Seasoning of Wood

by J. B. Wagner

Kiln-drying Gum

The following article was published by the United States Forestry Service as to the best method of kiln-drying gum:

Piling.—Perhaps the most important factor in good kiln-drying, especially in the case of the gums, is the method of piling. It is our opinion that proper and very careful piling will greatly reduce the loss due to warping. A good method of piling is to place the lumber lengthwise of the kiln and on an incline cross-wise. The warm air should rise at the higher side of the pile and descend between the courses of lumber. The reason for this is very simple and the principle has been applied in the manufacture of the best ice boxes for some time. The most efficient refrigerators are iced at the side, the ice compartment opening to the cooling chamber at the top and bottom. The warm air from above is cooled by melting the ice. It then becomes denser and settles down into the main chamber. The articles in the cooling room warm the air as they cool, so it rises to the top and again comes in contact with the ice, thus completing the cycle. The rate of this natural circulation is automatically regulated by the temperature of the articles in the cooling chamber and by the amount of ice in the icing compartment; hence the efficiency of such a box is high.

Now let us apply this principle to the drying of lumber. First we must understand that as long as the lumber is moist and drying, it will always be cooler than the surrounding air, the amount of this difference being determined by the rate of drying and the moisture in the wood. As the lumber dries, its temperature gradually rises until it is equal to that of the air, when perfect dryness results. With this fact in mind it is clear that the function of the lumber in a kiln is exactly analogous to that of the ice in an ice box; that is, it is the cooling agent. Similarly, the heating pipes in a dry kiln bring about the same effect as the articles of food in the ice box in that they serve to heat the air. Therefore, the air will be cooled by the lumber, causing it to pass downward through the piles. If the heating units are placed at the sides of the kiln, the action of the air in a good ice box is duplicated in the kiln. The significant point in this connection is that, the greener and colder the lumber, the faster is the circulation. This is a highly desirable feature.

A second vital point is that as the wood becomes gradually drier the circulation automatically decreases, thus resulting in increased efficiency, because there is no need for circulation greater than enough to maintain the humidity of the air as it leaves the lumber about the same as it enters. Therefore, we advocate either the longitudinal side-wise inclined pile or edge stacking, the latter being much preferable when possible. Of course the piles in our kiln were small and could not be weighted properly, so the best results as to reducing warping were not obtained.

Preliminary Steaming.—Because the fibres of the gums become plastic while moist and hot without causing defects, it is desirable to heat the air-dried lumber to about 200 degrees Fahrenheit in saturated steam at atmospheric pressure in order to reduce the warping. This treatment also furnishes a means of heating the lumber very rapidly. It is probably a good way to stop the sap-staining of green lumber, if it is steamed while green. We have not investigated the other effects of steaming green gum, however, so we hesitate to recommend it.

Temperatures as high as 210 degrees Fahrenheit were used with no apparent harm to the material. The best result was obtained with the temperature of 180 degrees Fahrenheit, after the first preliminary heating in steam to 200 degrees Fahrenheit. Higher temperatures may be used with air-dried gum, however.

The best method of humidity control proved to be to reduce the relative humidity of the air from 100 per cent (saturated steam) very carefully at first and then more rapidly to 30 per cent in about four days. If the change is too marked immediately after the steaming period, checking will invariably result. Under these temperature and humidity conditions the stock was dried from 15 per cent moisture, based on the dry wood weight, to 6 per cent in five days' time. The loss due to checking was about 5 per cent, based on the actual footage loss, not on commercial grades.

Final Steaming.—From time to time during the test runs the material was resawed to test for case-hardening. The stock dried in five days showed slight case-hardening, so it was steamed at atmospheric pressure for 36 minutes near the close of the run, with the result that when dried off again the stresses were no longer present. The material from one run was steamed for three hours at atmospheric pressure and proved very badly case-hardened, but in the reverse direction. It seems possible that by testing for the amount of case-hardening one might select a final steaming period which would eliminate all stresses in the wood.