FRONTISPIECE.-Photomicrograph showing a hypha of the blue-stain fungus penetrating several tracheid walls in northern white cedar: 1. Radial section stained with safranin. 2. The extreme right-hand portion of 1 enlarged, showing the constricted hyphæ in the bore holes made in the tracheid walls. 3. Photomicrograph showing sporelike development of hyph of Lasiosphaeria pezizula in persimmon wood: radial section, stained. 4. Enlarged portion of the hypha shown in 1. Note the swelling of the hypha at entrance and emergence and the constriction where passage is secured through the cell wall. 5. Photomicrograph showing hypha of the blue-stain fungus passing through a bordered pit in the wood of loblolly pine. Tangential section stained with safranin and lichtgrün

SAP STAINS OF WOOD AND THEIR PREVENTION

INTRODUCTION

The staining and molding of lumber and other wood products have for many years been serious sources of loss to the lumber industry, which is one of the largest in the country (1926), employing 921,000 wage earners and producing products valued at approximately $4,000,000,000. Aside from their relation to the vital national question of maintaining an adequate timber supply, large losses of this nature can not be suffered by such an important industry without serious efforts being made to prevent them. Research offers the only means of obtaining the information necessary to the development of effective preventive methods, and the first step in this direction is a study of causes and the factors influencing them.

Most of the discoloration or stains which lower the quality and value of forest products are caused by certain minute organisms of the lower forms of plant life known as fungi and are very widespread. Another type of stain believed to be due to chemical reactions is not so common but nevertheless causes considerable loss due to degrade.

STAINS CHEMICALLY PRODUCED

Among the discolorations which apparently result from some chemical changes on the surface or within the wood are several brownish to yellowish stains in both pine and hardwood lumber. The exact nature of the changes is not known. Some of the stains appear in logs stored for a considerable period, while others develop in the lumber during kiln-drying or air-seasoning. The more important of these stains are:

STAINS DEFINED

Yard brown stain, which occurs as a yellow to dark-brown stain in air-seasoned white pine (Pinus strobus), sugar pine (P. Lambertiana), and western yellow (Pondosa) pine (P. ponderosa). In sugar pine the brown discoloration is often found just below the surface of the sawed boards and therefore is usually undetected until after planing.

Kiln brown stain is a yellow to dark-brown discoloration developing during the kiln-drying of sugar pine and western yellow pine. It is a serious factor in the grading of lumber and causes an annual loss that is very appreciable through lowering boards from the more valuable grades. The terms "scorch" and "kiln burn" have often been used to describe these two brown stains.

Kiln burn is a very dark browning, scorching, or actual charring of kiln-dried stock, caused by the use of too high temperatures.

On the next heating not so much heat can be stored in the wood, because of its lowered moisture content; and this effect is shown in the following tabulation of several successive moisture stages (Table 3), based upon heating to 131° F. (not assuming any addition of moisture to the wood in heating) and cooling to 50° F., or boiling point, at a vacuum of 98.8, the initial moisture percentage of the wood being 68 per cent.

TABLE 3.'-Tabulation of several successive moisture stages, based upon heating to 131° F., and cooling to 50° F., or boiling point, at a vacuum of 98.8

1 In this table it should be noted that allowance for considerable amounts of heat, which is impractical to estimate, may be required to produce a movement of moisture through the wood.

It is, of course, obvious that, since the heat in the wood does the work, to reheat the wood if steam were used would require condensation of practically the same quantity of steam that had been evaporated in the previous stage of evacuation; and as this condensation would occur on the wood, it would result in the restoring of a like amount of moisture. The heating of the wood must therefore be done by a mixture of steam and air with a humidity which will not result in changing the moisture of the wood in either direction, which means that the air will contribute most of the heat.

When the boiling point is reduced by the vacuum below the existing temperature of the wood the effect is to cause evaporation throughout the interior of the wood as well as on the surface; but if the steam can not pass off freely, a local pressure accumulates which raises the boiling point locally to the existing temperature of the wood, when local evaporation can no longer occur. If the wood is heated to 131°, this represents a boiling point corresponding to 5 inches of mercury, or about 211⁄2 pounds pressure per square inch. This pressure, of course, tends to drive the steam out of the wood and also to drive out the interior moisture, and thus hastens diffusion.

At the stage of vacuum used a pound of water will produce nearly 700 feet of vapor, which has to be removed by the pump in order to maintain the vacuum. This exhaustion of steam produced from all the surfaces of the wood amounts to quite an active circulation of the rarefied atmosphere inside the retort.

It is obvious that the use of higher temperatures in heating the wood must produce higher interior vapor pressures at the vacuum and also tend to greater softening of the wood; and the resulting stresses might produce checking. The foreign specifications, how

ever, state that checking is chiefly produced by failure to heat the wood through; and it is apparent that this would produce faster drying near the surface and the consequent unequal shrinkage, which usually cause checks. If, however, the hea ng is thoroughly done, this system would seem conducive to uniform drying, because the portions of the wood having highest moisture content could store the greatest number of heat units effective for evaporation in the next evacuation stage.

The temperature limit set by the Swedish specification, 131° F., seems very moderate in comparison with maximum dry-bulb temperatures of American kiln-drying schedules, ranging up to 180°. It seems very probable that the damage mentioned as having occurred in using higher temperatures in the vacuum process did not result directly from the temperature but from steam-pressure differences developing in the wood from premature application of a too high vacuum. If, for instance, the wood was heated to 212° and a vacuum of 98.8 per cent were then applied, equivalent to a boiling point of 50° F., the tendency of interior moisture to turn to steam would not be restricted until the pent steam had developed to atmospheric pressure of 15 pounds to the square inch; and this might have a disruptive effect on the softened fibers of the wood at such a high temperature. If, however, a vacuum of 20 inches of mercury were first applied, the maximum interior pressure difference that could develop would be 5 pounds per square inch, probably not enough to cause any checking of the wood; and as the temperature reduced the vacuum could gradually be increased until the maximum was reached. For most woods a schedule could probably be developed of heating to 180°, practically equivalent to boiling point at a vacuum of 15 inches of mercury, and then applying a vacuum of 10 inches, reducing next to 5 inches, and then to the final high vacuum of 98.8 per cent. Using this higher temperature would accomplish more evaporation for each reheating, and 8 stages would be equivalent to the 10 shown in the above table.

BIBLIOGRAPHY OF LUMBER SEASONING, HANDLING, AND CARE FOR THE LUMBER MANUFACTURER

Most of the literature here listed is written in clear language which the lay reader can readily understand, even when dealing with technical subjects, Those publications of the Forest Products Laboratory which are for free distribution are subject to the following provision announced by the laboratory: "The Forest Products Laboratory reserves the right to furnish only those publications, available for distribution, which in its judgment will furnish the information requested. Blanket requests or requests for a large number of copies of any individual article will not be filled except in unusual cases."

KILN-DRYING OF LUMBER

Of the books for sale listed below, the first two are the most important, and a study of each is desirable. If only one is chosen, the later book will probably be selected. All three authors are of the staff of the Forest Products Laboratory:

The Kiln-Drying of Lumber. By Harry Donald Tiemann. Third edition, revised, 1920. Price, $5. J. B. Lippincott Co., Philadelphia.

The Kiln-Drying of Lumber. By Arthur Koehler and Rolf Thelen. Price, $3. McGraw-Hill Book Co., New York City, 1926.

Seasoning of Wood. By Joseph B. Wagner. Price, $4. D. Van Nostrand Co., New York City, 1917.

The Seasoning and Preservation of Wood. By Ernest G. Blake. Price, $3.50. Chapman and Hall (Ltd.), London, England, 1924. D. Van Nostrand Co., New York City.

Practical Kiln-Drying. By E. U. Kettle. Price, $3. American Lumberman, Chicago, 1923.

Timbers of the World. By Alexander L. Howard, with an appendix on the artificial seasoning of timber, by S. Fitzgerald. Price, $9. London, 1920. The MacMillan Co., New York City.

The following Government bulletins are for sale by the Superintendent of Documents, Government Printing Office, Washington, D. C., at the prices listed: Kiln-Drying Handbook. By Rolf Thelen. Department of Agriculture Bulletin 1136, May 12, 1923, and now under revision and republication, and will express the latest thought on the subject. The old price of 25 cents may or may not be applied to the new edition.

Manual of Design and Installation of the Forest Service Water-Spray Dry Kiln. By L. V. Teesdale. Department of Agriculture Bulletin 894, October 18, 1920. Price, 10 cents. A good manual for this type of kiln.

The Theory of Drying and its Application to the New Humidity Regulated and Recirculating Dry Kiln. By Harry D. Tiemann. Department of Agriculture Bulletin 509, March, 1927. Price, 5 cents. Relates to the same type of kiln as the previous listing but goes farther into theoretical considerations. The Control of Stain, Decay, and Other Seasoning Defects in Red Gum. By L. V. Teesdale. Department of Agriculture Circular 421, July, 1927. Price, 10 cents. A valuable report on this wood.

The Seasoning of Wood. By H. S. Betts. letin 552, July 9, 1917. Price, 10 cents.

Department of Agriculture Bul

Effect of Kiln-Drying, Steaming, and Air Seasoning on Certain Fungi in Wood. Department of Agriculture Bulletin 1262, August, 1924. Price, 10 cents. Dry-Kiln Practice. By H. L. Henderson. New York State College of Forestry Bulletin 16. This bulletin is a practical treatise on the kiln-drying of lumber and will be of great help to kiln operators. It can be secured by writing to the New York State College of Forestry at Syracuse, N. Y.