Q: In our manufacturing of mouldings, we use red oak and are in the process of trying to monitor casehardening. We have a few questions for you.
1. What is the easiest test to do for this? Prong? Cup? Other?
2. What is the frequency recommended for doing this test?
3. Should the supplier be able to provide us with this information normally based on their testing or drying process?
Let us know and thanks for your help...
A: You are probably more concerned about "across the grain" stress (and resultant across the grain warp when machining) than lengthwise stress (and resultant lengthwise warp). Therefore, the prong test, as documented in most drying manuals, is appropriate. You should probably detail the procedure for conducting the test for your suppliers, however, to make sure that they and you use the same procedure.
Make sure the prongs are about 6 inches long and are each 1/4 of the thickness of the lumber, with the middle half being removed. Note that the test is accurate only if there is no moisture gradient, so a kiln operation needs to use a microwave oven to eliminate the gradients in the prong before they read the results. By the time you get the lumber, all gradients should be gone.
I would check about 10 pieces in a load, chosen randomly throughout the load. If the prongs are straight (within 1/8 inch typically), then you are fairly certain that stress is not an issue. Certainly, more tests will increase your confidence. If any of the first 10 do not stay straight, then I would cut 20 more to ascertain if indeed there is a problem. Being fairly strict on stress is worthwhile when dealing with small pieces of wood, such as mouldings. For larger pieces, a little more, but not much, stress can be tolerated before creating a problem.
A good dry kiln operation should be able to eliminate all stress and should also keep records to document that they did so. Oftentimes, they get erratic stress removal when the final MC is not uniform, so they should also be able to document, and you should be able to confirm by making your own readings, that the MC is uniform. If you find widely varying MCs, expect poor stress relief.
Q: I have just found a source of rough-sawn lumber from overseas. Are there any big differences between domestic and foreign lumber with the same name? I have bought some maple and beech and it seems to have different coloring.
A: Most foreign lumber is a different species and sometimes a different genus than common U.S. lumber (except for Canadian lumber). Common names or trade names are often not a good guide for identification of the lumber you are purchasing. On the other hand, often foreign lumber does look and behave similarly to our native lumber. Pau marfim, also called guatambu, is often a good substitute for hard maple.
Even if you have the same species, lumber handling can be different overseas and that can result in different appearances. For example, for the white-colored woods, drying procedures will greatly affect the color even for the same species. European beech sometimes has a pinkish hue, compared to American beech. But even under 10x magnification, we cannot tell the difference between the two. The pink is a result of the slow drying (dry American beech slowly and you will have the same coloration) and not a difference that can be used for identification.
Q: I just received some kiln-dried hardwood lumber that has worm holes in it. Is there a risk of having this lumber with worm holes perhaps having living bugs in the holes, infecting other dried lumber in my warehouse?
A: The risk of infecting lumber and such an infection causing a problem is very, very small. You will understand this answer better if we review a few facts:
Almost all insects that infect wood prefer wet wood. There are only two common insects (in addition to termites) that infect dry wood the lyctid powder-post beetle (grainy hardwoods only) and the old house borer (softwoods, especially pine, only). Even these two insects that like dry wood prefer wood 8 percent MC or wetter. Drier wood is just too hard for them to eat. Most kiln-dried hardwoods are drier than 8 percent MC.
Second, kiln drying (if the temperature went over 130 F, which it does in 99 percent of the kilns) will kill all insects and eggs. So, when the lumber leaves the kiln, it is sterilized.
Third, it takes months to years for newly laid eggs to hatch, burrow into the wood and then burrow out, leaving the visible exit hole. When you see holes in lumber that has been recently kiln-dried, there has not been enough time to have a new infestation of insects make those holes.
Fourth, the powder-post beetle makes holes that are 1/16 to 1/32 inch in diameter. Any larger holes in kiln-dried hardwood lumber would not be a risk factor, as the insects are not "dry wood insects."
Fifth, even if you see small-diameter new holes with a living insect, it still takes "two to tango." The risk of breeding new insects is a rare event if sanitation (no wood debris, no socializing or breeding grounds) is good to excellent.
Sixth, once new eggs are laid, there is an incubation period of up to two years. Survival of the eggs in a piece of furniture machined or sanded so the surface fibers where the eggs were laid is gone, coated with a finish, heated in finishing ovens, and so on is quite rare.
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