SEASONING OF TIMBER Freshly cut timber frequently contains half its weight of water, or, stated otherwise, it contains 100 per cent of water based upon the absolutely dry weight of the wood. A large proportion of this excess water must be removed before the timber is in shape to use, and the process by which it is removed is called "seasoning." Seasoning usually increases the strength, stiffness, and hardness of timber, greatly reduces its weight, and renders it less likely to shrink in subsequent usage. Timber is used green only when absolutely necessary, since, among other undesirable qualities, it is more likely to decay than is seasoned timber.
There are two general methods of seasoning timber—the natural and the artificial, or airdrying and kiln-drying. Air-dried timber may contain from 15 to 30 per cent of moisture, depending upon kind, size, climate, and other factors. Kiln-dried timber usually contains 5 to 10 per cent of moisture; while in what is called "oven-dry" or "bone-dry" wood, the moisture content is less than 1 per cent of the absolutely dry weight of the wood.
For ordinary structural timber, studding, sheathing, and the like, air-drying is sufficient. For the more refined uses of timber where it is re-worked into flooring, finish, furniture, and other articles, thorough kiln-drying is necessary to reduce as much as possible the tendency to swell and shrink with atmospheric changes. Heavy material like vehicle stock may be airdried for two or three years, and then kiln-dried slowly for a long time to obtain the necessary seasoning with the least checking and warping.
Thin boards of any kind of lumber exhibit more or less tendency to check and twist during seasoning processes. This tendency is greater in the hardwoods than in the softwoods, because of the much more complex structure of the hardwoods. Commercial practice has, however, made such rapid strides in the last few years that almost any kind of timber is now successfully seasoned by either natural or artificial means. For many years, cottonwood and gum were rejected by sawmill operators, because of the general belief that they could not be satisfactorily seasoned. Now the manufacturers handle these woods with comparatively little trouble; and their products are popular for a multitude of purposes, some of which are most exacting.
Since most of the softwoods are very easily kiln-dried with little damage, many of them are artificially seasoned to reduce the shipping weight and save the time required for air-seasoning. Much of the Southern yellow pine and the Western fir and cedar go straight from the sawmill to the dry-kiln, and then into cars for shipment to market. As the hardwoods are more difficult to handle, they are ordinarily airdried by the lumber manufacturer, and kilndried at the factory where they are re-worked into flooring, finish, and other products.