Having finally reached the commercial stage, the lumber is shipped away
from the mill either by water or by rail to the lumber yards of the
country.
Here it should be seasoned. In the past this process consisted of
piling the planks in the open air in such a way that air could
circulate freely through the pile, allowing the sap to evaporate and
the wood to dry evenly. This was a sure but slow process, and in the
hurry of modern life quicker methods have been tried.
One of these is known as kiln drying, by which the time is reduced to
a few weeks. It consists of piling the wood in a room like a kiln and
drying it by artificial heat. The result is not so satisfactory as the
natural method, because the sap near the surface hardens and prevents
the inner moisture from escaping, so that kiln-dried lumber while dry
at the surface is "green" inside. When planed till part of the surface
is removed the green wood is brought near to the air again, and
warping is liable to occur.
Other methods have been tried, such as steaming to vaporize the
sap, and soaking in hot water for the same purpose. Of course these
processes all add to the cost of lumber, yet so valuable is time that
it is difficult to obtain good old-fashioned seasoned wood unless it
has lain for some time in a local yard.
In order to understand the phenomena of warping, shrinkage, checking,
shakes, etc., it is necessary to know something of how the tree grows.
Like all living organisms, it is made up of minute cells. The new cells
are formed on the outside of the tree under the bark, and here the
sap is most active. The cause of the flow of sap is not very clearly
understood, but it corresponds to blood in the human body, in that it
carries the nourishment that forms the cells. As a new mass or layer
of soft new cells forms each season, the layers may be distinctly seen
and counted, but the line of separation is not a sharply drawn one,
as we find by examining a cross section of wood with the microscope.
However, the layers or annual rings are distinct enough to be counted,
so that the age of the tree at the time it was cut down may be readily
discovered.
The new or sap wood, then, is further from the centre each year, and
while the old cells may not be dead, they contain less and less sap,
are therefore drier, and after a few years change colour, becoming
darker.
There is often a very great contrast between the colour of the heart
wood and that of the sap wood, although the latter may be represented
by several years of growth.
These annual rings are not actually circular, but very irregular, and
often wider in some parts than in others. The study of these rings
is very interesting, and it shows that the tree usually increases in
diameter more rapidly during the first few years than later. Very
often, after growing slowly for several years, the tree will apparently
grow rapidly again. The cause of this cannot be determined without a
knowledge of the tree's history.
It has been proved by experiment that thinning the forest increases the
growth of the remaining trees 18 per cent., and these peculiarities in
the rings may have been due to some like cause. The bearing of this
fact on the peculiarities of warping and shrinkage is that when cut
down the log is drier at the heart and more sappy at the outside, so
that evaporation occurs near the surface.
[Illustration: Fig. 239. Warping, wind, and shrinkage.]
The effect of it is shown in Fig. 239. The outside drawing together
has opened the wood, or "checked" it, most at the outside, diminishing
toward the centre. The evaporation would have occurred just the same
had the log been cut into planks, causing them to curl as shown at
_a_. This is known as warping, and it is one of the troubles of the
woodworker. In construction it must be constantly guarded against, and
overcome as far as possible. It cannot be entirely prevented, but if
the wood has been well seasoned before it is used a large part of the
warp will be taken out in the planing mill, or in the squaring up.
Twisting, winding, and warping are also caused by the two sides of a
board having been subjected to different degrees of heat, moisture,
etc. If a plank is laid on the floor, the upper part is more exposed to
the air and to changes of temperature and humidity; therefore it curls.
If a board is stood on end or placed in a rack where there is a free
circulation of air, the curling will be much less. Even in a rack,
if several boards are piled one on another, the top one will have
different conditions from the others and be apt to curl or wind.
Shrinkage is a term applied to the decrease in diameter of the tree,
due to sap evaporation.
In the case of the board it means a decrease in width, and it varies
greatly in different trees and their woods. As shrinkage is always
across the grain, its effect on a common joint may be illustrated in
Fig. 240. At _a_ is shown a middle lap joint just put together. If the
wood is not well seasoned, shrinkage will in time change it to the form
shown at _b_, which is exaggerated to make the meaning clear.
[Illustration: Fig. 240. Effect of shrinkage on lumber]
A square piece of timber, one corner of which is the centre of the
tree, will change from _c_ to _d_. Shrinkage as well as warping must be
taken into consideration in construction.
[Illustration: Fig. 241. The development of the panelled door]
The development of the panelled door is a good illustration. Suppose
we wish to close a space with a door, knowing little about shrinkage.
Let us construct it by the simplest method, say four vertical boards.
If the width of these boards equals the opening when the door is built
(Fig. 241) there will soon be an opening wide enough for the fingers
to enter and lift up a latch on the inside. The door is very much of
a failure. We notice, however, that there is no opening at top or
bottom. An idea! We construct a door with planks placed horizontally.
Now although we find after a while no opening at the sides we do find
openings at top and bottom. The panelled door is not constructed solely
for beauty but to overcome shrinkage as far as possible. Fig. 241 shows
the various parts. The rails maintain the width, the only shrinkage
being in the cross grain of the stiles, and they preserve the height
except for the small amount in the rails. The remaining spaces are
panelled, the construction being shown at _a_. Both stiles and rails
have a groove plowed out to receive the edges of the panel. This should
be free to shrink in the grooves, where it is invisible, but if the
mistake is made of fastening the panel edges rigidly in these grooves
the panel will shrink anyway and frequently split from top to bottom.
Many other forms of construction which we have seen daily as long as
we can remember have equally sound reasons for their form. No piece of
woodwork should be designed without considering how it will be affected
by shrinkage and warping.
In selecting lumber always look out for "shakes." This is a defect
caused by the separation of the annual rings. A tree may be considered
as a series of irregular cylinders of diminishing diameters. The
forest-grown tree is much more spindling, tall, and straight than the
low-crowned, heavy-branched specimen grown in the open, where there is
no crowding.
The swaying of the forest tree in the wind, especially when its
neighbours have been cut down, is sometimes sufficient to make the
rings separate and slide one within the other. This is more noticeable
in some species than others and it gives the wood a serious fault.
(Fig. 239).
"Winding" is the result when the ends and sides are no longer parallel.
Like all peculiar characteristics of wood, this varies greatly in
lumber of various kinds, and may be largely avoided by exposing both
sides to the same conditions, or keeping equally distributed weight on
it until used. When winding becomes excessive, the board is useless for
any kind of work.