ON SEASONING TIMBER BY NATURAL METHODS, VIZ. HOT AND COLD AIR; FRESH AND
SALT WATER; VAPOUR; SMOKE; STEAM; BOILING; CHARRING AND SCORCHING, ETC.
All timber must, whether it be sap-wood or heart-wood, be placed in
situations which will allow the sap to exude or evaporate, and this
process is the one technically known by the term “seasoning.” There are
natural and artificial modes of seasoning, both of which have their
recommendations; but the former has certainly the right of preference,
as it gives greater toughness, elasticity, and durability, and therefore
should always be employed in preparing timber for carpentry. As the word
“timber” has been frequently used, it may be as well to state that it is
derived, according to Dr. Johnson, from the Saxon, _timbrian_, to build:
hence the above definition. The legal definition of timber is restricted
to particular species of wood, and custom varies in different countries
as to the species ranked among the timber trees.
When a tree is felled, it encloses in its fibres as well as in capillary
channels a considerable quantity of sap, which is nothing else but water
charged with gummy, saccharine, saline, mucilaginous, and albuminous
matters. In this state, the latter are very liable to ferment, but they
lose their liability when, by the evaporation of the sap, they pass
to a dry and solid state; so that the first suggestion which naturally
presents itself to the mind, is to subject the timber to a lengthened
seasoning.
But the present demands for time will not admit of this, and therefore it
is imperative to resort to artificial and speedy methods.
With respect to the value of timber in the log, owing to its becoming
rent by the weather, it sells for 15 per cent. less the second year than
the first, and so on for less and less the longer you keep it.
A natural seasoning may be adopted for specimens of moderate thickness,
such as deals, planks, &c. At the end of eighteen months from the time of
importation they are scarcely dry enough for the consumer’s use.
When there is time for drying it gradually, all that is necessary to be
done on removing it from the damp ground of the forest, is to place it
in a dry yard, sheltered from the sun and wind, and where there is no
vegetation; and set it on bearers of iron or brick in such a manner as to
admit of a ventilation all round and under it. In this manner it should
continue two years, if intended for carpentry; and double that time,
if intended for joinery; the loss of weight which should take place to
render it fit for the purposes of the former being about one-fifth; and
for the latter about one-third. In piling it, the sleepers on which the
first pieces are laid should be perfectly level, and “out of the wind,”
and so firm and solid throughout that they will remain in their original
position; for timber, if bent or made to wind before it is seasoned,
will generally retain the same form when dried. Blocks of wood should
be put between the “sticks” of timber, and each piece directly over the
other, so that air may freely pass through the whole pile; for while it
is necessary to shield timber from strong draughts of wind and the direct
action of the hot sun, a free circulation of air and moderate warmth are
equally essential.
[Illustration: PLANS OF DIFFERENT BALTIC MODES OF CUTTING DEALS FOR THE
ENGLISH AND FRENCH MARKETS.
THE SMALLEST TREES ARE CUT FOR DEALS; THE LARGEST FOR LOGS.
OLD MODE OF CUTTING.
ENGLISH BATTEN. 7IN × 2½IN
ENGLISH DEAL. 9IN × 2½IN
ENGLISH DEAL. 9IN × 2½IN
ENGLISH BATTEN 7IN × 2½IN
9 × 2½ = 22½ × 2 = 45in
7 × 2½ = 17½ × 2 = 35/80
MODE PRACTISED UNTIL THE FRENCH MARKET IMPROVED.
ENGLISH BATTEN. 7IN × 2½IN
ENGLISH DEAL. 9IN × 3IN
ENGLISH BATTEN. 7IN × 2½IN
9 × 3 = 27 = 27in
7 × 2½ = 17½ × 2 = 35/62
MODE GENERALLY ADOPTED AT THE PRESENT TIME.
FRENCH DEAL. 9IN × 1¼IN
ENGLISH DEAL. 9IN × 3IN
ENGLISH DEAL. 9IN × 3IN
FRENCH DEAL. 9IN × 1¼IN
9 × 3 = 27 × 2 = 54in
9 × 1¼ = 11¼ × 2 = 22½/76½
_Notes on Deals._
_All deals are liable to dry rot, if placed in contact with damp
brickwork._
_If the pith of a tree be left in the centre of a deal, dry rot attacks
it._
_Dry rot first attacks the sapwood of a deal; and next, the pith._
_Stockholm, or Gefle deals are not liable to be affected with dry rot._
“STRONG” DEALS _rend. When sawn, they do not give saw dust, but the
fibres tear_.
BEST DEALS _are light, mellow, and exhibit a silky texture when planed_.
“BESTS”. _Wholly free from knots, shakes, sapwood, or cross grain, and
well seasoned._
“SECONDS”. _Free from shakes, and sapwood: small knots allowed._
“THIRDS”. _All that remains after “bests” and “seconds” have been picked
out._]
If timber is not used round, it is good to bore out the core; as, by so
doing, the drying is advanced, and splitting prevented, with almost no
sacrifice of strength. If it is to be squared into logs, it should be
done soon after some slow drying, and whole squared, if large enough, as
that removes much of the sap-wood, facilitates the drying, and prevents
splitting, which is apt to take place when it is in the round form, in
consequence of the sap-wood drying before the heart, from being less
dense. If it may be quartered, it is well to treat it so after some time,
as the seasoning is by that means rendered more equal. It is well also to
turn it now and then, as the evaporation is greatest from the upper side.
In France, the term “bois du brin,” means timber the whole size of the
tree, excepting that which is taken off to render it square.
To prevent timber warping to any serious extent, it should be well
seasoned before it is cut into scantlings; and the scantlings should be
cut some time before they are to be used, in order that the seasoning
may be as perfect as possible; and if they can be set upright, so much
the better, as then they will dry more rapidly. The white lowland deals
of Norway and the white spruce deals of Canada have the same disposition
to warp and split on drying. Du Hamel has shown that it is a great
advantage to set the timber upright, with the lower end raised a little
from the ground; but as this cannot always be done, the timber-yards
should be well drained and kept as dry as possible. “Ancient architects,”
observes Alberti, “not only prevented the access of the scorching rays of
the sun and the rude blasts of wind, but also covered the surface with
cow-dung, to prevent the too sudden evaporation from the surface.” The
warping of timber is attributable by some to the manner of its growth.
Boards cut out of a tree that is twisted in its growth will not keep
from warping; boards cut from trees that are grown in open situations
have another fault, in the heart of the tree not running straight like
forest-grown wood. In a plank cut from a tree of this kind in a straight
line, the heart will traverse it from one end to another. No treatment
will prevent it from warping or drying hollow on the side farthest from
the heart. Where the heart is in the centre of a plank, and each side has
an equal chance of drying, it will not warp; but there will be a shake or
crack upon each side, denoting the position of the heart.
Some deals, and particularly the stringy deals, are very hygrometric,
and never lose the property, however long they have been seasoned, of
expanding and contracting with change of weather. White Petersburgh deals
are said to have that property, however long they may have been kept,
so that if used in the panel of a door, the wood alternately enters and
recedes from the groove into which it fits, as the paint will show when
that kind of deal has been used for a panel.
The wood of the north side will not warp so much as the wood from the
south side. The face of the planks should be cut in the direction which
lay from east to west as the tree stood. If this be done, the planks will
warp much less than if cut in the opposite direction. The nature of the
tree, the soil upon which it is grown, the position of its growth, the
period of the year in which it is felled, and the length of time between
its felling and converting, are the principal points to be considered; a
thorough knowledge and study of which is the only true principle on which
we can hope to deal with the warping and converting of timber.
Wood, when it is cut into small pieces, very soon acquires its utmost
degree of dryness. Dr. Watson, Bishop of Llandaff, in the month of
March, cut a piece from the middle of a large ash tree that had been
felled about six weeks, and weighed it; its weight was 317 grains. In
seven days it lost 62 grains, or nearly one-fifth of its weight. It was
weighed again in August of the same year, but had not lost any more of
its weight; hence it had become perfectly dry in the short space of seven
days. He also found that the sap-wood of oak lost more weight in drying
than the heart-wood, in the proportion of 10 to 7.
The time that is required to season or dry a piece of timber obviously
depends upon its magnitude; as a _general rule_, large timbers will not
continue good so long as small ones, as sufficient time is rarely given
for a thorough seasoning. The time required to dry a piece of timber, all
other things being alike, will depend on the quantity of surface exposed
to the action of the air; therefore, while the quantity of timber
remains the same, the larger the surface, the sooner it will dry. Also,
if the quantity of surface remains the same, the time of drying will be
proportioned to the quantity of matter; as the greater the quantity of
matter under the same surface, the longer it will be in drying.
As drying proceeds most rapidly in small pieces, it is therefore
important to reduce the timber to its proper scantlings or size for
use; for however dry a piece of timber may be, when it is cut to a
smaller scantling it will shrink and lose weight, being always less dry
in the centre than at the surface; and the more rapidly the drying has
been carried on, the greater will be the difference. Nevertheless, in
the first stage of seasoning it is best that it should proceed slowly;
otherwise, the external pores shrink so close as not to permit the
full evaporation of the internal moisture, and the piece would split
from unequal shrinking; and lastly, it should be reduced to the proper
scantling, as already observed, some time before it is to be framed.
Various tables have been given by writers on timber, the result of
algebraical calculations, of the times of seasoning and drying for
different woods of different lengths, breadths, and thicknesses, in the
open air; but as wood even of the same description and quality varies
so much, this matter is best left to those who are well acquainted with
timber. It may, however, be stated that the time required for drying
under cover is shorter than in the open air, in the proportion of 5 to 7.
The English shipwright considers that three years are required to
thoroughly season timber. The timbers for ships are usually cut out
to their shape and dimensions for about a year before they are framed
together, and they are commonly left a year longer in the skeleton shape
to complete the seasoning, as in that condition they are more favourably
situate as regards exposure to the air than when they are closely covered
in with planking.
It is worthy of mention that all the harder woods require increased care
in the seasoning, which is often badly begun by exposure to the sun or
hot winds in their native climates: their greater impenetrability to
the air the more disposes them to crack, and their comparative scarcity
and expense are also powerful arguments on the score of precaution. Oak
timber requires to be very carefully seasoned, as it is generally used
in buildings for the best description of work, and should unseasoned oak
be used for “panelling,” any shrinkage will be fatal to the work. Mr.
George Marshall, timber merchant (see the _Builder_, January 20, 1872),
with respect to seasoning oak timber, observes: “I should select oak
trees known to be old and hearty, with clean, straight butts, from 15
inches to 20 inches in diameter. I should then have the bark taken off
as they stand, and leave them thus till the winter; the sap will then
partially dry out, and make the wood a rich brown colour. As soon as they
are cut down, have them sawn up at once into the lengths you require the
panelling, 6 inches or 8 inches wide and 1 inch to 1½ inch thick. Be
careful to cut all the heart shakes, by having one cut through the centre
of the log before edging the boards to the required width. With regard
to the drying process, stack the boards in a shed with a good draught
through it, and load them down, with slips between each board, to prevent
warping. If this be done they will be found to dry well and speedily, and
they will not require to be exposed to the weather.”
Sir. Robert Phillips, on seasoning oak for panelling, states: “If the
tree is large enough for the purpose, cut it into four, in sections, by
drawing a vertical and horizontal line across the end, meeting in the
centre. If too small for this, cut it into 4½ inch or 6-inch plank, as
soon as possible after felling, and then stack _on end_ out in the open:
do not lay on the ground, but stand it as nearly _vertical_ on its end as
possible, and keep it wet during the first three months. If the weather
is dry, well _wet_ it with water poured on the top, and allowed to run
down. Let the ends stand on a piece of quartering, to keep it out of the
dirt, or it will be stained some distance up. After standing thus for
some six months, after putting it in a dry place for some time, cut it
into the scantlings you require, always bearing in mind that oak will,
after this seasoning, shrink at least half an inch to a foot, in width
and thickness. They should then be stacked and stripped, and covered with
spare boards, and weighted on the top, for at least six months--as much
more as possible--in a covered shed, with plenty of air, occasionally
turned over and shifted, till they are dry enough to make dust when
planed, and not turn the shaving black. They will then be fit for use.
“I should advise for the panels to be cut _feather-edged_ boards, in
radial lines from the centre of the tree: it will be a waste of material,
but will repay in the beauty of the wood, and the way it will stand
without warping. Most of the panels of our old cathedrals were rent (_not
sawn_) in this way, and stand admirably. The butt of the tree should be
taken, the top being used for a rougher purpose.”
Mr. George Marshall and Mr. Robert Phillips might have mentioned that
the oak trees should be of the _Quercus Robur_ species, and _not_ the
_Quercus Sessiliflora_. They are easily distinguished when growing by the
following peculiarities: The acorn-stalks of the _Robur_ are _long_; the
acorns grow singly, or seldom two on the same footstalk; the leaves are
_short_. The acorn-stalks of the _Sessiliflora_ are _short_; the acorns
grow in clusters of two or three, close to the stem of the branch; the
leaves are _long_.
WATER SEASONING.
When there is not time for gradual drying, the best method, perhaps,
that can be adopted, especially for sappy timber, and if strength is not
principally required, is immediately on felling to immerse it in running
water; and after allowing it to remain there about a fortnight, to set
it in the wind to dry. Some persons prefer this method of seasoning
timber, as they say it prevents cleaving, and strips and seasons better
afterwards. This process has been adopted with good results by placing
the boards end on at the head of a mill race for fourteen or twenty days,
at most, and then setting the boards upright, and subject to the action
of the sun and wind; though it is questionable whether the sun will not
do them more harm than good. As they stand, turn them daily, and when
perfectly dry--which process will take about one month--it is considered
they will be found to floor better than timber after many years of dry
seasoning. The sap-wood of oak is said to be improved by this method,
being much less subject to be worm-eaten; and providing it is placed in
fresh running water, Mr. G. A. Rogers, the celebrated wood carver, is of
opinion that the colour of the oak is improved. The more tender woods,
such as alder and the like, are less subject to the worm when water
seasoned. Beech is said to be much benefited by immersion. It should be
remembered that the timber should be altogether under water (chained
down beneath its surface), as partial immersion is very destructive. Du
Hamel considers “that where strength is required, wood ought not to be
put in water.” Timber should never be kept floating in ponds or docks,
as in London; but it should be stacked, as at Liverpool and Gloucester.
Timber that has been lying for months in ponds or docks is sometimes cut
up, and in six or seven days fixed in a building; consequently, the usual
result takes place, viz. dry rot. After having been swelled by soaking
much beyond its former bulk, the baulk of timber is put on the saw-pit,
and cut into scantlings, and framed while in this wet state, therefore
it cannot be surprising that the dry rot soon appears as a natural
consequence.
Amongst wheelwrights the water seasoning is in general favour. It is
said that the colour of the white woods is improved by water seasoning,
boiling, or steaming. The Venetians place the oak used for gun-carriages
in water for two years before it is used, and the timber for sea service
two or three years under water. The Turks do not appear to pay any
attention to seasoning, for they fell their timber at all times of the
year without any regard to the season, and although they grow very good
oak, it is used so green and unseasoned that it not only twists, but
decays rapidly, as anyone may observe in the houses at Constantinople and
other Turkish towns.
Timber is rendered more durable by placing it in a stream of water,
saturated with lime, for eight or ten days, and it also makes it less
liable to the attack of worms; but it, however, becomes hard after being
dried, and is difficult to be worked; and therefore the process should
be applied to timber which has been sawn into scantlings, and is ready
for use. Mr. William Chapman, in 1812, considered that an immersion of
timber in hot limewash in deep ponds, exposing little surface to the air,
merited a trial; but in 1816, from experiments he had made, he was of
opinion that it had proved injurious to timber.
Evelyn states that green elm, if plunged for a few days in water
(especially salt water), obtains an admirable seasoning. According to
Society of Arts _Trans._, 1819, every trace of fungus was eradicated from
the ship “Eden,” by its remaining eighteen months under sea water. Salt
water is considered good for ship timber, but for timber to be employed
in the construction of dwelling-houses, fresh water is better. Pliny
notices, as a fact, that certain woods on being dried after immersion in
the sea acquire additional density and durability. M. de Lapparent, late
Director of the French Navy, considers that timber cannot be seasoned
in salt water, but in fresh, or at the most, in brackish water. The
condition of the timber which, at the port of Rochefort, is kept in
ditches filled with fresh water is in this respect most favourable; that
kept at Toulon, Brest, and Lorient, where the water is brackish, is much
less so; but to estimate their relative advantages, it would be necessary
to test the average density of these waters. It is, however, at Cherbourg
that this natural preparation of timber is the most inefficient, as the
beds-of sand in which the timber is buried, near the Pool of Tourlaville,
contain but a small quantity of water, which, being nearly always
stagnant, very quickly exhausts itself, and is very prejudicial.
At the Cologne International Agricultural Exhibition, in 1865, three
sleepers were exhibited from the Magdeburg Leipsiger Railway, from the
Salt Work Branch, at Stassfurt, laid in 1857. These were moistened by the
refuse of the salt which was lost from the load and by the rain. The jury
in their Report stated that these sleepers proved nothing, “because every
old table on which fish or meat has been salted, proves that a constant
moistening with salt water preserves the wood from decay, but as soon
as the process of salting is given up, the salted matter is immediately
given out, and the timber soon decays. In this case it would have been
important to have known that these sleepers, after having been salted,
had lain anywhere else than in the Salt Work Branch without getting fresh
salt applied, and then to have seen if they would have been as perfect as
they are now. They, indeed, prove nothing but the fact that if sleepers
be daily sprinkled with salt they will remain sound, but the price paid
for this durability might be very considerable.” As the use of salt as
a preservative agent will be considered in the next chapter, it will be
best to defer the consideration of salt-water seasoning until then.
In India, teak, sál, and blackwood, &c., improve by lying in water, or
in the soft black mud of an estuary: there is one exception, viz. heddé,
which deteriorates from steeping, and should be carted to its destination.
Evelyn states that he had found a fortnight’s immersion in river water
sufficient, and this opinion is held by Silloway, a North American
authority; but Dr. Porcher, a South American writer, recommends a six
months’ immersion in water, and a six months’ exposure to wind and shade.
Vitruvius and Alberti consider that timber should be left immersed in a
running stream thirty days.
It is considered that the longer wood has remained under water, the more
rapidly it dries; for instance, every one is aware that the firewood
brought out of the river is less green and burns better than that brought
by waggon or boat.
In 1817, Admiral Count Chateauvieux, a Sardinian naval officer, observed
to Mr. McWilliam that it was a custom at the Royal Arsenal, at Genoa,
as a preventive against the diseases of timber, to steep it about three
years in fresh water immediately after it is felled. Mr. James Dickson,
of Gottenburg, timber merchant (member of the firm Peter Dickson and
Company, London), many years engaged in the Swedish timber trade,
observed in 1835, “If square timber lies in the water two or three years,
it rends at the heart, but I should not say it would, perhaps, for the
first year; but the exterior part rends soon by exposure to the weather.”
In 1818, the Chevalier de Campugano, Secretary of Legation to the Spanish
Embassy, stated that in Spain, when timber is felled, it is generally
laid in water for a considerable time.
The sap in timber, by reason of the matters which it holds in solution,
is denser than pure water; moreover, it is enclosed in fibres or channels
permeable at the ends.
Supposing in submerged timber, the surrounding water to be flowing, or at
least changing, this water will conclude by occupying, if not altogether,
at least in a great degree, the place of the sap, which will have issued
forth, carrying with it the fermenting principle with which it is
charged. The timber, therefore, which has remained _sufficiently long_
in the water ought to be much less susceptible of fermentation than that
seasoned only by the atmosphere. Besides, as pure water evaporates much
easier than that which contains certain principles, this timber ought to
be seasoned much sooner than the other.
Of steeping generally, whether in cold or warm water, it must be
particularly observed that it dissolves the substance of the wood, and
necessarily renders it lighter; indeed, it is known that notwithstanding
wood that is carefully submerged remains good for a very long period
after the water has dissolved a certain soluble part, it is, when taken
out and dried, liable to be brittle, and unfit for any other work but
joinery.
SEASONING BY STEAMING AND BOILING, ETC.
For the purposes of joinery, steaming and boiling are very good methods,
as the loss of elasticity and strength which they produce, and which
are essential in carpentry, is compensated by the tendency to shrinkage
being reduced; the durability also is said by some to be rather improved
than otherwise, at least from steaming. If steaming be not carried on
too quickly it will answer, but if it be pushed with too much vigour
it is very apt to produce a permanent warping and distortion of the
material. Oak of British growth may be seasoned by this process, as
without this precaution it requires a long time to season. It has been
ascertained, that of woods seasoned by these methods, those dried soonest
that had been steamed; but the drying in either case should be somewhat
gradual, and four hours are generally sufficient for the boiling or
steaming process. The question of time will depend upon circumstances:
some persons consider that one hour should be allowed for every inch
in thickness. In some dockyards, salt water is used in the boilers, in
others fresh, from considerations of convenience; and the fact is, plank
boiled in salt water never gets rid of the salts that naturally enter the
pores of the wood in boiling; and such being the case, the ship in which
this plank is used is much more liable to the effects of damp than she
would have been if the plank had been boiled in fresh water.
Boiling and steaming are likewise employed for softening woods, to
facilitate the cutting as well as bending of them. Thus, in Taylor’s
patent machines for making casks, the blocks intended for the staves are
cut out of white Canada oak to the size of 30 inches by 5 inches and
smaller. They are well steamed, and then sliced into pieces ½ inch or
⅝ inch thick, at the rate of 200 in each minute, by a process far more
rapid and economical than sawing; the instrument being a revolving iron
plate of 12 feet diameter, with two radial knives arranged somewhat like
the irons of an ordinary plane or spokeshave.
How far steaming or boiling affects the durability of timber has not been
satisfactorily ascertained; but it is said that the planks of a ship
near the bows, which are bent by steaming, have never been observed to
be affected with dry rot. With respect to boiling, Du Hamel’s opinion is
not favourable as to its adding to the durability of timber; for when a
piece of dry wood was immersed in boiling water, and afterwards dried in
a stove, it not only lost the water it had imbibed, but also a part of
its substance; and when the experiment was repeated with the same piece
of wood, it lost more of its substance the second time than it did the
first. Tredgold--no mean authority--considers that “boiled or steamed
timber shrinks less, and stands better than that which is naturally
seasoned.” Barlow is of opinion that “the seasoning goes on more rapidly
after the piece is steamed than when boiled.”
At the close of the Crimean and Baltic campaigns the port of Cherbourg
was almost completely cleared of staves sufficiently seasoned for making
casks. The engineer at the head of the coopering department determined to
boil in fresh water the newly-cut staves, and compare the time of their
seasoning with that of other staves cut from the same forests, but not
prepared; and the result was that after four or five months’ exposure
to the atmosphere, the boiled staves were perfectly fit for working up,
while to bring the others to the same point fifteen months were barely
sufficient.
Steaming is understood to prevent dry rot. No doubt boiling and steaming
partly remove the ferment spores, but _may not_ destroy the vitality
of those remaining. For, according to Milne-Edwards, on ‘Spontaneous
Generation.’ he has seen tardigrades resist the prolonged action of a
temperature of 248° Fahr., and has known them to survive a temperature of
284° Fahr. That low forms of vegetation are fully as tenacious of life
cannot be doubted.
Boiling and steaming also coagulate the albumen at 140° Fahr. Although
coagulated albumen is insoluble in water, the water solution is by this
heating process sealed up in the wood, and the cohesion of the latter is
said to be diminished.
The first essays in the art of drying wood artificially carry us back to
a period now tolerably remote. Wollaston and Fourcroy both recommended
the drying of wood in ovens. Newmann, a German chemist, suggested another
method, which has since been adopted in a somewhat different form, i.
e. _steaming_ the wood. Newmann placed the wood to be dried in a large
wooden chest, taking care to leave spaces between the pieces, and then
turned on the steam from a boiler provided for the purpose. The condensed
steam, charged with albuminous matter taken up from the wood, or rather
from its surface, was run off from time to time, and the process of the
operation was judged by the colour of the water. When the latter was
clear and colourless the chest was opened, and the wood withdrawn for use
without further preparation. The process would have been useful enough
if _superheated_ steam, which would have dried the wood by absorbing
the moisture, could have been used, but the cost of the process would
doubtless have been too high to permit of its practical application.
In 1837, M. de Mecquenem devised a method of desiccation, in which the
pieces of wood to be dried were placed in a closed chamber, and subjected
to a current of hot air, heated for the purpose by a special apparatus,
and driven by a blower. The air entered by apertures in the lower part
of the chambers, and escaped at the top laden with the moisture absorbed
from the wood.
In 1839, M. Charpentier obtained a _brévet d’invention_ for a process of
drying wood in hermetically-closed chambers. The wood was subjected to
the action of air heated by contact with metal plates covering the flue
of a coke furnace. This air entered by conduits on the level of the floor
of the chamber, and escaped at the top through apertures leading into the
chimney of the furnace.
In the same year, M. Saint Preuve invented a process for forcing steam
into pores of the wood, and, by condensation of this steam in the pores,
sucking in a preservative preparation.
In 1847, MM. Brochard and Watteau’s process was introduced. It consists
simply of filling the cylinder with steam, and making a vacuum by forcing
in a cold solution of salt, &c.
The plan which has been for some years in use in England is the
injection, by means of a ventilator, of hot air into the drying stove
where the wood is placed: by this the temperature is gently and gradually
raised until it reaches boiling heat. But, as wood is one of the worst
conductors known of caloric, if this plan is applied to large logs, the
interior fibres still retain their original bulk, while those near the
surface have a tendency to shrink; the consequence of which would be
cracks and splits of more or less depth.
Timber may be dried by passing rapid currents of heated air through it
under pressure. This plan was carried out with the timber used for the
floorings of the Coal Exchange, London. The wood was taken in its natural
state, and in less than ten days it was thoroughly seasoned. In some
cases, from 10 to 48 per cent. of moisture was taken out of the wood,
and although the floorings have now been down a great many years, it is
stated that very little shrinkage has been found, except in the case of
a few pieces which were put down in the latter portion of the work, and
which had not been submitted to the seasoning process.
The process of desiccation, patented by Messrs. Davison and Symington,
in 1844, is of great practical value in reducing the time requisite
for seasoning timber. It is peculiarly applicable to the seasoning of
flooring boards and of the wood used in joiners’ work. Care must be
exercised when removing the timber from the stove to the building in
which it is to be used, that it be not exposed to the wet, nor even to a
damp atmosphere for any lengthened period. The advantage of this process
over the ordinary stoving consists in the temperature never being so
high as to scorch the wood, by which the strength of the fibres would
be injured; and in the facility for removing the vapour as fast as it
is expelled from the wood, in consequence of the air being propelled
through the stove at any required velocity and temperature. As compared
with furnace and steam-stoving ordinarily employed to desiccate woods,
the great superiority of this process is established by its seasoning
the wood quite as rapidly, but much more thoroughly; and instead of wood
being rendered brittle, as it is to some extent by stoving, this mode
does not reduce the strength and tenacity of the wood. The principle
of the invention is propelled currents of heated air; but the heat
has to be regulated according to the texture of the various woods.
Honduras mahogany might be exposed to a heat of 300°, and the whole of
the moisture can be taken out in three days. Timber 9 inches square is
considered by Mr. Davison a proper size for his invention. This process
is described as “A method or methods of drying, seasoning, and hardening
wood, and other articles, parts of which are also applicable to the
desiccation of vegetable substances generally.” The first or principal
part of the invention consists in drying, seasoning, and hardening wood
and other articles--among which other articles are included generally all
things made of wood, or chiefly of wood--by means, as has been stated,
of rapid currents of heated air. The manner in which these currents of
heated air are produced, is by an apparatus consisting of a furnace
and a series of pipes withinside of a core of brickwork. On each side
of the furnace, on a level with the fire-bars, is a horizontal tube;
communicating and springing from these tubes are a series of eighteen
tubes placed vertically and parallel to each other over the furnace. The
outer end of one of the horizontal tubes communicates with a fan or other
impelling apparatus for driving a constant stream of atmospheric air
through the tubes. As the air passes through the tubes it becomes heated
at a high temperature, and rushes out at the farther end of the other
horizontal tube, and is thus conveyed to the place where it is applied.
The materials to be subjected to the heated currents, such as logs,
deals, &c., by outward application, must be placed in closed chambers,
galleries, vaults, or flues, which are to be of any suitable form or
magnitude; but it is recommended that they should be made of fire-brick,
and have double doors or shutters for introducing or removing the wood.
Honourable Mention was made of Messrs. Davison and Symington’s process of
desiccation, by the jury, Class IV., Exhibition of 1851, England.
Some amusing instances are related of the efficiency of Davison and
Symington’s process. Thus, a violin had been in the owner’s possession
for upwards of sixteen years; how old it was when he first had it is
not known. Upon being exposed to this process it lost, in eight hours,
no less than five-sixths (nearly five and two-thirds) per cent. of its
own weight. This there is every reason to believe was owing to the
blocks glued inside, for the purpose of holding the more slender parts
together. A violin maker of high reputation, having an order to make
an instrument for one of the first violinists of the day, was requested
to have the wood seasoned by this process; only three days were allowed
for the experiment, in which the wood was seasoned and sent home. The
two heaviest pieces were reduced in weight 2½ lbs. It is ascertained
that, by this means of drying, the effect of age has been given to the
instrument made from the above wood, and it was, in 1848, _first fiddle_
in the orchestra of Her Majesty’s Theatre, London. The wood had been
in the possession of its owners for eight years, and it was sent from
Switzerland, in the first instance, as _dry wood_.
In proof of the value of this invention for the manufacture and cleansing
of brewers’ casks, it was stated, in 1848, that since its adoption at
Trueman’s brewery, Spitalfields, a saving of 300 tons of coals has been
effected annually.
Flues or chambers for the heated air may be constructed in parallel
lines, either in the floors or upright walls of a building, having narrow
openings through which the heated air may issue in thin streams, and
spread itself over the surface of the wood. If the openings are in the
floor, the wood will require to be placed in an upright position; but if
admitted in a horizontal direction, standards and skeleton shelves will
be necessary to lay it upon. The great object, in all cases, is to bring
the heated air as speedily as possible into contact with the wood, and to
allow it, after it has done its office, to pass away as speedily.
Furnaces and apparatus for the production of rapid currents of heated
air may be erected to prepare any quantity of timber or articles of wood
at one time, but care should be taken that whatever the size of the
outlet may be from the series of pipes or vessels by which the heat is
generated, an outlet of at least equal dimensions is left for the free
exit of the air and the vapours thrown off. It should also be observed,
in constructing the open space in the floor or upright walls for the
stream of heated air to pass towards the timber, that the superficial
area of the whole of them combined does not exceed the dimensions of the
principal outlet of the pipes at the extremity of the furnace, so that a
free current of heated air may be allowed to pass uniformly throughout
the chambers containing the wood to be prepared. The temperature proper
to be given to the air, and velocity to the current in each case, will
depend on the size, density, and maturity of the wood to be acted upon.
The inventors found by their experiments that wood generally may be
advantageously subjected to currents of air raised to a temperature of
400° Fahr., when the currents are impelled at the rate of 100 feet per
second. But when the wood is in a green state, it is better to commence
at a lower temperature, say from 150° to 200°, and gradually raise it to
the high degree before stated, as the desiccation proceeds, an object
which may, in some cases, be facilitated by carrying a cold-air drain
from the fanner or other propelling apparatus, and attaching a damper to
it, so that any quantity of cold air required to reduce the temperature
of the hot current may, from time to time, be admitted. When, again, the
wood is in the log or unconverted state, it should be bored or augured
out in the centre, and the current of hot air caused to traverse it as
well interiorly as exteriorly, whereby much time will be saved in the
process of desiccation, and a more uniform result obtained.
Woods treated in this manner, and with the above modifications when
requisite, part rapidly with their natural sap and any other aqueous
matter which they may contain, and the fibres are brought closer together.
With respect to the time required to season the wood upon this plan, much
must depend upon the original state of dryness it may be in, as well
as the quality and temperature of the heated air forced into contact
with it. It may suffice to remark that the wood may safely remain thus
exposed till any escape of moisture ceases to be perceptible. This may be
readily known, either by applying a mirror or any polished surface to the
outlet, or by calculating the quantity of moisture removed from the wood,
which will be found to range between ¼ and ⅟12th of its whole weight.
For the purpose of ascertaining more correctly the amount of moisture
removed from time to time, when the wood is placed in seasoning chambers
as already described, an opening should be constructed in the chamber,
in any convenient position, through which a specimen of the wood may be
withdrawn and weighed.
Between 1848 and 1853, Mr. Bethell, who had paid much attention to the
subject, obtained several patents, both in England and France, for stoves
for drying wood. In his English patent of 1848, and the subsequent French
one of 1853, we find a description of a peculiar kind of stove, on the
following plan:
It consisted of a rectangular chamber formed of three walls and vaulted
over, the whole in brickwork, with a certain thickness of slag in the
centre, to prevent loss of heat. One extremity of the chamber was open to
admit of the introduction of the wood by means of a truck running upon
longitudinal iron rails. The opening was closed with a double door when
the chamber was full. On the exterior of the opposite end of the chamber
was a furnace to burn coal, coke, wood, or tar, according as it was
desired to _dry_ the wood simply, or, in the words of the inventor, to
_smoke_ it, i. e., to impregnate it with the antiseptic gaseous matters
evolved in the imperfect combustion of certain tarry substances. The
heated air or smoke entered through a flue running along the floor and
branching at the end, and it escaped, or was pumped out, at the top of
the vaults. Bethell considered that the interior of the chamber should be
kept at a temperature of 110° Fahr., and that the duration of the process
should be regulated by the condition of the wood. His experiments showed
that this time varied from eight to twelve hours, the rapidity being
attained at the cost of a relatively large expenditure of fuel. In point
of fact, the draught was too great to permit of the utilization of the
full amount of heat contained in the gaseous matter, which escaped at a
temperature very little below that at which it entered. The heat produced
by the fuel was badly utilized, and it is open to question whether,
under any circumstances, large pieces of wood, such as sleepers, could
be dried in so short a time as eight or twelve hours. The drying could
only be effected by the use of a very high degree of temperature, tending
to split the wood and weaken its strength. This view was confirmed by
the results obtained in a long series of experiments made, in 1852-3,
by an English manufacturing company, known as the Desiccating Company. A
low temperature, and long continuance of the drying process, appear to
be the conditions essential to the success of artificial desiccation,
particularly with wood intended for cabinet-making, turning, joinery,
ornamental work, &c., in which it is desirable, as far as possible,
to prevent splitting, warping, and other changes of structure in
the material. These results, it would seem, were not secured by the
arrangements above described.
Some years since, a stove was constructed for Messrs. S. and J. Holme,
very extensive builders at Liverpool, for the purpose of drying timber
for floors, and other fittings of houses, &c., by the application of
Messrs. Price and Manby’s patent warming apparatus; the want of seasoned
timber, with the great number of men they employed, being a serious
inconvenience and loss. In their large undertakings Messrs. Holme found
a difficulty in keeping a stock of dry timber. The dimensions of the
stove in which the timber was to be dried was 43 feet long, 11 feet
wide, and 17 feet 6 inches high, and the cost of the apparatus was about
150_l_. It was calculated to hold about 30,000 superficial feet of 1-inch
boards, which, upon the steam-pipe system, occupied full three weeks
in drying. This apparatus of Messrs. Price and Manby, with rather less
fuel, was considered to thoroughly dry each stove-full in ten days, thus
saving a consumption of ten days’ fuel, independent of the advantages
of expediting business. The average temperature was 104°, and as the
continuous stream of pure air passing between the metallic plates was
divested of its moisture, it carried off the dampness of the timber in
an imperceptible manner. An experiment was tried, by having a flooring
batten, 7 inches by 1¼ inch, cut from a piece of timber which had been
floated, and was as full of water as it could be, placed in the stove;
and when the temperature was 102°, it remained there five days, and when
sawn down into ⅝ inch thick, and planed, it was found to be perfectly dry
throughout. The heat was so gentle, and the evaporation so equal, that
the timber was never rent, as when exposed to the air and a hot sun: in
short, Messrs. Holme considered it one of the most perfect timber stoves
that had been made.
It may be remarked with respect to desiccation, that the timber to be
artificially dried is generally exposed to a great heat for a short
time, rather than to a moderate heat for a lengthened one; and the air,
saturated with the vapour thus produced, is generally very imperfectly
removed. Wood so treated is almost sure to split, from the unequal
contraction to which it is exposed; and the pores are also very liable to
reopen on the wood being withdrawn from the stove, because there is no
gradual and permanent change in their mechanical structure. It is only
within the last few years past that the artificial desiccation of wood,
before its impregnation with an antiseptic preparation in closed vessels,
has been frequently adopted in practice.
We cannot give a better termination to the few remarks we have made about
“steaming and boiling timber,” than by quoting the opinion of the late
Sir Charles Barry, R.A., architect to the new Houses of Parliament, which
we propose doing in the following manner:
“YORK ROAD, LAMBETH, _Nov. 30, 1844_.
SIR,
In reply to your application, we beg to acquaint you that we
are willing to undertake the ordinary works required in the
finishings of the new Palace of Westminster. … The wainscot to
be used in the joiner’s work is assumed to be from the best
Crown Riga wainscot in the logs, and from pipe-staves of the
best quality, in equal proportions, to be _prepared for use by
steaming_, or otherwise.…
_Grissell & Peto._”
Charles Barry, Esq.
Sir Charles Barry recommended this tender to the Treasury for acceptance;
but we fancy that he was doubtful about the efficacy of steaming, as we
think will appear from the following extract from an “Agreement between
Sir Charles Barry and Messrs. Grissell and Peto, builders:
“_First._ That the wainscot is assumed to be from the log and pipe-staves
in equal quantities; the prime cost of which, in inch boards, _seasoned
by steam_, or other artificial means, so as to be fit for use, is
calculated at 6½_d._ per foot superficial.
“_Secondly._ That _if it should be found necessary to make use of
thoroughly dry wainscot boards_ for the whole or any portion of
the joiner’s work, seasoned by natural means (viz. exposure to the
atmosphere), the prime cost of such boards, with the addition of a
profit of 7½ per cent., is to be allowed for them, over and above the
price of 6½_d._ per foot superficial, the prime cost of wainscot boards
provided for in the contract, as above stated.” (Italics are our own.)
SEASONING BY SMOKE DRYING.
Smoke drying in an open chamber, or the burning of furze, fern, shavings,
or straw under the wood, is said to give it hardness and durability;
and, by rendering it bitter, destroys and prevents worms. It also
destroys the germ of any fungus which may have commenced. It is an old
and well-founded observation that smoke drying contributes much to the
hardness and durability of woods. Virgil appears to have been aware of
its utility, when he wrote the passage which is thus translated by Dryden:
“Of beech, the plough-tail, and the bending yoke,
Or softer linden, hardened in the smoke.”--GEORGICS, i., 225.
Beckman, in his ‘History of Inventions,’ quotes a passage from Hesiod to
the same effect; and adds, “as the houses of the ancients were so smoky,
it may be easily comprehended how, by means of smoke, they could dry and
harden pieces of timber.” In this manner were prepared the pieces of wood
destined for ploughs, waggons, and the rudders of vessels:
“These long suspend, where smoke their strength explores,
And seasons into use, and binds their pores.”--VIRGIL.
The late Brigadier-General Sir Samuel Bentham bestowed much time and
attention in endeavouring to ascertain the quickest and best means of
drying oak. In his letter to the Navy Board, 6th March, 1812, he says:
“By exposing block shells to the smoke of burning wood, they become in
the course of two or three days well seasoned in every respect, hard,
bright coloured, and, as it were, polished. But it was found in a very
short time that the acid with which the shells were thus impregnated very
rapidly corroded the iron pins which passed through them.
“In Russia many small articles, such as parts of wheels, wheel carriages,
and sledges, are prepared in this manner; so are wheels, at least in some
parts of America; and sabots and other small articles in France.”
In speaking of artificial heat, he says, in the same letter:
“From all the opportunities I have had of examining the state of timber
so prepared by artificial heat, the due seasoning without cracking has
appeared to depend on the ventilation happening to be _constant_, but
very _slow_, joined to such a due regulation of the heat as that the
_interior_ of the timber should dry, and keep pace in its contraction
with the outer circles.”
Mr. T. W. Silloway, in ‘American Carpentry,’ remarks: “If timber be dried
by heat, the outside will become hardened, and the pores closed, so that
moisture, instead of passing out, will be retained within.”
Bowden remarks, “that the timbers of a small ship underwent the process
of charring, either by suspending them over a fire of chips, or by
burning the exterior with red-hot irons, so as to char the external
surface. Air trunks were also formed between the timbers, for the
purpose of evaporating moisture. The state of this vessel was examined
five years after she was launched, and it appeared that, although the
timbers had been very strongly charred, fungi had grown to a considerable
extent on both sides abaft the fore channels, and that the plank near the
magazine was completely decayed.” _The power of vegetation broke through
the incrusted barrier against external affection._
A method is in operation at Tourlaville, near Cherbourg, for which the
inventor, M. Guibert, has taken out a patent, and it is said to give at
once more expeditious and sure results than those obtained from the use
of dry and hot air. It consists in filling the drying-stove with smoke,
produced by the distillation of certain combustible matters, such as
sawdust, waste tan, and smiths’ coals, &c. By means of a ventilator,
ingeniously arranged, a rotatory movement round the logs laid to season
is given to the smoke, so as to obtain an average uniform temperature in
every part. By this plan, as the distillation of combustibles is always
attended with a considerable discharge of steam, all cracks and splits
are said to be prevented.
There is much force in Sir Samuel Bentham’s observations respecting the
drying of timber by artificial heat: it is certainly not well to attempt
to dry it _too_ quickly, for if it be subjected to great heat, a large
portion of the carbon will pass off, and thereby weaken the timber.
Timber too suddenly dried cracks badly, and is thus materially injured:
planks of larch or beech are liable to warp and twist if their drying is
hastened.
STOVE DRYING.
In some of the large manufactories for cabinet work, the premises are
heated by steam pipes, in which case they have a close stove in every
workshop heated many degrees beyond the general temperature, for giving
the final seasoning to the wood; for heating the cauls; and for warming
the glue, which is then done by opening a small steam pipe into the outer
vessel of the glue-pot. The arrangement is extremely clean, safe from
fire, and the degree of heat is very much under control.
In some manufactories, the wood is placed for a few days before it is
worked up in a drying-room heated by means of stoves, steam or hot water,
to several degrees beyond the temperature to which the finished work
is likely to be subjected. Such rooms are frequently made as air-tight
as possible, which appears to be a mistake,[1] as the wood is then
surrounded by a warm but stagnant atmosphere, which retains whatever
moisture it may have evaporated from the wood.
Fire-stoves for drying the timber were placed in the magazine,
bread-room, and other parts of the ‘Royal Charlotte’ ship; and the evil
of this practice was soon shown, for the vessel became dry rotten in
_twelve months_.[2]
Wood sometimes undergoes a baking process for veneering. Fourcroy has
recommended baking timber in an oven, and he has asserted that it would
render timber more durable; “but,” says Boyden, “it should be subjected
to a very strong heat, lest in endeavouring to prevent vegetation,
we should give it birth.” Captain Shaw[3] observes: “Any artificial
heating which burns the air is most injurious to wood and all combustible
materials, and renders them much more inflammable than they would be if
only exposed to the temperature of the atmosphere.”
SEASONING BY SCORCHING AND CHARRING.
Scorching and charring are good for preventing and destroying infection
in timber, but have to be done slowly, and only to timber that is
already thoroughly seasoned; otherwise, by incrusting the surface,
the evaporation of any internal moisture is intercepted, and decay in
the heart soon ensues; if done hastily, cracks are also caused on the
surface, and which, receiving from the wood a moisture for which there is
not a sufficient means of evaporation, renders it soon liable to decay.
Charring has little or no control over internal corruption, though it is
a good preventive against external infection: it increases the durability
of dry, but promotes the decay of wet timber. Farmers very often resort
to this method for the preservation of their fence-posts; the charring
should extend a little above their contact with the ground. Unless they
discriminate between green and unseasoned timber, these operations will
prove injurious instead of beneficial.
We have already quoted Sir Charles Barry in favour of _steaming_ wood;
we now intend giving the opinion of a former pupil of his with regard to
_charring_ it. Mr. George Vulliamy, architect to the Metropolitan Board
of Works, in a specification for oak fencing which was fixed round the
boundaries of Finsbury Park, London, in 1867, writes as follows:--“Dig
out the ground for the upright standards where shall be directed, and
fill in and ram round same with dry burnt earth, stones, and rubbish (the
burnt clay will be provided); enclose the boundaries of Park, as shall be
directed, with _dry and well-seasoned_ heart of English oak, _wrought_
upright standards, 6 inches by 5 inches, and 8 feet 6 inches total
length, with cut and _splayed_ tops, holes drilled for oak pins, and
mortised for horizontal rails, as shown on detailed drawings; to stand 5
feet 3 inches out of ground, and _the ends in ground to be well charred
before fixing_.” (The italics are our own.)
Our ancestors used charcoal and charred wood, on account of their
durability, for landmarks in the ground between estates. The
incorruptibility of charcoal is well known. Amongst other advantages,
rats will not touch it; neither will the white ants nor cockroaches, so
common in the Indies, commit their depredations where charring has been
employed.
The ‘Revue Horticole’ states that it has been proved by recent
experiments, that the best mode of prolonging the duration of wood is to
char it, and then paint it over with three or four coats of pitch. Many
of the sleepers now laid down on the Belgian railways are charred, the
engineers preferring this process to any other.
The superficial carbonization, or charring of wood, as a preservative
means, has long been practised. The Venetians have used charring for
timber for a long period, particularly for piles. In France, M. de
Lapparent recently proposed to apply it to the timber used in the French
Navy. Some experiments, which were undertaken with a view to determine
its practicability, terminated satisfactorily; and the Minister of Marine
ordered the process to be introduced into the Imperial dockyards.
M. de Lapparent makes use of a gas blowpipe, the flame from which is
allowed to play upon every part of the piece of timber in succession.
By this means the degree of torrefaction may be regulated at will. The
method is applicable to woodwork of all kinds; and the charring, it is
said, does not destroy the sharpness of any mouldings with which the wood
may be ornamented.
In the ‘Journal des Savants,’ Feb. 15, 1666, appears the following: “The
Portugals scorch their ships, insomuch that in the quick works there is a
coaly crust of about an inch thick; but this is dangerous, it happening,
not seldom, that the whole vessel is burnt.” It is no wonder that the
Portuguese ships should frequently fire in the operation, as their plank
was charred an inch deep. A mere charring, if done properly, after the
timbers had been thoroughly seasoned by air, would have been sufficient.
Charring seasoned wood is known to be a most effectual mode of
preservation against rot in timber: thus do piles, when charred, last for
ages in water or moist soil. Charred wood has been dug up, which must
have lain in the ground for 1500 years, and was then found perfectly
sound. After the Temple of Diana, at Ephesus, was destroyed, it was
found to have been built on charred piles; and at Herculaneum, after 2000
years, the charred wood was found to be whole and undiminished. But we
find Sir Christopher Wren did not approve of charred piles, except in a
soil where they would be constantly wet. So, in order to attain a firmer
foundation for St. Paul’s Cathedral, he had the ground excavated to an
immense depth before a stone of the building was laid.
From time immemorial it has been the practice, particularly in France,
to burn the ends of the poles driven into the ground to preserve them
from decay. According to the remark of the celebrated Carlomb, we should
always take into serious consideration old and well-known customs; but in
this instance it is easy to admit the preserving effect of carbonization.
Mr. James Randall,[4] Architect, states that he “oxidated several
pieces of wood with nitric acid, and with fire,” and these processes
were attended with success. Nearly the last sentence in his work is,
“_oxidation only_ can be relied on, in all cases, as an effectual cure.”
In charring, the surface of the timber is subjected to a considerable
heat, the primary effect of which is to exhaust the sap of the epidermis,
and to dry up the fermenting principles. Here this is done by long
exposure to the air; and, in the second place, below the outside layer
completely carbonized, a scorched surface is found, that is to say,
partly distilled and impregnated with the products of that distillation,
which is creosoted; the antiseptic properties of which are well known.
When Mr. Binmer was examined before the Commissioners of Woods, Forests,
&c., in 1792, he stated “that all steamed plank should be afterwards
dried and _burnt_ to extract the moisture.”
To a spontaneous carbonization must be attributed also the
unchangeableness of that timber entirely black, which is met with
everywhere in digging up the ground, where it has laid buried for ages.
In the neighbourhood of St. Malo, France, these specimens are very
common, and there most of the espaliers and vine props are made of wood,
black as ebony, and famous for its durability. They have been cut from
the trees of an old forest, submerged in the eighth century by an inroad
of the sea, which formerly crossed a Roman road, leading from Brittany to
Cotentin.
Not long after the beginning of the eighteenth century, the method of
heating or charring timber, before it was worked up, and also that of
stoving--that is, of heating in kilns with sand--were practised in
the Royal dockyards. The ‘Royal William,’ one of the most remarkable
instances of durability that the British Navy has supplied, was built
either wholly or in part of timber that had been charred. It was launched
in 1719; never repaired until 1757; and then, when surveyed afloat, in
1785, it appeared that the thick stuff and plank had been _burnt_ instead
of being _kilned_; and that the ends of the beams, the faying parts of
the breast-hooks, crutches, resters, knees, &c., had been gouged in a
manner then practised, which was called _snail-creeping_; by means of
which the air was conveyed to the different parts of the ship.[5]
The reason this method has not been persevered in, but nearly abandoned,
is owing to many causes: the difficulty and danger of the means adopted
for charring, when either straw, fern, or shavings are made use of; the
serious objection of burning the timber too deeply; or the encumbrance
of the apparatus, and the length of time occupied, if sand-kilns
sufficiently heated are used; and, finally, to indifference, or that
system of routine, against which the wisest plans often contend in vain.
In house-building, the charring process should be applied to the beams
and joists embedded in the walls, or surrounded with plaster; to the
joists of stables, washhouses, &c., which, although exposed to the free
air, are constantly surrounded by a warm and moist atmosphere, an active
cause of fermentation; to the wainscotting of ground floors; to the
flooring beneath parquet work; to the joints of tongues and rabbets;
for carbonization by means of gas still leaves to the wood, for working
purposes, all the sharpness of its edges. Charring is particularly useful
in the junction of all broad surfaces, and more essentially in those
which are cut either transverse or oblique to the grain of the wood,
as the sap vessels are then exposed to the absorption of moisture. The
butts of timbers are peculiarly liable to rot, because of affording a
lodgment for moisture without a free passage for air. No seasoned timber
should have its tubular parts exposed, nor should any timber have the saw
marks upon it, because the torn filaments absorb and retain moisture.
Allusion has already been made to the process adopted, near Cherbourg,
for preventing the decay of timber by means of gas.
By carbonization, a practical and economical means is afforded to railway
companies of preserving, almost for ever, the sleepers, and particularly
oak, which cannot be impregnated easily by the injection of mineral
salts. Let us suppose, for instance, that after, say ten or fifteen
years, the sleepers on a line are taken up for the length of a mile,
and replaced by new ones; the old, when rasped and burnt again, will
serve for the replacing the following mile, and so on, one mile after
the other. It might be equally serviceable to apply the same process to
injected beech, for the reason that it is almost impossible to make the
preserving liquid penetrate thoroughly the mass of the timber.
SEASONING BY EXTRACTION OF SAP.
Mr. John Stephen Langton’s method of seasoning by extraction of the
sap was patented in 1825, but is now almost wholly discontinued. It
consists in letting the timber into vertical iron cylinders, standing in
a cistern of water, closing the cylinders at top; and the water being
heated, and steam used to produce a partial vacuum, the sap relieved
from the atmospheric pressure oozes from the wood, and being converted
into vapour, passes off through a pipe provided for the purpose. The
time required is about ten weeks, and the cost is about ten shillings
per load; but the sap is wholly extracted, and the timber is said to
be fit and ready for any purpose; the diminution of weight is, with a
little more shrinkage, similar to that in seasoning by the common natural
process.[6]
Mr. Barlow’s patent provided for exhausting the air from one end of
the log while one or more atmospheres press upon the other end. This
artificial aerial circulation through the wood is prolonged at pleasure.
However excellent in theory, this process is not practicable.
In October, 1844, M. Tissier proposed to place wood in a close vessel,
and subject it to a current of hot dry air; and in 1847, Mr. Miller
proposed to inject hot air through beams of wood to drive out the sap.
In 1851, M. Meyer d’Uslaw proposed to first dilate the pores of the wood
with steam, and then place it in a hermetically closed chamber, and make
a vacuum there.
The following system of preparing timber for the Navy was, not many years
since, adopted in South Russia. A full account of the practice will be
found in Oliphant’s ‘Russian Shores of the Black Sea,’ 1853. The only
name we can give it is
“‘SEASONING’ BY BRIBES.”
A certain quantity of well-seasoned oak being required, Government issues
tenders for the supply of the requisite amount. A number of contractors
submit their tenders to a board appointed for the purpose of receiving
them, who are regulated in the choice of a contractor not by the amount
of his tender, but of his bribe. The _fortunate_ individual selected
immediately sub-contracts upon a somewhat similar principle. Arranging
to be supplied with the timber for half the amount of his tender, the
sub-contractor carries on the game, and perhaps the eighth link in this
contracting chain is the man who, for an absurdly low figure, undertakes
to produce the _seasoned_ wood.
His agents in the central provinces accordingly float a quantity of
green pines and firs down the Dnieper and Bog to Nicholaeff, which are
duly handed up to the head contractor, each man pocketing the difference
between his contract and that of his neighbour. When the wood is produced
before the board appointed to inspect it, another bribe _seasons_ it; and
the Government, after paying the price of well-seasoned oak, is surprised
that the 120-gun ship, which it has been built of it, is unfit for
service in five years.
“Mark but my fall, and that that ruin’d me,
Corruption.”--SHAKSPEARE.
A few words can only be given to a most important matter, viz., the
_second seasoning_, which many woods require. If floor-boards are only
laid down at first on the joists of a building, and at the expiration of
one year wedged tight and nailed down, those unsightly openings caused
by shrinkage, which form a harbour for dirt and vermin, will be avoided,
as the wood will have had an opportunity of shrinking. Doors, sashes,
architraves in long lengths, will also be better if made up some time
before they are required for use. Many Indian woods require a second
seasoning--kara mardá, for instance, a favourite wood with Indian railway
engineers. Even sál and teak are not exempt. Teak shrinks sideways least
of all woods. In the ‘Tortoise,’ store ship, when fifty years old, no
openings were found to exist between the boards; yet Colonel Lloyd says
he found the teak timbers used by him in constructing a large room in
the Mauritius to have shrunk ¾ of an inch in 38 feet. Thus a space of ⅜
of an inch must have been left at each end of the beam, where moisture
could lodge and fungi exist, obtaining their nourishment from the wood.
If unseasoned teak is used for ships, dry rot will in time find a place.
It may be said that teak is a very hard wood, and very durable; yet “the
mills of the gods,” says an ancient philosopher, “grind slow, very slow,
but they grind to powder;” _and so do the fungi mills_.