The means taken to keep timber as far as possible from cracking or warping during the process of seasoning, are very various. They are partly connected with the treatment of the wood when it is cut up into timber, and partly with its treatment for any special purpose.
Trees should be felled when the sap is down or at rest. The best time is from the the middle of December to the end of February. Too much stress cannot be laid upon the importance of felling timber at the right time, for if felled at the wrong season, it will contain too much sap, which will make it very difficult to dry, render it much more liable to swell or shrink, and increase the risk of its becoming worm-eaten. In the case of needle-leaved trees excess of sap gives a bluish tinge to the surface of the timber.
When wood should be cut down.
The more slowly timber is dried the less it cracks, and timber felled at the proper season and allowed to dry slowly cracks very little. Barked timber, which dries more quickly than unbarked, often cracks so widely that it is quite unfit for slojd-work. When the bark is left on, the cracks may be numerous, but they will be small. Thick pieces crack more than thin pieces; logs or round wood more than split wood; sap-wood more than heart-wood. Care should be taken during seasoning that the air has free access to the wood on all sides. Wood which has been split with the axe is apt to crack at the ends; this may be prevented by pasting paper over them. Portions of timber containing the pith and the adjacent annual layers, always crack; such pieces are therefore unavailable for work. When round timber is split in order to facilitate seasoning, it should be divided through the pith.
Wood should be dried slowly.
Boards or planks are best dried in a drying shed, where fresh air can circulate freely round each piece. The best way is to place the boards on their edges, with sufficient space between, taking care that they are not twisted in any way. If they are piled one on the other, pieces of dry wood should be placed between them, in order to separate them. For obvious reasons, none of the timber should touch the ground.
Natural seasoning or seasoning by exposure to the air.
Timber which has been felled at the proper time, takes no harm from exposure to a little rain in spring and early summer, provided always that the air has free access, so that it may dry again quickly. Indeed, timber usually dries very rapidly out in the open air in early summer. The rain helps to wash out the sap, and the timber is thereby rendered more durable when thoroughly dried.
When wholly or partially finished planks are laid by for future use, care must be taken that they do not lie one close upon the other, but that both sides are fully exposed to the air, to facilitate further drying and prevent warping.
In the early stages of seasoning, evaporation goes on with tolerable rapidity, but afterwards it takes place more slowly, and timber must be kept in a dry and airy place for two or three years before it can be considered fully seasoned. Timber is said to be seasoned when the quantity of moisture it contains coincides with that contained in the atmosphere.
As has been said above, the amount of water in timber seasoned as indicated, never falls below 10 per cent, of its weight. To decrease the water still further, it is necessary to dry the timber in ovens constructed for the purpose, or in heated air, or else to keep it for a long time in a warm place.
When timber can be said to be seasoned.
Drying expels water only, not the essential elements of the sap, some of which part with great difficulty from water, and also take it up again with great readiness when the timber is once more exposed to moisture. These properties of the sap make seasoning much more difficult than it would otherwise be, and retard the process considerably in wood which abounds in sap - e.g., beech, birch, oak, and walnut.
Influence of the sap on seasoning.
To overcome this difficulty, the sap may either be removed altogether, or its action may be neutralised. The first is accomplished by immersing the wood in cold water for some time, or in boiling water for a shorter time ; or, what is still better, by steaming it. In the second case the timber is impregnated with substances calculated to counteract the destructive effects of the sap-e.g., a solution of common salt, vitriol, chloride of zinc, etc. These methods can, however, only be mentioned here incidentally, as any detailed description would be entirely beyond the limits of this work.
Removal of the sap.
2. Precautions necessary to prevent Warping and Cracking under special conditions.
As shrinkage is greater in tangential than in radial section, the wood for any special purpose ought to be sawn out or split in the direction of the radii of the stem, in order that the article may the better preserve its form and size. There are, however, some practical difficulties which render it impossible to carry out this principle in all cases.
Uniformity of texture, and consequently less tendency to crack or warp, is more easily secured in small pieces of timber than in large pieces, and consequently it is usual in the construction of articles to employ smaller pieces of wood than are required, and to joint them together; and these pieces may often, without any disadvantage, be chosen from different kinds of wood, and may have their fibres running in different directions. Hence it is better in making a broad plane surface to select planks which have been divided in two, than to make it of whole planks. Planks containing the heart-wood nearest the pith which is generally cracked, are always divided in two to get rid of this portion.
Jointing pieces of wood.
Jointing also permits large plane surfaces to shrink without injury to parts of the work already completed. For example, blackboards, the panels of doors, etc., which are set into a groove in a frame, are thus permitted to shrink without cracking. Table-tops are strengthened by blocks which lit into a groove in the framing, and are glued to the under part of the top. Broad pieces of wood are furnished on one side with clamps, the fibres of which run at right angles to those of the broad piece, and which are inserted in such a way that the wood of the broad piece can shrink without hindrance.
Frames and panels.