The knots liable to come loose are mostly incased knots. That means that they are the stubs of limbs that have remained on the tree, around which the wood has grown without having any direct connection with the wood of the knot. The knot goes through the board at a slight longitudinal angle, and the end grain of its wood is exposed on both faces of the board; in seasoning the knot dries out faster than the rest of the board because of this exposed end grain and thus shrinks earlier. Because of the dense wood it probably also shrinks to a greater degree than the surrounding wood, and this difference increases as the average moisture of the board is lowered; therefore more knots will loosen in lumber that has been dried to 12 per cent moisture than in that dried to 15 or 18 per cent, and this fact gives practically the only key to control of loosening

FIGURE 29.-End of pile of 4-inch oak unprotected from sun or wind, showing severe end checking. Photograph courtesy of Forest Products Laboratory

of knots-to avoid drying the lumber more than is necessary for the purpose. If common lumber is usually air-dried to 15 per cent, and drying it to only 18 per cent will materially reduce knot degrades, then the higher moisture content will be the more economical to use-not forgetting that, if later the board self-seasons further, the knots may then loosen. As loosening of knots largely occurs under the blows from planer knives, knots which remain tight when going through the planer at a moisture around 15 per cent may remain tight thereafter when the lumber seasons further.

It should also be taken into account that the loosening of knots because of the kiln-drying can be largely controlled by carrying a higher humidity in the latter part of the operation. This can be regulated so as to prevent the knots going much below the average moisture content of the rest of the board, even though it slows down the drying.

In the species in which knots are liable to loosen the damage occurs to much the same extent at a low moisture content regardless of whether seasoning was by air-drying or kiln-drying; and the only practical expedient as far as air-drying is concerned seems to be to avoid overseasoning. Slow sales often leave common lumber in pile for storage after it had reached a desirable stage of dryness, and whatever additional thorough seasoning it gets can not well be avoided. WARPING AND CUPPING

Warping is a tendency which varies greatly according to species. Practically all commercial softwoods and some hardwoods will dry straight and true, except per

haps an occasional piece with cross or twisted grain; some very valuable species require frequent sticker courses to hold them straight, though most of them, if seasoned straight, will hold their shape thereafter. When warp develops in the easier species it is usually because of glaring faults in piling.

There are three kinds of warp: Cup or warp crosswise, bow or warp flatwise, and crook or warp edgewise. A gradual sweep of either is negligible, because the wood is flexible enough to be straightened and fastened in place; a short kink is more serious and is often caused by crooked sticker placement. If a sticker is not firmly supported on the sticker below, it will, of course, sag or kink in the unsupported board on which it rests. A crooked sticker may

ends and yet be entirely to one side of the sticker below,

the crooked course of stickers has produced warping in the lumber. Photograph courtesy of Forest Products Laboratory

in the middle of the width of the pile. (Fig. 30.),

Local warp often occurs in the ends of lumber projecting too far beyond sticker support, especially if they also project from the ends of the pile; such projecting ends are also liable to excessive surface and end checking. The warp may take the form of either cup or twist; they are also subject to casehardening from their exposed location. If lumber piles are allowed to sag through lack of sufficient crosser support this sag reflects itself in a permanent gradual bow in the affected boards; this may not be a serious defect, but it makes extra work in handling and working the stock, even if it does not result in a certain amount of degrade.

PERIOD REQUIRED FOR ADEQUATE AIR-DRYING OF LUMBER

Lumber in pile loses weight slowly during the winter months, even in the South; there are regional differences in this respect which mostly depend upon the prevailing air humidity during these months. Lumber which does not complete its seasoning in the autumn usually goes over until March or April before reaching the desired condition. In the study of air seasoning of lumber in the Inland Empire there are various tables of drying rates for inch lumber of western white pine and western yellow pine (Pondosa pine) with different factors of foundation height, roof height, spacing, stickering, etc. These tables show that western white pine piled in March reached 15 per cent moisture content in 10 to 13 weeks; piled in May, in 8 to 12 weeks; piled in July, in 2 to 5 weeks; piled in August, in 6 to 32 weeks; piled in September, in 27 to 36 weeks; piled in October, in 11 to 26 weeks.

The survey of Air-Seasoning Practices in the Northern Hemlock and Hardwood Region says on this point (p. 12):

During the best drying season, through the spring and summer, 2-inch hemlock self-stuck is considered in shipping dry condition by some mills in 40 to 60 days, while at other mills it is piled till late fall, a period of 6 to 8 months. It is stated that even after reaching the shipping dry condition it is still wet at the center. The reason for cutting hemlock 2 inches thick is to avoid the dropping out of knots.

One-inch maple and birch are said to reach shipping dry weight, if dried during the spring and summer, in 60 to 90 days in some yards and 5 to 8 months in other yards. Ninety days is probably the average time for these species. Basswood is said to require about a month less.

Little drying, if any, occurs during the winter months, and stock piled in the fall must hold over until the spring months before it is dry.

The study of Air Seasoning of Lumber in the Douglas Fir Region (reprinted from the Timberman, p. 18 of reprint) gives a table of the estimated time required to season Douglas fir 1 and 2 inch lumber to different percentages when piled in different months, from which the following figures, showing the time required to air-dry to 14 to 16 per cent moisture content, are taken:

In the study of air seasoning of oak and gum in the southern hardwood region pile No. 1 at mill No. 1, of inch red oak on 1-inch stickers. piled May 27, reached 15 per cent moisture content in the upper half of the pile on July 22, or 87 days; the bottom half of the pile did not reach that stage until September 2, or 129 days. Pile No. 1 at mill No. 4, inch sap gum piled on stickers 2 inches thick, was started June 28 and reached 15 per cent moisture content by September 9,

73 days; pile No. 2 on 1-inch stickers reached only 28 per cent moisture content during the same period. Pile No. 2 at mill No. 3, of inch plain sap gum piled with 1-inch stickers on June 17, had reached 17 per cent moisture content by September 23, or 98 days, and had gone no lower at the end of the record on October 10, probably because of the lateness of the season. During the same period pile No. 3 on CHART 4

GRAPHIC AIR SEASONING CHART FOR DOUGLAS FIR REGION

PART FACTOR

JAN FEB MAR APR. MAY JUNE JULY AUG. SEPT OCT NOV DEG COMPARATIVE MONTHLY WEATHER CONDITIONS

DEGREES 50

TEMPERATURE

ACTIVE DRYING SEASON,

AVERAGE DRYING PERIODS FOR STOCK PILED IN DIFFERENT MONTHS AVERAGE NO. DAYS

TO REACH 4 100 80 55 35 25 20 20 15 220 190 160 130 20% MC

94

110 90 65 45 35 30 30 25 230 200 170 140

TO REACH 44 145 120 100 70 45 40 40 45 270 240 210 180

15% M.C.

4 165 140 115 90 60 55 55 320 290 260 230 200 APPROXIMATE MOISTURE CONTENT OF THOROUGHLY AIR DRY 44 STOCK BY MONTHS % MOISTURE CONTENT 26 24 22 18 16 15 12 13 15 16 22 26 AVERAGE PICK-UP OF MOISTURE BY YARD STOCK AT DIFFERENT SEASONS

NOTE: THIS CHART IS PRESENTED TO INDICATE GENERAL REGIONAL
CONDITIONS AND CONSEQUENTLY IS NOT APPLICABLE FOR SPECIFIC PLANT USE.
WEATHER DATA IS BASED ON AVERAGE RECORDS FOR REPRESENTATIVE
POINTS IN THE REGION.

M10705

DRYING PERIODS DUE TO SEASONAL AND LOCAL DIFFERENCES IN CLIMATIC
CONDITIONS MAY VARY 10 TO 20 DAYS.

FIGURE 31.-Graphic chart showing air-drying rate by months on Douglas fir. Photograph courtesy of Forest Products Laboratory

2-inch stickers reached 17 per cent moisture content on September 9 but lost little moisture after that, probably because it was nearly in balance with prevailing air humidity.

Among the photographs received through the courtesy of the Forest Products Laboratory are four air-seasoning charts, one of which (for Douglas fir) is reproduced in Figure 31. From the other

three charts the following table has been prepared, showing for the three species the number of days required to air-dry inch stock to 15 per cent moisture content, starting with each month of the year, and also the approximate moisture content of thoroughly airdry stock corresponding to each month:

All these charts carry stipulations like those at the bottom of the one reproduced, to the general effect that they show regional conditions in a general way which will be found to vary at individual plants. All the charts show curves similar to the chart reproduced with a humidity drop at the summer-temperature peak, except that for the redwood region, where the highest relative humidity occurs during the hottest summer period.

The above examples are adequate indication that the air seasoning of lumber is largely a seasonal process and does not fit in very well with all-the-year operation of a sawmill, if depended on for the main seasoning process. This is another reason for the increasing popularity of kiln-drying, which can be operated efficiently without regard to seasonal conditions.

SEASONING OF MISCELLANEOUS SAWMILL PRODUCTS

Among the miscellaneous products of a sawmill, in addition to lumber, are timbers, railroad ties, dimension stock, shingles, and lath, though none of these are products of every mill.

SEASONING OF TIMBERS

By timbers, under standard nomenclature, are meant stock 6 inches. and over in its least dimension; and sawmill product of this class can not be said to have any standard of sawmill seasoning, being shipped in any condition up to green from the saw, and a relatively small proportion of it in fairly air-dried condition. To secure an air-dry condition in stock of this size would take a year or more in most woods, and even this would be a superficial dryness in large

timbers.