:
1. Dry air heated. This is generally obsolete.
2. Moist air.
_a._ Ventilated.
_b._ Forced draft.
_c._ Condensing.
_d._ Humidity regulated.
_e._ Boiling.
3. Superheated steam.
Drying under Pressure and Vacuum
Various methods of drying wood under pressures other than atmospheric
have been tried. Only a brief mention of this subject will be made.
Where the apparatus is available probably the quickest way to dry wood
is first to heat it in saturated steam at as high a temperature as the
species can endure without serious chemical change until the heat has
penetrated to the center, then follow this with a vacuum.
By this means the self-contained specific heat of the wood and the
water is made available for the evaporation, and the drying takes
place from the inside outwardly, just the reverse of that which occurs
by drying by means of external heat.
When the specimen has cooled this process is then to be repeated until
it has dried down to fibre-saturation point. It cannot be dried much
below this point by this method, since the absorption during the
heating operation will then equal the evaporation during the cooling.
It may be carried further, however, by heating in partially humidified
air, proportioning the relative humidity each time it is heated to the
degree of moisture present in the wood.
The point to be considered in this operation is that during the
heating process no evaporation shall be allowed to take place, but
only during the cooling. In this way surface drying and
"case-hardening" are prevented since the heat is from within and the
moisture passes from the inside outwardly. However, with some species,
notably oak, surface cracks appear as a network of fine checks along
the medullary rays.
In the first place, it should be borne in mind that it is the heat
which produces evaporation and not the air nor any mysterious property
assigned to a "vacuum."
For every pound of water evaporated at ordinary temperatures
approximately 1,000 British thermal units of heat are used up, or
"become latent," as it is called. This is true whether the evaporation
takes place in a vacuum or under a moderate air pressure. If this heat
is not supplied from an outside source it must be supplied by the
water itself (or the material being dried), the temperature of which
will consequently fall until the surrounding space becomes saturated
with vapor at a pressure corresponding to the temperature which the
water has reached; evaporation will then cease. The pressure of the
vapor in a space saturated with water vapor increases rapidly with
increase of temperature. At a so-called vacuum of 28 inches, which is
about the limit in commercial operations, and in reality signifies an
actual pressure of 2 inches of mercury column, the space will be
saturated with vapor at 101 degrees Fahrenheit. Consequently, no
evaporation will take place in such a vacuum unless the water be
warmer than 101 degrees Fahrenheit, provided there is no air leakage.
The qualification in regard to air is necessary, for the sake of
exactness, for the following reason: In any given space the total
actual pressure is made up of the combined pressures of all the gases
present. If the total pressure ("vacuum") is 2 inches, and there is no
air present, it is all produced by the water vapor (which saturates
the space at 101 degrees Fahrenheit); but if some air is present and
the total pressure is still maintained at 2 inches, then there must be
less vapor present, since the air is producing part of the pressure
and the space is no longer saturated at the given temperature.
Consequently further evaporation may occur, with a corresponding
lowering of the temperature of the water, until a balance is again
reached. Without further explanation it is easy to see that but little
water can be evaporated by a vacuum alone without addition of heat,
and that the prevalent idea that a vacuum can of itself produce
evaporation is a fallacy. If heat be supplied to the water, however,
either by conduction or radiation, evaporation will take place in
direct proportion to the amount of heat supplied, so long as the
pressure is kept down by the vacuum pump.
At 30 inches of mercury pressure (one atmosphere) the space becomes
saturated with vapor and equilibrium is established at 212 degrees
Fahrenheit. If heat be now supplied to the water, however, evaporation
will take place in proportion to the amount of heat supplied, so long
as the pressure remains that of one atmosphere, just as in the case of
the vacuum. Evaporation in this condition, where the vapor pressure at
the temperature of the water is equal to the gas pressure on the
water, is commonly called "boiling," and the saturated vapor entirely
displaces the air under continuous operation. Whenever the space is
not saturated with vapor, whether air is present or not, evaporation
will take place, by boiling if no air be present or by diffusion under
the presence of air, until an equilibrium between temperature and
vapor pressure is resumed.
Relative humidity is simply the ratio of the actual vapor pressure
present in a given space to the vapor pressure when the space is
saturated with vapor at the given temperature. It matters not whether
air be present or not. One hundred per cent humidity means that the
space contains all the vapor which it can hold at the given
temperature--it is saturated. Thus at 100 per cent humidity and 212
degrees Fahrenheit the space is saturated, and since the pressure of
saturated vapor at this temperature is one atmosphere, no air can be
present under these conditions. If, however, the total pressure at
this temperature were 20 pounds (5 pounds gauge), then it would mean
that there was 5 pounds air pressure present in addition to the vapor,
yet the space would still be saturated at the given temperature.
Again, if the temperature were 101 degrees Fahrenheit, the pressure of
saturated vapor would be only 1 pound, and the additional pressure of
14 pounds, if the total pressure were atmospheric, would be made up of
air. In order to have no air present and the space still saturated at
101 degrees Fahrenheit, the total pressure must be reduced to 1 pound
by a vacuum pump. Fifty per cent relative humidity, therefore,
signifies that only half the amount of vapor required to saturate the
space at the given temperature is present. Thus at 212 degrees
Fahrenheit temperature the vapor pressure would only be 7-1/2 pounds
(vacuum of 15 inches gauge). If the total pressure were atmospheric,
then the additional 7-1/2 pounds would be simply air.
"Live steam" is simply water-saturated vapor at a pressure usually
above atmospheric. We may just as truly have live steam at pressures
less than atmospheric, at a vacuum of 28 inches for instance. Only in
the latter case its temperature would be lower, _viz._, 101 degrees
Fahrenheit.
Superheated steam is nothing more than water vapor at a relative
humidity less than saturation, but is usually considered at pressures
above atmospheric, and in the absence of air. The atmosphere at, say,
50 per cent relative humidity really contains superheated steam or
vapor, the only difference being that it is at a lower temperature and
pressure than we are accustomed to think of in speaking of superheated
steam, and it has air mixed with it to make up the deficiency in
pressure below the atmosphere.
Two things should now be clear; that evaporation is produced by heat
and that the presence or absence of air does not influence the amount
of evaporation. It does, however, influence the rate of evaporation,
which is retarded by the presence of air. The main things influencing
evaporation are, first, the quantity of heat supplied and, second, the
relative humidity of the immediately surrounding space.
Drying by Superheated Steam
What this term really signifies is simply water vapor in the absence
of air in a condition of less than saturation. Kilns of this type are,
properly speaking, vapor kilns, and usually operate at atmospheric
pressure, but may be used at greater pressures or at less pressures.
As stated before, the vapor present in the air at any humidity less
than saturation is really "superheated steam," only at a lower
pressure than is ordinarily understood by this term, and mixed with
air. The main argument in favor of this process seems to be based on
the idea that steam is moist heat. This is true, however, only when
the steam is near saturation. When it is superheated it is just as dry
as air containing the same relative humidity. For instance, steam at
atmospheric pressure and heated to 248 degrees Fahrenheit has a
relative humidity of only 50 per cent and is just as dry as air
containing the same humidity. If heated to 306 degrees Fahrenheit, its
relative humidity is reduced to 20 per cent; that is to say, the ratio
of its actual vapor pressure (one atmosphere) to the pressure of
saturated vapor at this temperature (five atmospheres) is 1:5, or 20
per cent. Superheated vapor in the absence of air, however, parts with
its heat with great rapidity and finally becomes saturated when it has
lost all of its ability to cause evaporation. In this respect it is
more moist than air when it comes in contact with bodies which are at
a lower temperature. When saturated steam is used to heat the lumber
it can raise the temperature of the latter to its own temperature, but
cannot produce evaporation unless, indeed, the pressure is varied.
Only by the heat supplied above the temperature of saturation can
evaporation be produced.
Impregnation Methods
Methods of partially overcoming the shrinkage by impregnation of the
cell walls with organic materials closely allied to the wood substance
itself are in use. In one of these which has been patented, sugar is
used as the impregnating material, which is subsequently hardened or
"caramelized" by heating. Experiments which the United States Forest
Service has made substantiate the claims that the sugar does greatly
reduce the shrinkage of the wood; but the use of impregnation
processes is determined rather from a financial economic standpoint
than by the physical result obtained.
Another process consists in passing a current of electricity through
the wet boards or through the green logs before sawing. It is said
that the ligno cellulose and the sap are thus transformed by
electrolysis, and that the wood subsequently dries more rapidly.
Preliminary Treatments
In many dry kiln operations, especially where the kilns are not
designed for treatments with very moist air, the wood is allowed to
air-season from several months to a year or more before running it
into the dry kiln. In this way the surface dries below its
fibre-saturation point and becomes hardened or "set" and the
subsequent shrinkage is not so great. Moreover, there is less danger
of surface checking in the kiln, since the surface has already passed
the danger point. Many woods, however, check severely in air-drying or
case-harden in the air. It is thought that such woods can be
satisfactorily handled in a humidity-regulated kiln direct from the
saw.
Preliminary steaming is frequently used to moisten the surface if
case-hardened, and to heat the lumber through to the center before
drying begins. This is sometimes done in a separate chamber, but more
often in a compartment of the kiln itself, partitioned off by means of
a curtain which can be raised or lowered as circumstances require.
This steaming is usually conducted at atmospheric pressure and
frequently condensed steam is used at temperatures far below 212
degrees Fahrenheit. In a humidity-regulated kiln this preliminary
treatment may be omitted, since nearly saturated conditions can be
maintained and graduated as the drying progresses.
Recently the process of steaming at pressures up to 20 pounds gauge in
a cylinder for short periods of time, varying from 5 to 20 minutes, is
being advocated in the United States. The truck load is run into the
cylinder, steamed, and then taken directly out into the air. It may
subsequently be placed in the dry kiln if further drying is desired.
The self-contained heat of the wood evaporates considerable moisture,
and the sudden drying of the boards causes the shrinkage to be reduced
slightly in some cases. Such short periods of steaming under 20 pounds
pressure do not appear to injure the wood mechanically, although they
do darken the color appreciably, especially of the sapwood of the
species having a light-colored sap, as black walnut (_Juglans nigra_)
and red gum (_Liquidamber styraciflua_). Longer periods of steaming
have been found to weaken the wood. There is a great difference in the
effect on different species, however.
Soaking wood for a long time before drying has been practised, but
experiments indicate that no particularly beneficial results, from the
drying standpoint, are attained thereby. In fact, in some species
containing sugars and allied substances it is probably detrimental
from the shrinkage standpoint. If soaked in boiling water some species
shrink and warp more than if dried without this treatment.
In general, it may be said that, except possibly for short-period
steaming as described above, steaming and soaking hardwoods at
temperatures of 212 degrees Fahrenheit or over should be avoided if
possible.
It is the old saying that wood put into water shortly after it is
felled, and left in water for a year or more, will be perfectly
seasoned after a short subsequent exposure to the air. For this reason
rivermen maintain that timber is made better by rafting. Herzenstein
says: "Floating the timber down rivers helps to wash out the sap, and
hence must be considered as favorable to its preservation, the more so
as it enables it to absorb more preservative."
Wood which has been buried in swamps is eagerly sought after by
carpenters and joiners, because it has lost all tendency to warp and
twist. When first taken from the swamp the long-immersed logs are very
much heavier than water, but they dry with great rapidity. A cypress
log from the Mississippi Delta, which two men could barely handle at
the time it was taken out some years ago, has dried out so much since
then that to-day one man can lift it with ease. White cedar telegraph
poles are said to remain floating in the water of the Great Lakes
sometimes for several years before they are set in lines and to last
better than freshly cut poles.
It is very probable that immersion for long periods in water does
materially hasten subsequent seasoning. The tannins, resins,
albuminous materials, etc., which are deposited in the cell walls of
the fibres of green wood, and which prevent rapid evaporation of the
water, undergo changes when under water, probably due to the action of
bacteria which live without air, and in the course of time many of
these substances are leached out of the wood. The cells thereby become
more and more permeable to water, and when the wood is finally brought
into the air the water escapes very rapidly and very evenly.
Herzenstein's statement that wood prepared by immersion and subsequent
drying will absorb more preservative, and that with greater rapidity,
is certainly borne out by experience in the United States.
It is sometimes claimed that all seasoning preparatory to treatment
with a substance like tar oil might be done away with by putting the
green wood into a cylinder with the oil and heating to 225 degrees
Fahrenheit, thus driving the water off in the form of steam, after
which the tar oil would readily penetrate into the wood. This is the
basis of the so-called "Curtiss process" of timber treatment. Without
going into any discussion of this method of creosoting, it may be said
that the same objection made for steaming holds here. In order to get
a temperature of 212 degrees Fahrenheit in the center of the treated
wood, the outside temperature would have to be raised so high that the
strength of the wood might be seriously injured.
A company on the Pacific coast which treats red fir piling asserts
that it avoids this danger by leaving the green timber in the tar oil
at a temperature which never exceeds 225 degrees Fahrenheit for from
five to twelve hours, until there is no further evidence of water
vapor coming out of the wood. The tar oil is then run out, and a
vacuum is created for about an hour, after which the oil is run in
again and is kept in the cylinders under 100 pounds pressure for from
ten to twelve hours, until the required amount of absorption has been
reached (about 12 pounds per cubic foot).
Out-of-door Seasoning
The most effective seasoning is without doubt that obtained by the
uniform, slow drying which takes place in properly constructed piles
outdoors, under exposure to the winds and the sun. Lumber has always
been seasoned in this way, which is still the best for ordinary
purposes.
It is probable for the sake of economy, air-drying will be eliminated
in the drying process of the future without loss to the quality of the
product, but as yet no effective method has been discovered whereby
this may be accomplished, because nature performs certain functions in
air-drying that cannot be duplicated by artificial means. Because of
this, hardwoods, as a rule, cannot be successfully kiln-dried green or
direct from the saw, and must receive a certain amount of preliminary
air-drying before being placed in a dry kiln.
The present methods of air-seasoning in use have been determined by
long experience, and are probably as good as they could be made for
present conditions. But the same care has not up to this time been
given to the seasoning of such timber as ties, bridge material, posts,
telegraph and telephone poles, etc. These have sometimes been piled
more or less intelligently, but in the majority of cases their value
has been too low to make it seem worth while to pile with reference to
anything beyond convenience in handling.
In piling material for air-seasoning, one should utilize high, dry
ground when possible, and see that the foundations are high enough off
the ground, so that there is proper air circulation through the bottom
of the piles, and also that the piles are far enough apart so that the
air may circulate freely through and around them.
It is air circulation that is desired in all cases of drying, both in
dry kilns and out-of-doors, and not sunshine; that is, not the sun
shining directly upon the material. The ends also should be protected
from the sun, and everything possible done to induce a free
circulation of air, and to keep the foundations free from all plant
growth.
Naturally, the heavier the material to be dried, the more difficulty
is experienced from checking, which has its most active time in the
spring when the sap is rising. In fact the main period of danger in
material checking comes with the March winds and the April showers,
and not infrequently in the South it occurs earlier than that. In
other words, as soon as the sap begins to rise, the timber shows signs
of checking, and that is the time to take extra precautions by careful
piling and protection from the sun. When the hot days of summer arrive
the tendency to check is not so bad, but stock will sour from the
heat, stain from the sap, mildew from moisture, and fall a prey to
wood-destroying insects.
It has been proven in a general way that wood will season more slowly
in winter than in summer, and also that the water content during
various months varies. In the spring the drying-out of wood cut in
October and November will take place more rapidly.
SECTION XI
KILN-DRYING OF WOOD
Advantages of Kiln-drying over Air-drying
Some of the advantages of kiln-drying to be secured over air-drying in
addition to reducing the shipping weight and lessening quantity of
stock are the following:
1. Less material lost.
2. Better quality of product.
3. Prevention of sap stain and mould.
4. Fixation of gums and resins.
5. Reduction of hygroscopicity.
This reduction in the tendency to take up moisture means a reduction
in the "working" of the material which, even though slight, is of
importance.
The problem of drying wood in the best manner divides itself into two
distinct parts, one of which is entirely concerned with the behavior
of the wood itself and the physical phenomena involved, while the
other part has to do with the control of the drying process.
Physical Conditions governing the Drying of Wood
1. Wood is soft and plastic while hot and moist, and becomes
"set" in whatever shape it dries. Some species are much more
plastic than others.
2. Wood substance begins to shrink only when it dries below
the fibre-saturation point, at which it contains from 25 to
30 per cent moisture based on its dry weight. Eucalyptus and
certain other species appear to be exceptions to this law.
3. The shrinkage of wood is about twice as great
circumferentially as in the radial direction; lengthwise, it
is very slight.
4. Wood shrinks most when subjected, while kept moist, to
slow drying at high temperatures.
5. Rapid drying produces less shrinkage than slow drying at
high temperatures, but is apt to cause case-hardening and
honeycombing, especially in dense woods.
6. Case-hardening, honeycombing, and cupping result directly
from conditions 1, 4, and 5, and chemical changes of the
outer surface.
7. Brittleness is caused by carrying the drying process too
far, or by using too high temperatures. Safe limits of
treatment vary greatly for different species.
8. Wood absorbs or loses moisture in proportion to the
relative humidity in the air, not according to the
temperature. This property is called its "hygroscopicity."
9. Hygroscopicity and "working" are reduced but not
eliminated by thorough drying.
10. Moisture tends to transfuse from the hot towards the
cold portion of the wood.
11. Collapse of the cells may occur in some species while
the wood is hot and plastic. This collapse is independent of
subsequent shrinkage.
Theory of Kiln-drying
The dry kiln has long since acquired particular appreciation at the
hands of those who have witnessed its time-saving qualities, when
practically applied to the drying of timber. The science of drying is
itself of the simplest, the exposure to the air being, indeed, the
only means needed where the matter of time is not called into
question. Otherwise, where hours, even minutes, have a marked
significance, then other means must be introduced to bring about the
desired effect. In any event, however, the same simple and natural
remedy pertains,--the absorption of moisture. This moisture in green
timber is known as "sap", which is itself composed of a number of
ingredients, most important among which are water, resin, and albumen.
All dry kilns in existence use heat to season timber; that is, to
drive out that portion of the "sap" which is volatile.
The heat does not drive out the resin of the pines nor the albumen of
the hardwoods. It is really of no advantage in this respect. Resin in
its hardened state as produced by heat is only slowly soluble in water
and contains a large proportion of carbon, the most stable form of
matter. Therefore, its retention in the pores of the wood is a
positive advantage.
To produce the ideal effect the drying must commence at the heart of
the piece and work outward, the moisture being removed from the
surface as fast as it exudes from the pores of the wood. To
successfully accomplish this, adjustments must be available to
regulate the temperature, circulation, and humidity according to the
variations of the atmospheric conditions, the kind and condition of
the material to be dried.
This ideal effect is only attained by the use of a type of dry kiln in
which the surface of the lumber is kept soft, the pores being left
open until all the moisture within has been volatilized by the heat
and carried off by a free circulation of air. When the moisture has
been removed from the pores, the surface is dried without closing the
pores, resulting in timber that is clean, soft, bright, straight, and
absolutely free from stains, checks, or other imperfections.
Now, no matter how the method of drying may be applied, it must be
remembered that vapor exists in the atmosphere at all times, its
volume being regulated by the capacity of the temperature absorbed. To
kiln-dry properly, a free current of air must be maintained, of
sufficient volume to carry off this moisture. Now, the capacity of
this air for drying depends entirely upon the ability of its
temperature to absorb or carry off a larger proportion of moisture
than that apportioned by natural means. Thus, it will be seen, a cubic
foot of air at 32 degrees Fahrenheit is capable of absorbing only two
grains of water, while at 160 degrees, it will dispose of ninety
grains. The air, therefore, should be made as dry as possible and
caused to move freely, so as to remove all moisture from the surface
of the wood as soon as it appears. Thus the heat effects a double
purpose, not only increasing the rate of evaporation, but also the
capacity of the air for absorption. Where these means are applied,
which rely on the heat alone to accomplish this purpose, only that of
the moisture which is volatile succumbs, while the albumen and resin
becoming hardened under the treatment close up the pores of the wood.
This latter result is oft-times accomplished while moisture yet
remains and which in an enforced effort to escape bursts open the
cells in which it has been confined and creates what is known as
"checks."
Therefore, taking the above facts into consideration, the essentials
for the successful kiln-drying of wood may be enumerated as follows:
1. The evaporation from the surface of a stick should not
exceed the rate at which the moisture transfuses from the
interior to the surface.
2. Drying should proceed uniformly at all points, otherwise
extra stresses are set up in the wood, causing warping, etc.
3. Heat should penetrate to the interior of the piece before
drying begins.
4. The humidity should be suited to the condition of the
wood at the start and reduced in the proper ratio as drying
progresses. With wet or green wood it should usually be held
uniform at a degree which will prevent the surface from
drying below its saturation point until all the free water
has evaporated, then gradually reduced to remove the
hygroscopic moisture.
5. The temperature should be uniform and as high as the
species under treatment will stand without excessive
shrinkage, collapse, or checking.
6. Rate of drying should be controlled by the amount of
humidity in the air and not by the rate of circulation,
which should be made ample at all times.