Properly kiln dried lumber stored, dead packed in an enclosed, unheated shed may pick up moisture. Redrying may be necessary. Drying stresses do develop during redrying and should be relieved.
Properly kiln dried and stress relieved 4/4 red oak was placed dead packed in an enclosed, unheated shed. Average initial moisture content was 5.5%. Some checks were observed. After several months, moisture samples showed 9 – 11%. Stress sections showed no stress (Sample A in Figure 1.)
The lumber was stickered and placed into the kiln. The following schedule was used:
|Dry Bulb, °F||Wet Bulb, °F||EMC%||Time|
These mild conditions were used for the purpose of reducing the re-opening or deepening of existing checks. The length of time was excessive, but time was not of essence in the test and small moisture content variability was an objective. After redrying, the lumber had stress. (Sample B in Figure 1). Average moisture content was about 5.5%.
The following conditioning schedule was applied.
|Phase||Dry Bulb, °F||Wet Bulb, °F||EMC%||Time|
|Warm Up||170||140||5.7||2 hours|
Total time after set points were reached was 34 hours. Average moisture gain was 1.25%. Stress tests cut while the lumber was hot showed slight outward turning of the prongs as expected. After three days, prongs straightened, showing no stress. (Sample C in Figure 1.) The major reason why conditioning was done in two steps was to avoid too rapid of an initial moisture gain. If such would happen, there may be chances of reverse case hardening, resulting in open checks.
Publication date: Sept. 1, 1991
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