On the principle that prevention is always better than cure, we will deal first with the seasoning of timber, which is the next thing in importance to be attended to after the wood has been procured from a fully matured tree. The object of seasoning timber is to get rid of the sappy moisture which, as has previously been pointed out in these notes, is a great source of decay. This can be done by either drying up or driving out the superfluous moisture, a proceeding which of course reduces the weight of the limber. Tredgold maintained that the timber was not seasoned and fit for carpenter's use until it had lost one-fifth of its original weight; and for joiner's work he considered that it! should have parted with moisture to the extent of one-third its weight before it was fit for best work.
There are several methods of attaining this end, the chief of which are as follows: -
Natural seasoning is a very simple procedure, and consists of stacking the timber, after it has been converted, edgewise on perches, so that, while the air can hive free access all round it, the sun and wet can only touch the edge.
The method of operating is briefly explained thus: One length of timber is reared edgewise, and alternately from the sills of B and D (fig. 264), which should protect the bottom of the board from the damp arising to the cross-piece C; by which means they are secured from moving at the top, and the air can circulate all round the wood, except just immediately over the cross-piece, without exposing the width, the greater surface, to the weather.
About 1/3 Scale.
Floor-boards and other thin woods are sometimes stacked on the flat, on bearers raised off the ground, with a lath between each row of boards, thus (fig. 265). They are generally covered over, on the top only, with felt or other waterproof material.
In large yards the timber, of all sizes, is stacked under covered sheds, whereas it is best perched in the open, if in a shady position.
The seasoning process requires, for complete drying, about a year, for every inch in thickness, for converted and sawn oak or other hard woods, when perched or stacked under ordinary circumstances, and for fir and other soft woods about half as long.
Next to the natural seasoning comes the water seasoning, which is a simple method of washing the sap out, though this is generally applied only to logs. It consists of submerging the whole of the timber under either ordinary or salt water; the former being preferable, as the latter water has a tendency not only to harden the wood, but also to attract moisture when in the work, inasmuch as salt is generally affected by any excess or variation of humidity in the atmosphere. The logs are kept wholly under the running water until the sap has had time to get washed out, then the logs are taken out, dried by the air, and cut to sizes; for unless they are dried before use they will be subject to dry rot.
Boiling and steaming or the hot-air method, are similar: that is to say, in the one the sap is supposed to be driven out by the hot water, and in the other by the steam or hot air. They are all quicker processes than either water or natural seasoning; but they are certainly more expensive, and, moreover, they have injurious effects on the timber, by reducing its strength and elasticity.
It may be as well to mention that many kinds of timber require more than one seasoning, because each time the surface is planed, or anything is taken off, the wood is apt to shrink, as the air seems to harden the outside, and form a casing to the inside, and thus prevent its penetrating the wood under ordinary circumstances. This more particularly applies to floor boards and joinery, where the least shrinking is noticeable.