Question: The recent articles on hysteresis effect, moisture and movement have been very helpful. Can you give a little more detail on annual moisture in furniture and cabinets? 

Answer: Let’s consider a practical example. Assume that a cabinet is made with wood at 7.5% MC. Before going into detail, it is important to understand that there are two sets of data relating relative humidity (RH) to moisture content (MC). One set, and the set we see published all over, is for wood that is losing MC, or drying. A second set of data is supposed to be used when wood is gaining MC. But I would guess that probably less that ten readers have the data for MC gain, so this second data set for moisture gain is all too often ignored.

The First Winter. It is reasonable to assume that the first winter, the air around this cabinet will be dry, which means drying the wood at 6.0% MC at 30% RH. So, the cabinet wood will dry from 7.5% to 6.0% MC. As we all know, when losing moisture, wood shrinks; in this example, with a wood like oak, we would see about 1/2% shrinkage — an amount that will likely create cracks and warp. To minimize or avoid this risk of damage, it is necessary to dry the wood before manufacturing to 6.8% MC in most typical situations. The wintertime shrinkage from 6.8% to 6.0% MC is small enough to avoid serious problems.

The First Summer. As outside conditions warm in spring and summer, the humidity conditions around the cabinet increase to around 52% RH, which is for drying wood (the published data incorrectly used for gaining moisture) 10% MC in the wood. People who ignore hysteresis will say that the wood will go from 6.0% MC to 10% MC, wintertime to summertime. They will estimate over 1-1/4% % swelling. BIG MISTAKE.

Here is what actually occurs, due to hysteresis. As the 6.0% MC wood is exposed to humidity above 30% RH, there is no immediate increase in MC. There is a “dead zone” of about 1% MC or 5% RH (approximately). Of course, with no MC change in the cabinet’s wood, there is no swelling initially.  Further, the widely published moisture and humidity data that is widely reproduced relating RH to MC is only valid for MC loss. For moisture gain, such as wintertime to summertime, there are different numbers (see figure), due to hysteresis, that most so-called experts incorrectly ignore.

So, although drying data says 50%RH is 10%MC, the moisture gain data at 50% RH is around 7-1/2%, depending on wood species. The truth is that a 1-1/4% MC change over four months or so is virtually of no concern with respect to swelling. (The expert not considering hysteresis would base movement on a 4% MC change and not 1-1/4% MC.) In summary, the first summer is so mild, that we have no concern, thanks to hysteresis.

Second Wintertime. As might be expected, in the future wintertime as the air around the cabinet dries out, wood goes from 7-1/4% MC (summertime) to 6.0% MC (wintertime). At first, when the RH begins to drop, there is a drop in MC, but there is a dead zone where nothing happens to the MC in the wood. Eventually, the 1-1/4% MC loss, summertime to wintertime, is so small and over months, it does not cause any issues.

Future. This dead zone, gain or loss, followed by small MC changes (including the hysteresis effect with two sets of data, MC loss and MC gain), it should be fairly clear why the first winter is the riskiest time, but can be controlled by targeting 6.8% MC in the dry kiln; 7.5% MC is too high, and too risky for many species. 

After the first winter, the hysteresis effect (dead zone and two sets of data relating RH to MC when losing and when gaining MC) will “protect” the wood as the MC cycles annually.

Q: We are ripping 4/4 kiln-dried oak and also resawing into three 1/4-inch (about) thick pieces.  The final pieces are not flat, end to end. Why?

A: One reason for warp is stress from the tree (growth stress), but this would cause warp in the lumber.  In other words, do not expect flat cuttings from warped KD lumber.

A second reason is that there are moisture gradients within the lumber, especially shell to core (outside to inside) gradients.  We would prefer no moisture gradient for lumber being heavily resawn.  Uniform interior MC is achieved in kiln drying.  I prefer never letting the kiln at any time go under 5.5% EMC, meaning the lumber will never drying under 5.5% MC. 
 
A third reason, especially for lumber that was not air dried but went into the kiln over 35% MC, is that stress relief (also called conditioning) was not done with low pressure, wet steam.  Your particular use requires more care than many other uses.