Q. Due to a recent severe storm with lots of rain, some of our kiln-dried lumber in storage has increased its moisture above the acceptable moisture content. How can we dry this out?
A. Redrying lumber that has increased its MC due to a severe storm or from being kept in uncontrolled storage requires great care to achieve the required low MC. Redrying is best done in a standard kiln. The first step is to re-sticker the lumber using normal stickering procedures.
Two temperature steps are suggested for the redrying operation.
The first step should be about 18 to 24 hours long at 125 degrees F to 140 degrees F. The lower temperature settings are for thicker and hard-to-dry species. The humidity, which is set after the correct temperature is achieved, is at an EMC that is numerically halfway between the current MC of the lumber and the MC desired. (Every kiln operation has a chart of EMC values, relative humidity values and temperatures; use this chart to find out the correct settings for your equipment controls.) Do not use steam spray or water spray during kiln warm-up.
The second step should be at the final temperature of the normal schedule, usually 150 to 160 degrees F. This temperature can be achieved by using several smaller steps (each about an hour long), rather than making a single 20 degree F or greater jump in temperature. Set the humidity controls to give an EMC 2 percent below the desired final MC. Then, when the wettest kiln sample reaches the desired MC, stop the drying. There is no need for stress relief or conditioning if the lumber was dried correctly before it was rewetted.
Redrying is very risky if the species being dried is prone to surface checking. The risk is that surface checks, that were small and tightly closed, may become permanently opened, or may grow into internal hairline checks (sometimes called honeycomb). This is a problem with oak and a few other hardwoods, but not with easy-to-dry hardwoods and not with softwoods, like pine. With these risky species, go a bit cooler and make changes in conditions more slowly than suggested above. Avoid rewetting at all costs; that is, do not use higher than suggested humidities and do not add moisture before the correct temperature is achieved.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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