Seasoning of Wood

by J. B. Wagner

Humidity

Humidity is of prime importance because the rate of drying and prevention of checking and case-hardening are largely dependent thereon. It is generally true that the surface of the wood should not dry more rapidly than the moisture transfuses from the center of the piece to its surface, otherwise disaster will result. As a sufficient amount of moisture is removed from the wood to maintain the desired humidity, it is not good economy to generate moisture in an outside apparatus and force it into a kiln, unless the moisture in the wood is not sufficient for this purpose; in that case provision should be made for adding any additional moisture that may be required.

The rate of evaporation may best be controlled by controlling the amount of vapor present in the air (relative humidity); it should not be controlled by reducing the air circulation, since a large circulation is needed at all times to supply the necessary heat.

The humidity should be graded from 100 per cent at the receiving end of the kiln, to whatever humidity corresponds with the desired degree of dryness at the delivery end.

The kiln should be so designed that the proper degree may be maintained at its every section.

A fresh piece of sapwood will lose weight in boiling water and can also be dried to quite an extent in steam. This proves conclusively that a high degree of humidity does not have the detrimental effect on drying that is commonly attributed to it. In fact, a proper degree of humidity, especially in the loading or receiving end of a kiln, is just as necessary to good results in drying as getting the proper temperature.

Experiments have demonstrated also that injury to stock in the way of checking, warping, and hollow-horning always develops immediately after the stock is taken into the kiln, and is due to the degree of humidity being too low. The receiving end of the kiln should always be kept moist, where the stock has not been steamed before being put into the kiln. The reason for this is simple enough. When the air is too dry it tends to dry the outside of the material first—which is termed "case-hardening"—and in so doing shrinks and closes up the pores of the wood. As the stock is moved down the kiln, it absorbs a continually increasing amount of heat, which tends to drive off the moisture still present in the center of the stock. The pores on the outside having been closed up, there is no exit for the vapor or steam that is being rapidly formed in the center. It must find its way out some way, and in doing so sets up strains, which result either in checking, warping, or hollow-horning. If the humidity had been kept higher, the outside of the material would not have dried so quickly, and the pores would have remained open for the exit of moisture from the interior of the wood, and this trouble would have been avoided.

Where the humidity is kept at a high point in the receiving end of the kiln, a higher rate of temperature may also be carried, and in that way the drying process is hastened with comparative safety.

It is essential, therefore, to have an ample supply of heat through the convection currents of the air; but in the case of wood the rate of evaporation must be controlled, else checking will occur. This can be done by means of the relative humidity, as stated before. It is clear now that when the air—or, more properly speaking, the space—is completely saturated no evaporation can take place at the given temperature. By reducing the humidity, evaporation takes place more and more rapidly.

Another bad feature of an insufficient and non-uniform supply of heat is that each piece of wood will be heated to the evaporating point on the outer surface, the inside remaining cool until considerable drying has taken place from the surface. Ordinarily in dry kilns high humidity and large circulation of air are antitheses to one another. To obtain the high humidity the circulation is either stopped altogether or greatly reduced, and to reduce the humidity a greater circulation is induced by opening the ventilators or otherwise increasing the draft. This is evidently not good practice, but as a rule is unavoidable in most dry kilns of present make. The humidity should be raised to check evaporation without reducing the circulation if possible.

While thin stock, such as cooperage and box stuff is less inclined to give trouble by undue checking than 1-inch and thicker, one will find that any dry kiln will give more uniform results and, at the same time, be more economical in the use of steam, when the humidity and temperature is carried at as high a point as possible without injury to the material to be dried.

Any well-made dry kiln which will fulfill the conditions required as to circulation and humidity control should work satisfactorily; but each case must be studied by itself, and the various factors modified to suit the peculiar conditions of the problem in hand. In every new case the material should be constantly watched and studied and, if checking begins, the humidity should be increased until it stops. It is not reducing the circulation, but adding the necessary moisture to the air, that should be depended on to prevent checking. For this purpose it is well to have steam jets in the kiln so that if needed they are ready at hand.