Q: We received some lumber and checked it for stress and moisture. Some pieces were a bit high in moisture and we also noted some stress that we call casehardening in these wetter pieces. The supplier said it was fine when it left his plant and he thinks that moisture regain in transit caused the higher MCs and also the stress. Lumber drying seems to be a bit of black magic. Can you help us, please?
A: Drying lumber is indeed a specialized process. Each manufacturer and sometimes each customer have a special set of requirements for the dried wood. These requirements include target final average moisture content (MC), the spread of the average MC from piece to piece, the variation of MC within a piece (sometimes called the shell-core variation), the amount of transverse (across the grain) stress (also called casehardening) and the amount of longitudinal (lengthwise) stress. A lumber supplier has to guess oftentimes as to what is the exact level of quality that you want in these areas.
To assure that this final quality doesn't change between drying and manufacturing, the lumber must stored at a particular relative humidity (RH) condition. Such storage is intended to hold the lumber at the proper MC and not to dry the lumber further or to relieve any stresses.
Equalized dry kiln
With respect to the particular situation that you asked about, it would be important that in the dry kiln the lumber is equalized (which means that all the pieces are brought to the same, or nearly the same, final moisture content). The kiln drying operation determines, by controlling the equalization process, how much variation of final MC exists. The smaller the variation desired, the longer the process of equalization that must be used. It is possible with proper kiln drying to achieve final MC variation that is so small that subsequent treatments or storage cannot minimize the variation further.
Once the lumber has been properly equalized according to the requirements, the lumber is then stress relieved, which is also called conditioning. The stresses can only be relieved by exposing the lumber to a rapid regain of moisture at the surface at high kiln temperatures. It is impossible to relieve any stress at room temperature. It's also impossible to relieve any stress without exposing the lumber to a high RH condition.
Special note: If the lumber is not properly equalized or if the lumber changes moisture after drying while in storage, the moisture gradient present will make the lumber appear to have stress. Such false stress will disappear if the lumber is stored at a constant RH condition for a long time. It's certainly more appropriate to equalize properly and then store the lumber at a controlled RH so that such storage is not necessary. In any case, the lumber with a gradient is stress free. I think your supplier is suggesting that this change in moisture is the cause of your problems . . . he may indeed be correct.
Store at proper RH
Once lumber is dried, if it is not held at the proper RH condition, it will change its MC in order to achieve equilibrium. For example, for 42 percent HR, the lumber will seek to achieve 8 percent MC, if it is not already at this level. Therefore, the lumber must be stored, after drying and during transit at the proper RH to avoid changes in MC.
Note that temperature is not a factor in the MC of lumber. Further, note that lumber that is wrapped in plastic, even if exposed to incorrect RH outside the plastic, will not change MC at all as no moisture can get into the out of the wrapping. So, lumber shipped on a truck or even on a ship will arrive at the same moisture content that it was prior to shipping.
Here's the key: use a pin-type moisture meter and check the core MC. The core moisture will not change appreciably during transit. If the core is indeed dry, then your supplier is "off the hook." If the core MC is high, then the kiln was not proper equalized and subsequently stress relief could not be done perfectly.
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