Q: We built the Virginia Tech solar-heated kiln that you designed. It went beautifully, but the main problem seems to be removing moisture from the room efficiently enough to prevent mold from growing on the lumber. Can you offer any suggestions?

A. The kiln that I designed (the plans are widely available on the Internet as the Virginia Tech kiln and other names) was designed basically for oak lumber, which needs high humidities and also dries quite slowly. When lumber that releases its moisture quickly is put into the kiln green from the saw, the kiln will not be able to vent fast enough resulting in high humidities, and the solar heat will all be used for evaporation, so little heating will occur. Also, unless you have fairly large fans, the air will move slowly through the pile and the humidity will increase as the air moves through. 

The easiest way to solve this problem is to air dry the lumber prior to loading it into the kiln.  The source of moisture that mold needs to grow will be eliminated. 

A second option would be to use large vents. When starting with wet lumber, open the vents wide open and then use the dryer as a single pass dryer; that is, the outside air will enter at the top vents, go through the fans, pass down the clear roof where it will be heated slightly if the sun is shining, will then pass through the lumber and then exit the dryer. As the outside air will average about 65 percent RH, this will keep the moisture in the kiln from getting too high. Of course, turn off the fans when the RH is high, such as during foggy or rainy weather, or late at night. 

If you still have mold problems, then we can look at increasing the fan size. 

 

Q. We bought some white hard maple lumber that looked pretty good until we planed it. When we planed it, we found discoloration marks that looked that tracks from the saw blades; the marks lined up with the saw blade marks from the sawmill. One person in our shop called it bruising. Can you tell me what this is and who caused it? 

A. Indeed, this is sometimes called bruising and results from extra pressure that is caused by the teeth on the saw on green, freshly sawn  wood. Sometimes I have seen bruising from the pressure of rollers and forklift forks on green lumber. Dried lumber is not susceptible to this damage. As you noted, the damage does not show up on the surface, but rather shows up below the surface and can go 1/8 to 1/4 of an inch deep. I believe that the wood cells are damaged by this pressure during sawing and then they damaged cells are susceptible to oxidation during drying. The products of this oxidation are grey in color most of the time. Sometimes oxidation stains can be removed with oxalic acid (wood bleach), although in maple the bleach can also cause pinking of the unstained normal wood. So, the bottom line is that in your case the wood is defective and the defect cannot be repaired or removed (unless you want to plane off 1/4 inch from top and bottom faces). 

We do not know why this stain shows up from time to time, but most of time is absent. I believe it has to do with the tree and the chemistry within the tree. Perhaps a delay in sawing into lumber after the tree has been cut makes this stain more likely. I do not believe the sawmill had any idea that the lumber was at risk, or that even after drying they knew this stain existed. Yet, because the wood is essentially useless to you, the mill should be willing to exchange it for some different, unstained wood.

 

Q.  We are making a small thin product that must be kept flat, similar tot he slat of a wooden blind.  We are having trouble with some twisting.  What advice can you offer?

A.Twist is one form of warp. (Cupping, side bend or crook, and bow are the others.) If a piece is flat initially, but warps over time (hours to months), we know that the moisture content has changed. In other words, if the moisture content is kept constant (or changes only a percent or two (wood in a home is often 6 percent MC in the wintertime and 9 percent MC in the summertime), then wood will not warp. So, you need to spend considerable time getting your incoming wood supply at the correct MC. In most cases, the correct value is 6.5 to 7.0 percent MC. Do not believe a shipping invoice or word of mouth; measure the MC yourself using a meter that costs about $200 or more. Cheaper meters are often not accurate enough and or not well calibrated.

Twist occurs when the angle of the grain (or the angle of the wood cells that normally run vertically in the tree, are at an angle in the lumber rather than parallel to the face and side. The grain can be twisted in the lumber because the log is crooked, because the log is put on the equipment at an angle or because the tree itself has angled grain. With a critical product such as you have, you can assure that every piece will stay flat, even with MC changes, if you split the log first and then saw the lumber parallel to the split faces. In other words, a split will follow the grain. Sawing parallel to a split face assures you that the grain in the lumber will be straight. This splitting technique is used on pool cues, shakes and shingles and a few other straight, flat products. It is interesting that if you go to Colonial Williamsburg and visit some of their woodworking shops, you will see this splitting technique applied to many situations. There is something to be said about the “old” techniques for making furniture, cabinets, and other fine wood products.

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