Microwave Drying of Ponderosa Pine Cross-Sectional Disks

Disks 1 and 2 inches thick and 12 inches in diameter cut from a ponderosa pine tree were stored in a water bath which increased their MC from 100 to 140 percent of their ovendry weight. Cross sections were dried in a microwave oven operating at a frequency of 2450 MHz with variable microwave on-off time. One-inch disks subjected to wave energy 100 percent of the oven time dried in about 45 minutes without checking. However, continuous wave energy caused checking in the 2-inch disks. Reducing wave energy to 90 percent of the oven time as the 2-inch disk approached fiber saturation point (about 30 percent) caused checking in about one-third of the samples. Decreasing the wave energy to 70 percent as the disk approached its fiber saturation point eliminated checking in the 2-inch samples. For the 2-inch disks, raising the temperature of most of the free water and the wood to about 212?F took the first 30 minutes of drying time. Free-water flow and evaporation occurred for about 120 minutes of oven time, then bound-water diffusion and evaporation took place until the end of the drying period. Careful application of microwave energy to disks of moderate thickness from species with uniseriate rays, having a specific gravity of 0.40 or less, may provide an efficient method of preparing undistorted specimens, displays for exhibits, specimens for plaques, and similar items.

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