How to re-dry -- kiln dried lumber that has been re-wetted
Reprinted with permission from Modern Woodworking Magazine.
by Dr. Fred M. Lamb
Kiln dried lumber that is improperly handled, stored or transported can be re-wetted either by high humidity, rain or condensation. Of all the possible causes, the major problem is usually re-wetting by high humidity due to improper storage. (The storage of kiln-dried lumber will be the subject of a future article.) Even though high humidity is the major problem, it should be emphasized that kiln dried lumber should never be exposed to rain. This inevitably leads to severe surface checking, a loss in color quality and many other forms of degrade depending on the species. One absolute way to ruin the quality of kiln-dried lumber is to let it get rained on.
For lumber that has been re-wetted because of exposure to high humidity, everyone seems to have his or her own special way of re-drying this material. However, most everyone agrees that re-drying kiln-dried lumber that has been re-wetted takes a lot of care and a little luck. This is especially true for check-prone woods such as oak and beech. The major concerns when re-drying lumber are the opening of existing checks, creating new checks (or hairline cracks), or driving the existing checks deeper, thus making the checking more severe.
Moisture Content Sampling
I would suggest sampling several boards (3 to 4) in every pack of lumber. If the variation is great, you may wish to sample even more. Use an insulated pin resistance moisture meter and take shell and core moisture content readings. It is important to know how wet the shell is and how large the gradient is between the shell and the core (that is, whether the lumber is wet only in the shell or re-wetted throughout the thickness and by how much). Be sure you find the wettest and driest boards. You may cut samples and use the oven-dry method of determining the moisture content, but unless the moisture content is near to or exceeds about 28 percent, a moisture meter is faster. (Note: You still need to cut moisture sections and sample boards to control the kiln or choose appropriate samples for your kiln control probes.) The procedures outlined above are meant to determine the extent and magnitude of the re-wetting problem and to choose the appropriate course of action.
Minor Moisture Regain or Readjusting the Moisture Content
For minor moisture regain (or minor readjustment of the moisture content) of dead piled lumber, the material can be placed in the kiln dead piled. It does not have to be re-stickered. Use a beginning temperature of 110° F for hard-to-dry, check-prone species or thick lumber; 120° F for moderately-hard-to-dry species and 130° F for easy-to-dry species. Keep the steam spray off and the vents shut.
Use normal airflow (about 300 feet per minute through the load). Once the kiln has reached the temperature set point, set the EMC (equilibrium moisture content) in the kiln to one or two percent below the final or target moisture content of the lumber. If the lumber quality is good, the temperature can be raised, but keep the EMC the same. Use good drying practices regarding the temperature: Raise it if appropriate or keep the lumber cooler if necessary to maintain quality.
Major Moisture Regain
Follow good stacking, stickering, kiln loading and sample board selection practices.
Begin kiln drying immediately after re-stacking. Do not let the re-stacked lumber sit around.
Use a beginning temperature of 110° F for hard-to-dry, check-prone species or thick lumber; 120° F for moderately-hard-to-dry species and 125° F for easy-to-dry species. Keep the steam spray off and the vents shut. Use normal airflow (about 300 feet per minute through the load). Note: If the material is badly damaged or shows signs of extensive degrade, reduce the temperature by 5° or 10° F.
Once the target temperature is reached in the kiln, set the EMC. (You can convert EMC to wet bulb temperature and depression or relative humidity or to whatever your kiln controller uses by using the standard EMC tables.) Set the EMC in the kiln to a setting half way between the average current moisture content of the lumber and the desired final moisture content. For example: If the average current moisture content is 15 percent and the desired final moisture content is 7 percent, the initial kiln setting should be an EMC of 11 percent. It is probably best not to set the EMC any higher than 17 or 18 percent regardless of the result of the calculation.
Maintain this initial temperature and EMC condition for about 24 hours. Hard-to-dry species or thick lumber may require even longer times. Fast drying species may require less.
After this initial drying period, raise the dry bulb temperature by 10° F and lower the EMC to a setting half way between the current EMC and 2 percent below the final target moisture content for the lumber. For example, if the current EMC is 11 and the final target moisture content is 7, then 2 percent below the target is 5 percent. Thus, the new EMC setting is 8 percent, half way between 11 and 5. Hold this new temperature and EMC setting for about eight hours. For hard-to-dry species, thick lumber or badly damaged lumber, you may wish to hold this condition longer.
The final step is to raise the temperature by 10° or 15° F but no higher than 150° F. Easy-to-dry species that have little or no damage can go up to 160° F with little risk. Hard-to-dry species or lumber exhibiting significant damage should be kept much cooler. Use good drying judgment regarding raising the dry bulb temperature. Lower the EMC to 2 percent below the final target moisture content. For example, if the final target moisture content is 7 percent, the EMC in the kiln is set to 5 percent.
Minor Readjustment of Moisture Content
Continue at this step until the lumber is dry. Be sure to check shell and core moisture contents and check for drying stresses. It is possible that the lumber will contain sufficient drying stresses to require conditioning. Be sure to check for both casehardening (transverse) stresses and longitudinal stresses.
Some Additional Tips
Fred M. Lamb is Professor and Extension Specialist, Department of Wood Science and Forest Products, Brooks Forest Products Center, Virginia Tech, Blacksburg, Virginia. Dr. Lamb can be contacted at 540-231-7256; Fax: 540-231-8868
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