Kiln-Drying of Lumber

KILN-DRYING OF LUMBER The artificial seasoning of lumber has made such rapid strides in recent years that it is now claimed, on good authority, that lumber of almost any kind can be kiln-dried in comparatively short time, with less damage than results from air-drying. However, many users insist that only air-dried lumber is fit for the most exacting purposes. This opinion is due very largely to the poor work done by the early types of kilns; which were neither scientifically constructed nor properly operated.

The rate at which lumber seasons is determined by three factors—temperature, humidity, and air circulation. All of these factors admit of regulation in a kiln; hence it is fair to assume that it is feasible to obtain favorable combinations of them which will rarely be found under natural conditions.

Kinds of Kilns.

Kilns for drying lumber are of three general types: (1) The dry air kiln, which is now generally obsolete because it produced so much case-hardened and honeycombed lumber.* (2) The moist air kiln, of which there are several modifications according to the methods used to regulate circulation and humidity.

(3) The kiln which uses superheated steam.

Whatever make or type of kiln is used, its successful operation requires adherence to the following principles according to the authority of the United States Forest Service: (1) The timber should be heated through before drying begins.

(2) The air should be very humid at the beginning of the drying process, and be made drier only gradually.

(3) The temperature of the lumber must be maintained uniformly throughout the entire pile. For this an exceedingly large circulation of air is essential.

(4) Control of the drying process at any given temperature must be secured by controlling the relative humidity, not by decreasing the circulation.

* Case-hardening and honeycombing may be explained thus: Suppose a block of wood is very wet, and is placed in a kiln at too high a temperature and too low a humidity. The surface begins to dry and tends to shrink, but is prevented from doing so by the wet interior. Being plastic, it yields to this resistance and becomes stretched. If not plastic, it will check open. As drying proceeds, the surface hardens and sets in an expanded condition, and acts as a strong shell. The interior now dries very slowly, does not become set, but shrinks; and, as the exterior is already hard, it opens up or "honeycombs." When the exterior once becomes set or "case-hardened," the interior is almost certain to become honeycombed, whether the drying takes places in the kiln or a long time afterward. The only remedy is to moisten the exterior by steaming or soaking before it is too late. Air-dried material may also case-harden and honeycomb.

(5) In general, high temperatures permit more rapid drying than do lower ones. The higher the temperature of the lumber, the more efficient is the kiln. It is believed that temperatures as high as the boiling point are not injurious to most woods, provided the humidity of the surrounding air is great enough. Some species, however, may not be able to stand as high temperatures as others.

(6) The degree of dryness attained, where strength is the prime requisite, should not exceed that at which the wood is to be used.

Kilns which most nearly conform to these principles of operation yield a product which is superior to ordinary air-dried lumber, since it warps, checks, and stains less in the seasoning process, and will reabsorb from the air from 15 to 25 per cent less moisture than air-dried lumber. This reduction in the ability of the wood to absorb moisture—or, as it is technically called, its "hygroscopicity"—is very important, because it means a reduction in the extent to which the wood will swell or shrink under atmospheric changes.