Quality and Strength Properties of Douglas-Fir Dried At High Temperatures

Studies of the kiln drying of Douglas-fir at high temperatures lead to the following conclusions: 1) The three influential factors, temperature, humidity, and drying time, as applied in the high temperature ranges reduced the strength properties of 2- by 8-inch Douglas-fir as compared with tests of their conventional counterparts. 2) Superheated steam (temperatures up to 222?F dry bulb) affected strength properties more adversely than high-temperature air-stream mixture. 3) Temperatures above 225?F resulted in heavy degrade. 4) Strength was reduced more when the high-temperature portion of the drying time was increased. 5) High temperature apparently lowered the hygroscopicity of the wood. 6) Under a high-temperature air-stream mixture drying condition the fiber stress at the proportional limit was reduced 7 percent, modulus of rupture 13 percent, modulus of elasticity 8 percent, and maximum crushing strength 2.5 percent. 7) Under superheated steam, fiber stress was reduced in the range of 5 to 16 percent, modulus of rupture between 9 and 17 percent, modulus of elasticity 3 to 7 percent, and maximum crushing strength 6.3 to 12.7 percent. 8) The decrease in strength properties was less when material of higher density was used.

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