As it is impossible to season wood by natural means so that it will not shrink when put in a building that is to be kept warm and dry, it is necessary to dry all lumber that is to be used for finishing or in the manufacture of furniture by artificial means.
For this purpose a tight chamber called a dry kiln is constructed, and a constant current of air heated from 1500 to 1800 F. is made to pass over the lumber.
Pine, spruce, cypress, cedar, etc., may be put in the kiln fresh from the saw, allowing four days for 1-inch boards. Hard woods, especially oak, ash, maple, birch and sycamore, should be air-seasoned from three to six months to allow the first shrinkage to take place gradually before it is put in the kiln, and should then be exposed to the above temperatures for from six to ten days for 1-inch lumber.
Steaming lumber is often resorted to in order to prevent checking and "case-hardening," and also to make it bend more easily when bent pieces are required.
"The rapidity with which water is evaporated, that is the rate of drying, depends on the size and shape of the piece and on the structure of the wood.
White pine dries faster than oak. Water also evaporates faster from a cross section than from a longitudinal section and twice as fast from a radial section as from a tangential section."
Dry wood when soaked in water soon regains its original volume, and wood that has been kiln-dried at once takes up water from the air, even in the dryest weather; hence the necessity of having the building dry before delivering the finishing lumber and of keeping it dry thereafter.
This property of wood of absorbing moisture may be lessened by boiling and steaming, and also by exposure in dry air to a temperature of 3000 F. for a short time, but cannot be entirely overcome.
Case-Hardening. - Rapidly dried, in the kiln, the wood of oak and other hard woods "case-harden," that is, the outer part dries and shrinks before the interior has a chance to do the same, and thus forms a firm shell or case of shrunken, commonly checked wood around the interior. This shell does not prevent the interior from drying, but when this drying occurs the interior is commonly checked along the medullary rays, as shown in Fig. 5. In practice this occurrence can be prevented by steaming the lumber in the kiln, and still better by drying the wood in the open air or in a shed before placing in the kiln. Since only the first shrinkage is apt to check the wood, any kind of lumber which has once been air-dried (from three to six months for 1-inch stuff) may be subjected to kiln heat without any danger.
The only reliable measure of dryness is that of weight. Professor J. B. Johnson, engineer in charge of the U. S. Timber Tests, offers the following recommendation, which appears to have much practical value:
It would be well for architects to specify definite maximum percentages of moisture which would be allowed in lumber to be used in various kinds of interior finishing work instead of the usual specification of "thoroughly seasoned lumber" or "kiln-dried lumber." As such terms as these usually have to be interpreted by the judgment of different individuals, and as the legal determination of the fact always rests upon the testimony of various witnesses, each having his own interpretation of the meaning of such terms, it follows that the standards so established are extremely indefinite and unsatisfactory. If the architect would specify a particular percentage of moisture for flooring, for instance, by saying that it should not contain more than 10 per cent, of its weight in water, this is a specification which is perfectly definite and the fulfillment of which is easily determined. Thus, when the flooring is delivered at the building, the architect may select a half dozen flooring boards from the lot and cut sections from their central portions about 1 foot long and take them to the nearest grocery or drug store and have them carefully weighed. He can then dry them out by putting them into an ordinary cook stove oven and keeping them there for a few hours at a temperature somewhat greater that boiling. He can then weigh them again, quickly, before they have re-absorbed atmospheric moisture, wrapping them up carefully if it is necessary to carry them any distance for the purpose of weighing. The difference between the two weights divided by the dry weight gives the percentage of moisture in terms of the dry weight. (Thus, if a piece weighs 44 ounces when first weighed and 40 ounces when taken from the oven, the percentage of moisture would be 4÷40, or 10.)
Fig. 5 - " Honeycombed" Board. The checks or cracks form along the pith rays.
Since the moisture in the air in inhabited buildings is rarely less than 10 per cent., this may be taken as the standard moisture for " thoroughly seasoned lumber." Twelve per cent, of moisture would probably not be detrimental, and in buildings that are not warmed above 68° F., even 15 per cent, may be allowed.
As a check on the fulfillment of the specifications, kiln-dried lumber should be tested for moisture immediately upon its delivery, for if the building is in the least damp the lumber will quickly absorb additional moisture from the air.
Framing timber and outside finishing lumber may be considered as well seasoned when it only contains 15 per cent, of moisture.