tilation. Sawdust should not be scattered in the yard, and if this has been done it should be covered with soil, cinders, or other material which is not susceptible to decay.

Foundations.-Foundations for piles, tramways, etc., which are in direct contact with the soil should be of durable wood, treated with a preservative or replaced by concrete, brick, or some other material not subject to decay. In sections where the humidity is high, stock should be piled 18 to 24 inches from the ground to facilitate air circulation beneath and in the interior of the piles.

Piling and stickers.-Stock should be loosely piled to facilitate circulation and rapid drying. When a pile is torn down, the stickers should not be placed on the ground but should be piled on sound foundations and kept as dry as possible. Stickers should be well seasoned and are most satisfactory when made from the heartwood of the various species which are resistant to decay, such as oak, chestnut, and pines saturated with resin.

KILN-DRYING

Probably no phase of the lumber industry has received as much attention or has been characterized by such improved practices as has the process of kiln-drying. Thirty years ago little was known regarding the theory of kiln-drying. The value of humidity in the dry kiln was not understood. Lumber was dried in buildings which were "hot boxes" rather than kilns, since they were equipped only with heating coils and with chimneys for the removal of moisture. Nothing was known of the correct proportions of the factors of heat, humidity, and circulation.

Through the efforts of the United States Forest Products Laboratory, other research institutions, and dry-kiln manufacturers great improvements have been made in kiln construction, equipment, and operation during the last two decades. Kiln operation has developed from a hit-or-miss proposition to a scientific process in which the variable factors of heat, humidity, and circulation may be accurately controlled. Drying schedules have been developed for various thicknesses of all of our commercially important lumber species, and all of these species have been successfully kiln-dried green from the saw. Although it is not yet common practice to kiln dry all hardwoods green from the saw, it is expected that this practice will be established in the near future. Numerous instrument manufacturers have specialized in kiln-drying equipment and have developed some very efficient recorders and controllers for the indication and regulation of temperatures and humidities. An automatic drying controller which regulates kiln conditions according to the changing weight of a sample board is now being used.

Practical kiln operating, from being an odd job around a sawmill, has become a technical position which requires a man of intelligence, training, and practical experience. Exact tests have been developed for moisture content and distribution, casehardening, temperature, humidity, and rate and direction of circulation. Instruments based on electrical resistance and on the sensitiveness of membranes to the presence of moisture have been developed for measuring moisture content directly.

PILING

The methods of piling small dimension stock for kiln-drying depend on the size, species, and moisture content of the material being dried. For example, short stock which is easily dried is usually supported with a sticker at each end of the piece, while if the same stock is rather refractory it may be held in place with three stickers. Green stock should be piled more openly and with better sticker support than material which has been air seasoned. As in piling rough lumber, the size of the stickers should be in proportion to the size of the stock.

Figure 27 illustrates a method of piling small dimension stock. It will be noted that the stickers are carefully aligned and that three flues are built in the stack to furnish adequate circulation. In many cases only one central flue is used. In kilns having cross circulation the material may be piled without a central flue.

STICKERS

The width of the stickers should be one and a half times the thickness to prevent rolling while the stock is being piled. Stickers may be made from nearly any species of wood. Yellow poplar, southern yellow pine, the oaks, gums, and basswood are all commonly used in the southern and Appalachian regions. In any case stickers should be thoroughly air dried or kiln dried before being used.

Sticker spacing and alignment are two important items in piling. Warping, twisting and cupping are often caused by the careless placing of stickers. When piling thin stock-1/2 to 3/4 inch thick— or green stock up to 6/4 inch thick, it is advisable to space the stickers not farther than 8 to 12 inches apart. In stock 1 inch thick and thicker, which has been well air seasoned before kiln drying, a maximum spacing of 24 inches is allowable. The correct spacing of stickers varies with the different species of wood. Woods which have a greater tendency to warp should be stacked with closer sticker spacing.

Stickers should be placed flush with the ends of the stock. Because of uneven trimming this may be impossible in all cases, but stickers may be placed flush at one end and pulled in at the other to allow for variations in trimming.

Stickers should be aligned one above another, so that the load of dimension is supported at a number of points equal to the number of vertical rows of stickers rather than at different points in different parts of the pile. When it is desired to have the weight distributed directly over the kiln trucks in crosswise piling or over the cross supports in endwise piling, two lines of stickers may be started close together over the point of support and gradually separated until the desired spacing is obtained,

FLUES

In piling hardwood dimension for kilns having natural circulation it is common practice to leave a flue or chimney in the center of the pile. The width of these chimneys varies from 8 to 16 inches at the bottom of the stack and from 0 to 4 inches at the top. The purpose of the flue is to increase the rate of circulation through the

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FIGURE 27.-Piling small dimension stock for kiln drying. A shows the side and B the end view of a kiln stack of hickory-spoke billets. Note the careful placing of stickers, the presence of flues to permit good circulation, and the fact that the pile is "tied" with a course of lumber running horizontally through the center of the stack as well as with a vertical board at each end of the pile

interior of the pile and, by gradually closing the flue, to force the warm air to the right and left throughout the pile.

DRYING REFRACTORY STOCK

In drying refractory stock special care must be exercised in piling and in treatment in the kiln. In piling oak furniture squares the plain-sawed surfaces may be brought close together in each layer of the stack and the quartered surfaces placed toward the top and bottom of the pile, as explained on page 50. Refractory stock should be given in a mild treatment and should be conditioned frequently.

THE USE OF RACKS

In piling small items of dimension stock for kiln drying the handling cost is usually prohibitive. For this reason stock shorter than 12 inches which is not of the highest grade is sometimes dried in racks. These racks are built to conform to the size of the trucks and pile bottoms being used for the stacking of lumber or larger sizes of ready-cut stock. A flue is usually built into the rack. The material to be dried is thrown into these racks by hand and therefore is loosely arranged to the extent that fairly good circulation is obtained. However, this method of drying may produce stock that is unevenly seasoned. Very small items of dimension stock may be placed in bags and dried in a rack similar to the one described above.

KILN-DRYING SCHEDULES

In

The dry-kiln schedules that have been evolved may be used successfully in drying all thicknesses of our commercial species of lumber under various conditions of original moisture content. It is often assumed that these schedules can be successfully applied to drying small dimension stock of the same species and thickness. some cases this assumption may be correct, but the extent to which these lumber schedules can be applied to small dimension stock_still remains to be demonstrated. The United States Forest Products Laboratory has done considerable work on the development of drykiln schedules for small dimension stock and gives in the United States Department of Agriculture Bulletin No. 1136, Kiln Drying Handbook, two schedules for drying the following sizes of oak artillery wheel spoke blanks: 234 by 234 by 26 inches and 334 by 33⁄4 by 26 inches. Schedules for artillery wheel rims of bent oak for sizes 32 by 312 by 56 inches and 334 by 334 by 60 inches are also given. The laboratory has also prepared a schedule for the drying of black walnut gun stocks and recommends a schedule for kiln drying maple shoe-last blocks. The laboratory expects to release specific information concerning small dimension stock dry-kiln schedules within the next two years.

With the exception of the references made in the above paragraph there is no information available on the subject of drying schedules for ready-cut stock and no recommendations can be made. In the absence of this information, it has been common practice for kiln operators to use schedules given for the drying of rough lumber of the same species and thickness, and to increase or decrease the severity of these schedules depending on the results obtained.

KILN TYPES

All dry kilns may be divided into two types, namely, compartment and progressive. Each type has advantages and disadvantages under different conditions and in drying different classes of material.

Progressive kilns.-In progressive kilns the green stock enters at one end, moves gradually forward, and is presumable dry when it reaches the discharge end of the kiln. The conditions of temperature and humidity remain approximately uniform at any given point and vary in severity from the charge to the discharge ends.

The progressive kiln is most adaptable to conditions in which large quantities of stock of the same species, thickness, and moisture content are being consumed regularly; for example, the kiln drying of parquetry flooring dimension stock. A continuous supply of stock of approximately uniform moisture content for kiln drying is prerequisite for efficient progressive kiln operation. Progressive kilns must have a minimum length of 100 feet for efficient operation. Greater lengths are recommended; most kilns of this type vary in length from 120 to 150 feet.

Most progressive kilns are of the natural circulation type in which the outside air enters the kiln below the heating coils at the discharge end, is heated and moves upward and toward the charge end of the kiln. In some types of progressive kilns the rate of circulation is increased or controlled by means of fans, steam sprays, or steam jet blowers.

The temperature of these kilns is approximately controlled by a concentration of heating coils at the discharge end of the kiln. The concentration decreases toward the charge end of the kiln; for example, three banks of coils may extend 30 feet, two banks 60 feet, and one bank 90 feet from the discharge end of the kiln. Each set of coils may be controlled as a separate unit. Steam sprays are commonly at three or more points; that is, one set near the charge end, one near the discharge end, and one near the center of the kiln. In operation the two sets of sprays last mentioned are ordinarily used only for conditioning treatments, while the sprays located near the charge end are used to control the normal operating kiln humidity. Obviously, exact control is impossible in a kiln of this type. Temperature can be controlled only at the discharge end of the kiln, the temperatures at other points varying with the length of kiln, rate of circulation, outside temperature, condition of stock, etc. Humidity control may be more exact through the use of sprays located at various points in the kiln, but it also varies with the conditions mentioned above.

Compartment kilns.-Compartment kilns are designed to operate on a definite drying schedule by changing kiln conditions from time to time rather than by moving the lumber into different conditions of temperature and humidity. The kiln is completely charged with lumber of any moisture content; drying begins and continues until the entire charge is dry and is removed in one operation.

Compartment kilns are usually much smaller than kilns of the progressive type and are best adapted to conditions in which a variety of sizes and species must be dried at the same time, for example, custom drying. Since compartment kilns are built in various sizes