A live tree contains a great amount of water in the walls and cell cavities of the fibers of the wood. When the log is cut into lumber and exposed to the air, the fibers shrink and dry out as the water in the wood is evaporated. The seasoning process consists of the elimination of water from the wood pores. If the drying process is not done properly the wood checks, warps, splits and is subject to stain and insect attack. The degree to which the water content is removed from the wood depends upon the purposes for which the seasoned lumber is to be used. For some purposes, such as for boxes or crating, the wood need not be thoroughly dry, while wood used in the manufacture of furniture, fixtures, etc. must be thoroughly dry and very well seasoned. In general the wood used by the aver-age home craftsman must be thoroughly seasoned.
There are two principal processes used in seasoning wood, namely air drying and kiln drying. As a rule the quantity of lumber of any particular kind used by the home craftsman is comparatively small and his lumber is obtained from a local lumber yard or from a dealer specializing in woods for his use. He should insist upon getting thoroughly seasoned lumber and, if possible, obtain dry-kilned supplies.
There are a number of publications available regarding the specialized processes of seasoning lumber, and since the average home craftsman is not equipped for seasoning lumber by the dry-kiln process, this discussion will be limited to a brief description of air drying.
Air drying consists of piling the lumber, usually outdoors, in such a way that the air can circulate throughout the pile and thus absorb the moisture from the wood through evaporation. The atmospheric conditions in the locality and the way in which the lumber is piled influences the degree and speed with which the moisture in the lumber is removed. It usually requires many months to properly season lumber by air drying. If the lumber is carefully and properly piled and stripped with narrow pieces called "stickers", air drying can be accomplished so as to avoid warping, excessive checking, stain, decay or insect infestation.
A proper base should be prepared upon which the lumber is to be piled so that there will be proper drainage and the air can pass freely underneath, around and through the pile. The boards or lumber should be laid in rows with air spaces between each piece and narrow strips or stickers placed from 2 to 4 feet apart and spaced uniformly throughout the pile, directly over each other and at both ends placed flush or slightly beyond the ends of the board. The layers are given a pitch of about one inch to the foot for drainage. The pile should be situated so as to take full advantage of the prevailing winds and should be sloped from the front to the rear for proper drainage during rainy weather. A "roof", or top protective covering should be put over the lumber pile consisting of a double layer of boards placed crosswise to the pile and overlapping each other. The roof should be stripped so that it will be approximately five or six inches above the first layer of lumber and provide ample air circulation. It should overlap one to two feet in front but only a few inches in the rear, sufficient to reasonably deflect the rain from the pile.
If lumber is dried too rapidly excessive checking and warping occurs. This can be controlled by using thinner stickers, making wider piles, narrowing the spaces between the boards in the piles and also between piles, placing covers over the piles as a protection against the weather and reducing the flow of air through the vents.
Some lumber companies stand the lumber on end and at an angle, leaving spaces between the boards and stripping between the rows, and without a roof covering. This system is followed to some extent in the Southern states where the rainfall is comparatively light. Piling is also done in this manner by some concerns in the Pacific Northwest with certain species of lumber. However, more satisfactory and thorough results are obtained by air drying lumber in piles as above explained.
It may be found more convenient to the average craftsman to air-dry his lumber in a building or open shed to avoid excessive exposure to the weather.
Quite frequently the home craftsman desires to season a small section of a log or limb without sawing it into pieces of lumber. Sections from a small or medium sized tree or limb can be dried conveniently by placing a good covering of paraffin over each end of the piece and over each cut where a branch has been cut off, and storing it in a dry place where the air can freely circulate all around it. A sealer such as is used by nurserymen when trimming trees may also be used in lieu of the paraffin. Ample time should be allowed for the moisture to be completely evaporated from the piece. Usually this will require several months to a year or more.
Any craftsman desiring to study the subject of wood seasoning in more detail may obtain a number of bulletins on the subject at small cost from the Superintendent of Documents, Government Printing Office, Washington, D.C.