the other by shed piling so that the sides of the shed serve as end shields, and where the piles abut each other the ends are so close together as to protect them from air movement to a considerable degree, though the stickering permits free movement of air sidewise through the piles. Figure 39 shows another method of bulk ricking of short squares with the various tiers tied together by end-lapping. The short lumber in Figure 40 is probably for box purposes rather than dimension, but the ingenious method of piling it without stickers and yet with considerable ventilation is worthy of illustration.

FIGURE 38.-Squares shed piled with ends butted close to reduce end checking. Photograph courtesy of Forest Products Laboratory

Figure 41 is a general view of a storage yard for stave bolts, and figure 42 shows a closer view of one of the square cribs or ricks, from which it will be seen that the smallest corner lap is given that will be secure, so that much of the width of each bolt overhangs or projects.

Technical Note B-10 of the Forest Products Laboratory offers a suggestion for the piling of squares to reduce surface checking, advantage of which should be taken on all species liable to develop this defect. It occurs chiefly on the flatsawed surfaces; but if these are turned toward each other and crowded together in the layers, they will dry out more slowly and check less. Figure 43 has been drawn to show the idea.

THE KILN-DRYING OF LUM-
BER AT THE SAWMILL

The average loss per thousand feet on air-seasoned lumber because of degrade in drying can be cut down at least two-thirds by improved methods. The degrade loss on lumber kiln-dried by the manufacturer is somewhat less per thousand feet than that which occurs in air-drying, but a larger proportion of it is preventable loss. Good kiln-drying causes less lumber degrade than the best air-drying prevailing weather will permit. Commercial kiln-drying in connection with lumber manufacture usually is not technically good, and it errs chiefly on the side of using too little humidity in proportion to the temperatures and sometimes in drying the stock to an unnecessarily low moisture content.

Sawinill kiln-drying is almost entirely in the softwood field; hardwoods are yard dried at nearly all sawmill plants; in most cases where hardwood manufacturers operate dry kilns there is a flooring factory or other secondary industry operated in connection with the sawmill, and the lumber is kiln-dried chiefly in preparation for this secondary operation, which may convert it into flooring, or hardwood interior finish, or even into furniture or other manufactured product. Even such kilns work largely on lumber that has been first air-dried.

The greater portion of our production of hardwood lumber is sold to factories that have their own dry kilns and kiln-dry their product to suit their own needs; and they buy this product in air-dry condition, largely at 20 to 25 per cent

moisture content, though much of the movement of some species is at higher moisture percentages. A much smaller proportion of softwood production goes into industrial uses; and while there are shop or cutting-up grades in softwoods as well as hardwoods, much of the softwood lumber goes into the yard grades used for building lumber. If the product is an item usually kiln-dried for the building trade, the factories will get it in kiln-dried form, as it is diverted from the same common source of supply. There are large industries like the box industry which use quantities of softwoods without redrying, and most of it in air-dry condition.

FIGURE 39.-Dimension in large stacks for air-drying. Note method of lapping the ends. Photograph courtesy of Forest Products Laboratory

There are two main reasons why the kiln-drying of softwoods by the manufacturer is on a somewhat lower technical plane than the kiln-drying of hardwoods, which is done chiefly by the factory user. The first reason is that softwoods are generally easier to kiln-dry than hardwoods. Their soft flexible texture does not develop so heavy unequal shrinkage stresses as the harder and less yielding species and tend to readjust and absorb such stresses without the amount of checking and deformity that would occur in hardwoods under like conditions.

The second reason is that the primitive type of dry-kiln equipment built some years ago still is used at many softwood sawmills. These kilns are operated with little control of temperature and practically no control of humidity, and many of them have a sluggish air circulation, which slows down further if any attempt is made to keep

Photo

Figures computed for 383 kiln units indicated the following age averages of kilns for the various species:

The oldest kilns reported were built 25 years ago, and there are 16 of them in the California pine region and 18 in southern pine. Of those 22 years old, all 6 are in southern pine; of those 20 years old, 15 are in the redwood region and 6 in southern pine. These are, of

FIGURE 43.-Squares, surface-check most on the flat-sawed surfaces, and this sketch shows method of piling with such surfaces faced close together to retard surface drying and reduce checking. This can not be carried out with the pieces on the outside of the layer nor with those used as crossers; poorer pieces should be used here, or those in which the annual rings are as near cornerwise of the piece as possible

course, not conclusive as to actual complete averages for all kilns in all species.

Factory kiln buildings are almost entirely of fireproof construction; there are some sawmill kilns of that type, but some of them have wooden walls and roof framing. The chief added fire danger is the exposure hazard; wooden kilns burn with the rest of the plant, while brick or concrete kilns may come through undamaged. Interior kiln fires rarely occur, unless the kilns are being operated as bake ovens instead of under proper conditions of interior humidity; in such cases the heat of steam pipes may char woodwork near by until it is in a fair condition for spontaneous combustion to occur; at least some kiln fires have been assigned to such a cause. Ceiling sprinkler installations are sometimes made in kilns, but flat piling of lumber