Date: 2010-07-23
Gene Wengert, gwengert@wattnet.net
Q: We have finally learned that using the correct moisture content (MC) of the lumber prevents end checking and warp in our glued up panels, especially in the wintertime. Our supplier of lumber says that wood is variable and so we cannot get the uniform MC that we need. What can we do?
A: First, it is good to hear that you have learned what so many in our industry have yet to learn. I am sure that the lack of defects in manufacturing and also in the field means a substantial financial improvement for your company.
There are two parts to your solution. First, is achieving a low final MC in your lumber (usually nothing wetter than 7.5 percent MC for hardwood lumber and sometimes nothing wetter than 7.2 percent MC) and second is making sure that the variability of final MC is low (oftentimes every piece within 1 percent MC of the target MC).
The way we achieve both of these objectives is in a process called equalizing. This process is done in the dry kiln near the end of the drying cycle. I am sure if you ask, we will find that every kiln operator says that he uses an equalizing step. The problem is that most operators equalize incorrectly. If your kiln operator says something like "I equalize every load for 24 hours." then you know that it is being done incorrectly.
Equalizing, which is the way we get all the lumber down to the specified final MC and minimize any variation in MC from piece to piece, may require six hours in one load and four days in another. Proper equalizing also prevents over-drying. (Over-dried lumber machines poorly, glues poorly and is often cupped excessively.)
As the lumber in the kiln approaches its final MC, the operator must know the MC of the driest piece of lumber in the kiln. This knowledge is gained by using short pieces of lumber, called kiln samples, that are prepared when drying first begins. When the driest piece of lumber is 2 percent MC below the target MC, then we set a humidity condition in the kiln that will stop this driest piece from drying any further. Technically speaking, the EMC condition in the kiln is set to 2 percent EMC under the target MC. (Example: If the target is 7.0 percent MC, then when the driest piece reaches 5 percent MC, the kiln is set to 5 percent EMC. This means, with 5 percent EMC, that it is impossible for any piece of wood in the kiln to dry under 5 percent MC.) Of course, any pieces that are wetter than 5 percent MC, will continue to dry. We hold this 5 percent EMC condition for as long as it takes for the wettest lumber in the kiln (based on kiln samples that represent this wettest lumber) to reach the target MC. (In our example, this means 7.0 percent MC.) If properly done (which includes proper MC sampling), all the lumber in the kiln will now be between the target MC and 2 percent MC below the target. This is a standard deviation of 0.3. This equalizing time is variable depending on the range of MCs encountered.
Note that we are talking about MCs measured to the closest 0.1 percent MC. Anyone that says their lumber is "6 to 8 percent MC" should make you question if they are really being as precise as you need them to be.
You may ask why the lumber is slightly below the target MC after equalizing. The reason is that after equalizing, we will condition the lumber to relieve stresses. In this stress relief process, the lumber will regain about 1 percent MC.
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.
Updated: Oct 06, 2010
This article appeared in CabinetMakerFDM, October 2010. ©Copyright 2013, All Rights Reserved.