DEPARTMENT BULLETIN No. 1425

Washington, D. C.

October, 1928

THE AIR SEASONING OF WESTERN SOFTWOOD LUMBER

By B. V. FULLAWAY, Jr., Formerly in Charge Office of Forest Products, Missoula, Mont., HERMAN M. JOHNSON, Assistant in Forest Products, Portland, Oreg., and C. L. HILL, in Charge Office of Forest Products, San Francisco, Calif., Forest Service

A real need exists in the lumber industry for a concrete presentation of efficient air-seasoning methods for western softwood lumber, including a review of the general principles that govern the drying of wood and their application to yard drying. Air-seasoning losses in the West, both in quantity and quality of the product, can be reduced. The average drying time can be shortened, with a consequent faster overturn of stocks. Shipping weights can be lowered and final moisture-content requirements more nearly attained. Moreover, highly competitive conditions in industry make essential all practicable improvement and economy in lumber manufacture, as well as a product of the highest possible utility and one that meets consumer requirements satisfactorily.

The import of such considerations to the lumber industry of the West is evident in view of the fact that 35 per cent of the total annual cut in its four 1 principal producing regions is air seasoned, or approximately 4,500,000,000 board feet, at a rough valuation of $120,000,000.

Douglas fir region of western Oregon and western Washington, "Inland Empire," California pine region, and redwood region.

1

TABLE 1.-Average annual production of western softwood lumber by regions1

1 Figures (rounded) based on five-year production records; Douglas fir region includes western Oregon and Washington; Inland Empire pine region includes Montana, Idaho, eastern Oregon and eastern Washington; California pine region includes California, Nevada, and Clamath Falls district of Oregon; Califor nia redwood region includes redwood belt in California.

2 Includes the following species: White fir (Abies concolor), lowland white fir (A. grandis), silver fir (4. amabilis), red fir (A. magnifica), alpine fir (A. lasiocarpa), and noble fir (A. nobilis).

3 West Coast hemlock.

4 California sugar pine.

Idaho white pine.

Pondosa pine, California white pine.

The air seasoning of lumber is, however a complex problem if anything like real efficiency is to be obtained. In working out the proper solution five distinct objectives must be kept constantly in mind:

1. Minimum depreciation of stock.

2. Rapid rate of drying.

3. Low, uniform final moisture content.2

4. Economy in operating cost.

5. Reasonable yard space.

Other complications of the problem must also be recognized. The various species, grades, and sizes of stock require individual consideration. Owing to climatic and other differences, the answer for one yard will not always hold for another. The effects of seasonal weather variation must also be provided against by each yard separately.

Maximum attainment of any one of the five principal objectives may often preclude the full realization of the others. Since actual efficiency in air seasoning must in the final analysis be measured by the profit-and-loss yardstick, it is necessary that these five objectives be adequately balanced to the best advantage of the producer.

The fact that wood shrinks and swells with changes in moisture content makes it highly desirable that seasoning result in a final moisture content suitable for the conditions of final use, but absolute attainment in this direction is hardly possible on account of the varied purposes for which wood is employed and the wide range of atmospheric conditions under which it is used. This can be illustrated in a concrete way. Wood thoroughly air-dried has a moisture content at Galveston, Tex., of about 17.5 per cent and at Phoenix, Ariz., of 7.5 per cent. In the general Middle West territory wood, to give the most satisfactory results, should have a moisture content of 6 to 8 per cent for interior work and 12 to 15 per cent for outside use. These differences indicate forcibly that the final moisture-content problem is a difficult one, but they also emphasize its importance to the operator.

A great variety of air-seasoning methods have come into use during the long period of development in the lumber industry. Observations of much value have been accumulated by those of experience in the industry; and yet to-day, after many years of air-seasoning practice, a wide difference of opinion exists among operators as to the relative value of many of the basic principles involved. There is urgent need for authentic and usable information based on careful studies.

It is obvious that the solution of the air-seasoning problem can not be found in any set of "cut-and-dried" rules. The chief aim of this bulletin is to present those general principles which can be applied by the lumberman in the manner that will best meet his own. specific conditions and problems. No attempt is made to present the detailed data in substantiation of the conclusions given. The conclusions are based on surveys and study of current practice and on intensive air-seasoning investigations by the Forest Service within the four western lumber-producing regions.3

IMPORTANT PRINCIPLES OF WOOD DRYING AND THEIR GENERAL APPLICATION

OCCURRENCE OF MOISTURE IN WOOD

Moisture in wood, or sap, is chiefly water with small percentages of organic and mineral matter present in soluble form. In the sapwood these materials are largely sugars, but in the heartwood they are principally tannins, resins, and dyestuffs. For all practical purposes in the drying of wood sap can be considered as water, since only very small quantities of the other materials pass off in evaporation.*

3 Acknowledgment is made by the authors to other members of the Forest Service, particularly those of the Forest Products Laboratory, who have contributed in a large measure to present knowledge relating to the principles of drying wood. Acknowledgment is also made to C. Burdette Green, formerly a member of the Forest Service, for his very considerable contributions of air-seasoning data from California.

The amount of moisture in wood, or the moisture content, is expressed in terms of percentage of the oven-dry weight of the wood. Thus, if the moisture content of a green board is 71 per cent, there are by weight 71 parts of water to 100 parts of oven-dry wood. Similarly, should the moisture content of a board happen to be exactly 100 per cent, the weight of the moisture and that of the oven-dry wood would be equal. The average moisture content of a lot of lumber may be determined in the following manner: Select representative pieces-about 1 out of every 100 to 500 pieces-with a fair representation of both heartwood and sapwood.

At a point about 2 feet from one end of each piece, cut out a section three-fourths to 1 inch wide, making the cut at a place free from knots, rot, pitch streaks, or other defects. Trim off all slivers from this section or sample.

Weigh the samples immediately and carefully on a delicate balance. This is the original weight.

Place samples in an oven heated to 212° F., or, if an oven is not available, on hot steam pipes; but do not scorch or bake them.

When samples have reached a constant weight, as can be determined by repeated weighing, remove them from the oven. (After a little experience the time required to reach constant weight can be estimated, and thus repeated weighings may be avoided. Twentyfour hours should be the maximum time necessary with softwoods.) This final weight is the oven-dry weight.

Subtract the oven-dry weight from the original weight. The difference is the loss in moisture. Divide the difference by the oven-dry weight and multiply by 100. This gives the percentage of moisture contained in the wood based on the oven-dry weight.

Original weight=284.7 grams.

EXAMPLE

Oven-dry weight=180.2 grams.

284.7 grams-180.2 grams 104.5 grams, or the moisture lost.

104.5180.2.580 × 100=58.0 per cent moisture originally in the wood.

(For convenience and accuracy the gram is often used as the unit of measurement, but other units, such as the ounce, may be employed.)

Moisture is held in green or wet wood in two ways. It is contained within the otherwise practically empty cell cavities, and it is absorbed in the cell walls. The cell water is called "free" water; that in the cell walls may be termed "imbibed " water. Free water is found in the cell cavities only when the cell walls are fully saturated.

Shrinkage of wood takes place only with a loss of moisture and swelling with the absorption of moisture. But all loss of moisture is not accompanied by shrinkage. As wood dries, it first gives up its free water. After the cell cavities become empty, the moisture in the saturated cell walls is drawn off. Wood does not start to shrink until the cell walls begin to lose moisture.

The point at which the cell cavities are empty but the cell walls are still saturated is thus an important one in drying. It is known as the fiber-saturation point. The moisture content at this point varies from 20 to 35 per cent, but for most woods is between 25 and 30 per cent. In actual practice, of course, the cells near the surface fall below this point before those on the interior have reached it, and the outer wood tends to shrink before the inner. Such a state is often the cause of serious drying troubles.

Free water is present in both the heartwood and sapwood of most living trees but in greatly differing quantity. Sapwood usually contains more moisture than heartwood. Butt logs ordinarily have a higher moisture content than top logs. Contrary to common belief, the quantity of moisture in green wood has little seasonal variation. Species and locality of growth, however, have an important bearing upon it.

Variation of moisture content was very marked in the many determinations of green wood made in the air-seasoning investigations upon which this study is based. Differences between species were, of course, large, but in all species the select grades contained more moisture than the common grades, owing to the greater proportion of sapwood in the better class of stock. The moisture content of western white pine averaged about 84 per cent for selects and 75 per cent for common; that of sugar pine 190 and 75 per cent; that of white fir 200 and 90 per cent; and that of redwood 200 and 70 per cent. The moisture content of coast Douglas fir probably ranges from 53 to 32 per cent and that of western hemlock from 120 to 28 per cent. Variation resulting from locality of growth is well illustrated by the moisture content of western yellow pine. The moisture content of stock from California ranged from 185 to 100 per cent, whereas that in stock from the Inland Empire ranged from 115 to 80 per cent.

MOVEMENT OF MOISTURE IN WOOD

As already stated, wood upon drying loses first its free water and then that which is absorbed in the cell walls. The pores themselves have very little to do with drying or the movement of moisture in wood. The moisture does not flow out of the pores of wood to the surface, but comes to the surface only along the cell walls. Thus, because of the nature of wood structure, the end grain of wood loses moisture more rapidly than does the side grain, and flat or plain

sawed faces lose it more rapidly than do vertical-grain or quartersawed faces.

For an understanding of the air-seasoning process, this general description of the movement of moisture in wood is sufficient. It can be assumed that the moisture tends to distribute itself evenly through the wood, moving from the moist regions to the drier ones. The really important facts are that the temperature and humidity of the atmosphere at the surface of the wood are controlling factors, and that circulation of the air is of extreme importance in maintaining and modifying these.

EFFECT OF HUMIDITY ON DRYING

Wood possesses the property of giving off or taking on moisture from the surrounding atmosphere until the moisture in the wood comes to a balance with that in the air. The humidity or water vapor in the air is, therefore, very important in the drying of wood, and a general understanding of this relationship between humidity and moisture content of wood is essential.

The weight of the water vapor contained in a cubic foot of air is the absolute humidity and is usually expressed in number of grains. This does not, however, indicate the drying capacity of the air, for the ability of air to hold water, or its saturation point, varies greatly with the temperature, as is illustrated by Table 2. This ability of air to dry wood, or any other substance, varies according to the additional moisture it can hold before becoming saturated. The vapor in the air expressed as a percentage of the saturation point for the same temperature is called the "relative humidity" and indicates the comparative drying capacity of air. The lower humidities represent dry air and the higher ones moist air. As used in this bulletin, the term "humidity" alone refers invariably to relative humidity.

TABLE 2.-Cubic foot moisture capacity of air at different temperatures

Marked changes in relative humidity are evident from season to season, and also the usual daily fluctuations must be taken into account. Because of its tendency to come to definite balance with the surrounding air wood is, under ordinary atmospheric conditions, practically always undergoing at least slight changes in moisture content. This same tendency accounts for the differences in final moisture content of thoroughly air-dry wood at different times of the year. The pick-up in moisture content of lumber left piled in the yard over winter is likewise explained. Table 3 shows the ultimate moisture content of wood if kept under exact humidity and temperature conditions.