// ^<"-' ^ FOREST PRODUCTS LABORATORY 1 FOREST SERVICE U. S. DEPARTMENT OF AGRICULTURE SPECIAL METHODS OF SEASONING WOOD INT3AR5D RADIATION The successful use of infrared radiation for hardening paints and enamels on ;aetal has caused some speculation that it might be used to advantage in seasoning wood. In drying paints or enamels on metal "by this method a battery of lamps is directed toward the object so that radiation from adjacent lamps overlaps to provide uniform coverage. The conduction of the metal and movement of the object past the lamps tends to equalize surface temperatures rapidly. Seasoning of lumber, however, presents a basically different problem be- cause the heat must penetrate the wood without raising the surface tempera- ture to such a point that drying defects occur. When wood is heated by . infrared radiation the depth of penetration is slight and heat applied in this manner is carried into the wood by conduction only. Since infrared radiation penetrates wood only to a slight degree it does not ap ear advantageous to use it as a source of heat for seasoning wood because the rate at which the interior wood heats will depend upon the sur- face temperature and not upon whether the surface temperature was established by absorption of infrared radiation or by contact with hot cir- culating air. The time required for heating wood in air in a dry kiln to the desired drying temperature is relatively small in comparison with the total drying time. To heat green wood 1 inch thick to the kiln temperature in a modern forced-air circulation kiln takes only approximately 1 hour, but several days or weeks may be needed to dry the lumber because of the slow diffusion rate of moisture at permissible kiln temperatures and relative humidities. All this time the air is circulating throughout the lumber pile and reaches all surfaces of each piece. Keat supplied by infrared radiation, in contrast, would reach only the surfaces in direct exposure. To apply infrared radiation to all surfaces of each board would, seemingly, require the single-file, board-by-board passage of the lumber through a tunnel on a traveling chain, a highly impractical ani costly method that would have to compete in output with the commonly provided kiln capacities of 20,000 to 60,000 board feet that are standard in the process of drying lumber by heated air. In drying lumber by any method, temperatures and relative humidities must be under accurate control for most woods. Green oak, for example, is particu- larly susceptible to surface checking caused by initial relative humidities below 80 percent and green oak that is 2 inches or more in thickness, especially in a forced-air circulation kiln, should have an initial relative humidity as high as 85 or 90 percent. Oak is also susceptible to honey- combing caused by the use of temperatures much above 115° 7. during the time the moisture content of the core is above 30 percent. Some species, such as basswood dry easily and higher temperatures and lower relative humidities can be used. P.ept. HO.HL665-4 t Maintained at Madison 5, Wisconsin in cooperation with the University of Wisconsin UNIVERSITY OF FLORIDA M 1 1 III III 1 1 3 1262 08924 3413 The relati humidity cannot be controlled within tl i in d limits at the surface of the ••■ood whi surface it: heated to a temperature greater than that of the surrounding air because tne wood then heats tr.e adjacent us lovers the relative humidity at the wood surface. Vith ;ir?d radi:iticn, the wood surface is always heated above the temperature Lng air. Relative humidity decreases rapidly as te. ea:-es. Starting with a temperature of 80° F. ana a relative humidity of 50 p I . , the relative humidity of the air when heated to 150° 7. will re j: Ly 11 perct. . It might seem that infrared radiation is well suited for the d. jf er be." or is so thin, tut r.ere again practical procedures do not elves to this method of heating. In drying veneer, restraint be provided against wrinkling and buckling by drying it between heavy wire ., springs, closely spaced iron rolls, or plates. '■'•' en .nese dj are used t .eer come? from them reasonably flat and in good condition for storage, shipment, or manufacture into plywood. In dr. ; 1th infrared ation t the restraining mechi . would interfere with the application of heat or it would be difficult to maintain the lamps if used • the drying equipment nov; available. Even though practical solutions were found fcr the difficulties of using infrared radiation for the seasoning of wood, the cost of electricity would •renter in most localities than the cost of steam. forest products laboratory madisgh 5, wisc ' 1947 Ho.Rl665-4