Effect of radio-frequency/vacuum drying and mechanical press-drying on shrinkage and checking of walnut log cross sections

The occurrence and severity of shrinkage and checking in walnut log cross sections (disks) were investigated during radio-frequency/vacuum (RF/V) drying from green to about 10 percent moisture content (MC) and during RF/V drying from green to about 50 percent MC followed by press-drying to about 8 percent MC. The effect of end-covering (covering the exposed end surfaces) of both the sapwood and heartwood areas (ECS/H) or end-covering only the sapwood area (ECS) was also investigated. Heart checks were significantly reduced by RF/V drying. The RF/V drying process alone, however, was unable to completely control the occurrence of v-cracks (v-shaped checks that radiate from the pith all the way to the periphery of the disk). Inversely, RF/V drying to 50 percent MC followed by press-drying was very effective in controlling v-cracks, but was not effective in controlling heart checks. End-covering of both the heartwood and sapwood exposed surfaces was very effective in controlling heart-checking in RF/V-dried disks, but was not as effective in controlling v-cracking during RF/V drying as end-covering only the exposed sapwood. Neither ECS/H nor ECS was effective in controlling heart checks during RF/V followed by press-drying. The study suggests that RF/V drying combined simultaneously with mechanical press-drying may be most effective in controlling checking in log cross sections.

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