Development of Collapse in Aspen Lumber

Collapse of wood in an intermediate zone between sapwood and heartwood is a serious defect developing in aspen lumber during kiln-drying, particularly when rapid or severe kiln schedules are used. The intermediate zone, often called “wetwood,” which is so subject to collapse has a very high moisture content when compared to heartwood and sapwood. In two separate log samples the moisture contents of sapwood, “wetwood,” and heartwood were found to be 100.5, 127.2, and 107.0 percent and 91.3, 113.4, and 80.1 percent, respectively. “Wetwood” was also found to be heavily infected with bacteria, very impervious to the passage of air and water, and both the “wetwood” and heartwood were weaker than the sapwood in toughness and compressive strength. The specific gravity of heartwood was 10 percent lower than that of sapwood. “Wetwood” was readily detected by its darker color and wet appearance. Losses from collapse can be reduced by sorting and separate drying (either air-drying or kiln-drying with a mild schedule) of the boards containing such wood.

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