Q. At our plant we periodically have spurts of wood with high mineral content. These components, when cooked in the radio frequency glue press, cause the wood to turn white and look and smell of high moisture. We have done numerous moisture tests on mineral sticks, but the data does not confirm that mineral sticks contain high moisture. These sticks can feel heavy and cold, but produce a normal moisture reading. Can you help explain this?

 

A. The definition of mineral is not real precise, but in general, mineral is a region within the wood that has a lot of mineral in it (mineral means chemicals that do not burn, and that are not organic, such as silica and calcium) that was deposited during the tree growth. Sometimes, however, the region we call mineral includes dark streaks that are the result of an insect getting into the wood and then subsequent bacteria and fungi entering in this same location and staining the wood. At times, the discoloration is water soluble, or at least part of it is.

When a mineral region is tested for MC, it's possible that the mineral will conduct electricity and create a high MC reading, even when there is not high MC. At the least, the mineral confuses the electrical MC readings. For oven-dry weight readings, the mineral does add to the oven dry weight of the wood, so that if the wood is actually at 10 percent MC, the extra weight of the mineral will slightly lower the calculated MC when using an oven dry test. (For example, add 5 percent to the weight of the oven-dry weight and then make a calculation again. Wet weight = 107.00 and OD = 100.00 with no mineral, then MC = 7.00 percent MC. Now if there is 5 grams mineral, then 112.00 and 105.00 are the two numbers and then MC = 6.67 percent MC.) There is no special reason why mineral wood that has been kiln-dried will have a higher MC than regular wood.

The way to determine mineral content is by weighing the stick, burning it completely and then weighing the ash.

The white color you observe is some "mineral" chemical that is water soluble being carried by water to the surface of the wood where the water evaporates and then the chemical is left behind. As more water moves toward this surface, it brings more chemical with it. The source of water is water that is naturally in the wood, as well as moisture that is in the adhesive.

 

Q. We make high-end cherry edge-glued panels, and I know everything has to do with moisture. Here is my question: I have a customer who is having problems with sunken joints, but it's not all the time. Why does it happen and why does it happen mainly after coating?

 

A. You are correct that the issue is related to moisture. One of the basic truths when working with wood is that wood doesn't shrink or swell or change size or shape from one hour to the next, one day to the next or one month to the next unless its moisture is changing.

So, let's look more closely at an edge-glued panel made of several strips of wood and glued together with an adhesive that has some water in it. When the panel is first glued together, within an hour or two, the wood right around the joint (within 1/8 inch nominally) will swell in thickness due to the water in the glue that moves into the wood fibers right near the glue line. So, right after gluing, there is actually a small bump at the glue line. As soon as this water evaporates, the swelling will go down and the wood is back to "normal."

The problem arises when we machine this panel before the water has left and the swelling has gone down. Right after sanding or planing, the panel will be perfectly flat, but then as the rest of the excess water leaves, there will be a slight amount of shrinkage right at the glue joint. You will find it nearly impossible to see this slight depression with your eye until you put a smooth finish on the wood. In fact, the glossier the finish, the more that this small imperfection, called a sunken joint, will show up.

Incidentally, we usually see more sunken joints in the winter, as the wood is drier so there is overall more shrinkage. However, I've seen it happen in the summertime, too.

Obviously, the cure is to wait until the moisture has left the joint area. This is typically 72 hours if the panel is kept at 70 degrees F. Cooler storage takes longer; hotter, shorter.

The above discussion is by far the most common cause of sunken joints, but in the summertime, if the wood is very dry and the humidity in the plant or shop is very high (especially in the panel storage area just before finishing), then sunken joints may also appear. This rarely occurs, however.

Here's what is happening in this case: when, after the panel has been glued up, the dry wood in the panel gains moisture, there will be swelling, and overall the panel thickness will increase. However, right at the joint, the glue, which is somewhat rigid, will inhibit expansion, giving a sunken joint appearance. Again, this will be seen only after finishing with a fairly glossy surface.

In both cases above, if the panel is resurfaced smooth after the problem is seen, the problem will be gone forever under normal conditions of use.

 

Q. Does moisture settle to the bottom of a piece of lumber when the lumber is drying? If so, would it pay to flip the lumber upside-down after a few weeks?

 

A. Gravity plays no important role in drying of lumber; wood is not that porous. So, the top side of lumber is not drier than the bottom, unless the humidity is lower or temperature or velocity is higher on the top than on the bottom.

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