NC State Extension Publications

Problem

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.

Test Material

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.)

Figure 1.

Figure 1. A = From dry storage shed (9-11%MC) B = After redrying (5-1/2%MC) C = After conditioning (6-3/4%MC)

Redrying Schedule

The lumber was stickered and placed into the kiln. The following schedule was used:

Dry Bulb, °F Wet Bulb, °F EMC% Time
126 95 5.5 6 days
140 110 5.8 5 days

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%.

Conditioning

The following conditioning schedule was applied.

Phase Dry Bulb, °F Wet Bulb, °F EMC% Time
Warm Up 170 140 5.7 2 hours
1 170 149 7.6 6 hours
II 170 156 9.6 28 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.

Authors:

Associate Professor and Extension Specialist
Wood & Paper Science
Superintendent, Hodges Wood Products Laboratory
Wood & Paper Science

Publication date: Sept. 1, 1991

North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons without regard to sexual orientation.