The alleviation of brown stain in hem-fir through manipulation of kiln-drying schedules

A brown stain often develops when drying hem-fir, leaving the lumber with an unsightly appearance that greatly reduces its value. The stain is believed to be caused, in part, by the oxidative polymerization of extractives during kiln-drying to darkly colored chromophores. Green lumber from a source that had proven to be especially susceptible to stain development was dried using various presteaming treatments in an attempt to remove some of the extractives and thus improve the appearance of the dried lumber. Small end-matched specimens were presteamed for periods of 0, 2, 8, and 16 hours and dried at temperatures of 130?, 150?, and 180?F (54?, 66?, and 82?C) and relative humidities of 40 and 80 percent. Visual examination of the specimens indicated that 8 hours of presteaming produced the greatest reduction in the extent of stain formation. Larger kiln-drying trials were performed using two schedules (regular and mild) with three presteaming treatments (0, 12, and 24 hr.). Judged in terms of the extent of stain formation alone, a presteaming period of 12 hours seemed to improve the appearance of the dried lumber, whereas extension of presteaming to 24 hours deteriorated the appearance.

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