Water In Timber. As has been explained, wood is composed of cells of different forms and of different functions with reference to the life of the tree. These contain more or less water, which may occur in three conditions: (1) it forms the greater part of the contents of the living cells; (2) it saturates the walls of all cells; and (3) it partly fills the cavities of the lifeless cells, fibers, and vessels. In some cases the water in growing timber makes more than half the weight of the wood.

Sapwood contains more water than heartwood; hence there is more water in the upper portion of a tree trunk than in its lower portion, more in limbs than in trunk, and most in the roots. Different trees of the same kind differ in the amount of water they contain, thrifty trees having more than stunted ones, and young ones more than old, while the moisture in the wood of all trees varies with the season of the year. The popular idea that trees contain more water in summer than in winter, however, is not always correct, tests recently made by the United States Bureau of Forestry showing that the greatest weight of certain trees is in the winter.

293. The Process of Seasoning consists in driving out of green wood, either by natural or artificial means, a considerable portion of the water contained in the walls and cavities of its cells. Seasoning thins the walls of the cells and makes the wood appear more porous. The rate at which it will season, or dry, depends upon the kind of timber, the size of the piece, the part of the trunk from which it is taken, and the character of its exposure to drying influences; pine, for example, dries faster than oak, small boards faster than large ones, and sapwood faster than heartwood.

Wood newly cut from a living tree, when exposed to ordinary atmospheric conditions, gradually dries, and in so doing changes its weight and dimensions. Green lumber, therefore, is unstable with reference to these qualities and is not serviceable for many purposes until it has been seasoned. All lumber designed for the manufacture of furniture, cabinetwork, and machinery should be thoroughly seasoned before it is used.

The method employed in seasoning must be such that the timber will not only dry, but will also be preserved from injury during the process. Some of the harmful effects due to improper ways of seasoning are the formation of cracks, or " checking," and a loss of strength caused by injury to the wood structure; these must be taken into account in deciding upon the method to be used. Those most common are air seasoning, steam seasoning, water seasoning, boiling in oil, and kiln-drying.

294. Air Seasoning is the cheapest and probably the best of the methods mentioned, although it is slow and must be carefully conducted or there will be much injury by decay and by checking. It consists in piling the lumber out of doors where the air may circulate freely about it. Under these conditions the moisture is given off and the solid constituents of the sap gradually harden and become incapable of further change; the lumber is then regarded as seasoned. Air drying demands the exercise of considerable care. If green lumber is piled without proper air spaces, it is sure to decay; while, on the other hand, if exposed to sun and wind, the moisture in the outer portions of each piece thus exposed evaporates faster than that in the inner portions, and that in the ends faster than that at the middle, with the result that shrinkage proceeds unequally and cracks are formed. Both decay and checking may be prevented by piling the timber properly and protecting it from the sun and rain. It should be so placed that the air may circulate freely, not only on all sides of the piles but also about each piece. Fig. 315 shows a pile of railroad ties as arranged for seasoning. Sawed lumber may be piled in a similar way, and with material of uniform dimensions the pile may be carried to a considerable height. The time required to air-dry lumber depends upon the size of the pieces, a longer time being allowed for large sticks than for smaller ones. Sometimes lumber which has been piled but a few months is regarded as seasoned, and for some purposes it may be safely used, but the drying is only partial. For complete air-drying from two to four years are required.

Fig. 315

295. Steam Drying is employed when it is desired to season boards quickly, or when it becomes necessary to soften wood in large pieces for the purpose of bending it, as, for instance, in shipbuilding and furniture making. As a seasoning process it is objectionable, because the high temperature required is likely to injure the wood structure to such an extent as to decrease the strength of the material. The process consists in exposing the wood to an atmosphere of steam under considerable pressure. The steam enters the cells of the wood and dissolves the sap, leaving water in its place; when the water is dried out, the wood is left well seasoned. The softening of the fibers by the steam during this process, and the uniform conditions of heat overcome all tendency toward checking, which is so likely to occur in air seasoning. The steaming process occupies but a few hours.

296. Water Seasoning is accomplished by allowing the timber to remain for a considerable time in water. By this means the sap is dissolved away and replaced by water, which evaporates rapidly when the timber is piled for drying. Timber seasoned in this way usually shrinks uniformly, exhibiting but slight disposition to check. Logs which are designed for the spars of ships are invariably water-seasoned. They are usually stored in water, with the bark on, for many months, and are thus kept in a soft and workable condition until such time as they may be removed for finishing.

297. Kiln Drying is a common method of artificial seasoning. It requires far less time than the processes already mentioned, and, with the exception of air-drying, is the one to which most lumber is subjected. Dry-kilns are to be found in connection with nearly all sawmills and planing mills, and also with those manufacturing establishments which consume large quantities of wood, such as furniture and car factories. Air-seasoned lumber, designed for inside finish, when received at the sawmill is often piled for a few days in the kiln to remove moisture which it may have gathered from the atmosphere.