Kiln seasoning originated with at tempts to prevent warping and checking in special pieces. In the United States, nearly all hardwoods, save those in large con struction pieces, are now cured by this method. Drying pro ceeds rapidly and details can be controlled in kilns as they cannot be in natural-seasoning or in water-seasoning.
There are many details and combinations, but the factors that influence design and operation in all cases are temperature, mois ture, and circulation.
Temperature. Heat may be dry or wet. In both cases, high tem peratures should be avoided. Dry heat in excess of two hundred and twelve degrees is sufficient to expel some of the volatile constituents of the wood, which then becomes weak and brittle. The equivalent of this temperature in moist heat is not known. Temperatures of from one hundred degrees to one hundred and twenty degrees Fahrenheit are used in connection with green oak and some other difficult woods, while temperatures of from one hundred and sixty degrees to one hun dred and eighty degrees Fahrenheit are employed with pine and cedar.
The temperature of the entire charge must be raised to a point at which the drying is to take place. The surface of wood heated in warm, dry air is liable to shrink before the heat has penetrated and acted upon the moisture moisture that is within. Wet heat or steam adds to the moisture but assists because it keeps the surface soft and swollen until the heat has penetrated to the interior.
Moisture. Natural moisture or sap must be distinguished from moisture that may be absorbed after the tree has been eat down. Most of the natural moisture is in the outer sapwood and this moisture is often retained, or even added to, with advantage, so that the outer wood will not shrink before the heat has penetrated to the moisture further in. This is particularly necessary in the case of oaks and other woods char acterized by complex cellular arrangements.
Ability to season woods successfully in kilns depends upon ability to manipulate the moisture. Heat, circulation, and the kilns themselves are designed or directed with this end in view. Some processes include the addition of steam while others use only the moisture that has been evaporated from the wood. In other processes the moisture from the wood is removed by condensation upon the surfaces of pipes filled with cold water. Moisture is sometimes introduced by piling snow upon the limber as it enters the "greenwood ends" of the kilns. Pieces must be piled so as to facilitate the escape of the moisture.
Circulation. The air within a kiln does not remain motionless. On the contrary, it moves naturally because of the heat, or else the move ment is induced by means of fans, and, in both cases, drying may be hastened or retarded by hastening or retarding the circulation of the air currents. Air currents may pass in at the bottom and out through the sides of the kilns, or they may pass through the kilns from one end to the other.
The principal features of all kilns are (1) the drying chambers in which the wood is stored,' (2) the fur naces for heating air or making steam, and (3) the devices for causing the air to circulate within the drying chambers.
There are kilns within which charges remain stationary, and others within which charges move through from end to end.
When the wood remains stationary, in what is known as a "charge kiln" or "apartment-kiln," the moisture in the air is removed little by little, and the mass is finally exposed to a current of warm, dry air. When the charge moves forward, in what is known as a "progressive-kiln," it advances through an air current that contains most moisture near the entrance or "green wood end" and least moisture at the other extremity where the wood emerges.
Kilns are also grouped according to the origin of the draft. which may he natural, if caused by pipes or radiators placed beneath the drying chambers; or it may be forced if the draft. heated outside the kiln, is forced in by a fan. As stated already, the air currents may pass in at the bottom and out through the sides of the kilns, or they may pass through from end to end.
Kilns designed for natural draft are often known as "moist air kilns," and those designed for forced draft as "hot-blast kilns," but these names refer to details of operation, rather than to methods of construction, since moist air may be used in kilns of either kind, regardless of the source of the draft.
Natural-draft kilns and forced-draft kilns are also sometimes referred to as "radiator-kilns" and "blower-kilns" respectively.
A scheme for a radiator or moist-air kiln designed for progressive operation is shown in the picture. The pieces that are to be seasoned are arranged upon trucks, which are then rolled into the kiln until it is full. After a sufficient time in the kiln, one truck with its charge is removed from the "dry-wood end," the others are moved forward, and a new charge is admitted at the "greenwood end" of the kiln. The ventilator shaft at the right is often dispensed with. The general direc tion followed by the air currents is shown by the arrows.
A scheme for a blower-kiln is also shown. The humidity is main tained by using the same air repeatedly. The saturated air drawn from the "greenwood end" at the left of the picture, deposits part of the moisture upon the cold water coils located at the left of the fan. The air then passes on through the fan and is re-heated by the steam pipes toward the right. The air is relatively dry and warm as it re-enters the dry end of the kiln.' Operation. The several parts of the process employed within a kiln of any kind may be grouped as they relate (1) to prepara tion and (2) to drying. First, the temperature of the charge must be raised to the point at which the drying is to take place, while the surfaces of the pieces that make up the charge remain or are rendered soft and permeable. Second, the drying is forced by means of the draft, but at such a rate that the moisture from within the pieces can move out fast enough to replace the moisture that escapes from their surfaces.
Preparation. The charge or apartment kiln both prepares and then dries the wood before it is removed. Once within this kiln the wood is not removed until the process is completed. In the progressive system the wood is sometimes, but not always, prepared before it is admitted to the drying chamber. The auxiliary kiln, that is here sometimes used, should he placed as near as possible to the greenwood end of the prin cipal kiln so that the charge will not be unduly chilled during transfer.
Drying. The draft must be held back until the heat has penetrated within the pieces. Even then, it must not be forced unduly, or the surface moisture will escape too rapidly, and cause the surface wood to shrink before that inside has had time to dry. Case-hardening, honey combing, checking, warping, or twisting may then result.
Difficulties.In kilns, where drying is hastened, the difficulties that have been mentioned in connection with slower methods of seasoning become more pronounced. The situation has been expressed as follows:' "In drying chemicals or fabrics, all that is required is to provide heat enough to vaporize the moisture and circulation enough to carry off the vapor, and the quickest and most convenient means to these ends may be used. In drying wood, whether in the form of standard stock or finished product, the application of the requisite heat and circulation must be carefully regulated throughout the entire process, or warping and checking are almost certain to result. Moreover, wood of different shapes and thicknesses is very often differently affected by the same treatment. Finally, the tissues composing the wood, which vary in form and physical properties and which cross each other in regular directions, exert their own peculiar influence upon its behavior during drying. With our native woods, for instance, summer wood and spring wood show distinct tendencies in drying, and the same is true in less degree of heartwood as contrasted with sapwood. Or, again, pro nounced medullary rays further complicate the drying problem. Plain oak and quartered oak require different treatment. Even in mahogany and similar tropical woods, which are outwardly more homogeneous, various kinds of tissues are differentiated." The presence of knots, windshakes, frostshakes, and other defects add to the problem, which is to dry without distortion rather than simply to dry. The comparatively simple cellular structure of coniferous woods makes it easier to dry the woods of that series. The broadleaf woods as a group are more difficult, and some of them, such as the oaks, are particularly hard to dry.
Time Required. The time required to kiln-season lumber depends upon the sizes, shapes, and species of the individual pieces. Some operators dry one-inch white oak planks in four or five days, while others require one or two weeks for the same woods, and still others need twice as long. Plain oak and mahogany dry with about the same speed; these woods require less time than quartered oak, and longer than ash, birch, and basswood.