High-Temperature Kiln-Drying Red Maple Lumber–Some Options

High-temperature drying (HTD) is attractive to many hardwood lumber processors because of shorter kiln residence, which saves energy and provides faster turnover of inventory. However, in some species more drying defects have been observed when kiln temperatures of 230? to 240?F were used on green fresh-sawn lumber. Partial drying of the green lumber at temperatures not exceeding 180?F lessens the drying degrade, but extends kiln residence time. In this study, 4/4 red maple lumber was dried by the conventional schedule (T8-D4) to average moisture contents (MCs) of 45, 30, and 20 percent, before HTD to a final MC of 6 percent. Drying degrade of this lumber was evaluated and compared to lumber dried to 6 percent MC by straight HTD or a conventional schedule. Starting HTD at 30 percent, 45 percent, and green (70 percent) MC substantially increased end checks/honeycomb in the first 4 inches from the board end, when compared to lumber where HTD was not started until MC was at 20 percent or where HTD was not used. From 4 inches and farther from the board ends, there was little difference in the number of end checks/honeycomb when beginning HTD at 45, 30, or 20 percent MC. Areas of mineral streak, discoloration, knots, and pith in the boards were more likely to develop checking and honeycomb. No difference in crook or bow was found between the five schedules, although less twist occurred using HTD schedules beginning at or above fiber saturation. Depending on the amount of end checking or end trimming acceptable to the user, options are suggested to offset drying degrade against drying time. Raising the temperature to 230? at 20 percent MC will reduce kiln time 30 percent, while increasing by 8 percent the number of boards with end checks. A 60 percent reduction in kiln time is obtained by starting HTD at 45 percent MC, provided a trim of 3 inches per board end and 15 percent fewer end-checked boards than the control can be tolerated.

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