37. The process of evaporating the sap, or the drying out of lumber, is effected after it has been sawed into planks, joists, studs, etc., and two methods known respectively as seasoning and kiln-drying are recognized as suitable and efficient for the purpose.

In the first of these, the boards are placed in the open air in large square piles, with narrow strips between the layers; a free circulation thus takes place throughout each pile, and the lumber remains in this position from two to four years, according to its ultimate purpose - two years being considered adequate for joists, studs, sheathing, and other ordinary framing material, while work intended for trim, doors, sashes, and other products of the joiner's skill, should season for four years, or even more, according to the class of material.

Kiln-drying is effected by piling the lumber, as above described, in chambers, or kilns, within which a circulation of air is maintained at a temperature of about 140° F. and at a speed of about 40 miles per hour. Vacuum pumps are used to produce this rapid circulation and to remove the moisture as it evaporates from the boards.

In this manner, lumber not over 2 inches in thickness can be thoroughly dried in about forty-eight hours, which is certainly a great saving of time, but the result is acquired at the expense of a loss of vitality of the material.

38. Kiln-dried lumber lacks the toughness and elasticity retained in the seasoned material, has a greater affinity for atmospheric moisture, and is often subject, especially in the softer woods, to what is known as dry shrinkage - that is, a shrinkage caused by the gradual closing together of the cell walls from which the moisture was evaporated in the kiln, leaving the cell in a vacuous, or hollow, condition. This dry shrinkage does not take place until after the material has been worked, and regardless of the position of the zones, or annual rings, the wood becomes concave on its freshly cut surface.

The cause of this is that the outside, or surface, tissue of the material is dried first, and thus forms a sort of casing, or crust, which holds the inner fibers in position, and when this surface is removed through the agency of the saw or plane, the interior fibers, being thus relieved of their protecting casing, gradually close on the exposed side and cause the wood to bend, or warp.

This will also occur in weather-seasoned wood which has been placed in one position for a long period and remained uncut or unworked. Thus, the top of an old table will almost invariably become concave if it is planed off to get a new surface.

39. Good results are obtained by the process of subjecting weather-seasoned boards of two or three years' exposure to the kiln-drying process before they are used in a building, or immediately after they are worked into flooring, siding, ceiling, etc. Boards thus treated and kept perfectly dry thereafter can be used in the trim and finish of a building, and if primed and painted, or filled and varnished as soon as possible after they are in place, a good, durable, and unchanging job can be assured.