Of all the terms used to describe wood, I think that casehardening ranks near the top of my list of words that are misleading and poorly understood. (Correctly classifying lightweight balsa wood as a hardwood because the tree has leaves and classifying dense Southern pine that bends nails as a softwood makes the terms hardwood and softwood perhaps the very top of my bad list.)
What is casehardening?
Casehardening has nothing to do with having the case or shell of a piece of wood harder than anything else. Casehardening is actually a term used to describe dried lumber that has drying stresses.
There are two forms of casehardening. One, called transverse casehardening, runs across the grain and will cause cupping from edge to edge during machining, such as planing or resawing (cutting a piece into thinner pieces). The second, called longitudinal casehardening, runs along the grain or length of the lumber and causes lengthwise warp when ripping the lumber into narrower pieces.
Why is casehardening to be avoided?
These drying stresses do not affect a piece of lumber until we begin to machine the piece. When we machine this casehardened piece of lumber, we remove some of the wood and some of the stress that it is in the wood. This is likely to unbalance stresses in the remaining piece, and these unbalanced stresses result in immediate warp. The key word is "immediate." In contrast, moisture changes cause warp over time and not immediately when machining.
Note: If we were able to remove equal amounts of wood at the same time from both faces of lumber (or from both edges), we would actually keep the stresses balanced and there would be no warp. But certainly, even removal of wood is next to impossible. In short, if we need flat, straight pieces of wood, then we need stress-free lumber.
It is a simple, final step when kiln drying lumber to remove casehardening stress. This procedure is called conditioning (or stress relief). It is done by quickly adding a small amount of moisture back to the surface of the lumber at temperatures above 160 F. Conditioning time is 4 to 18 hours typically. Most often, steam is injected into the kiln atmosphere and thereby moisture is added to the lumber's surface. (A few operations use a fine mist of water rather than steam.) If moisture is not added quickly or if the kiln is not hot enough, stress relief may not be thorough or satisfactory.
It is almost impossible to remove casehardening after the lumber leaves the kiln. Heating alone or just waiting for a year or more does nothing.
The flooring industry discovered years ago that when they machined the top surface of flooring planks they unbalanced the stress because they did not machine the back side as much. The result was that the flooring strips crowned from edge to edge. To offset this, they then machined several grooves on the back side, which it turns out is fairly effective in re-balancing the stress. I have also seen this technique used for wide moulding pieces. If there is no stress, the grooves do not hurt, but if there is stress, this will help to restore flatness. Of course, the best way to remove stress is by steaming, as mentioned.
Checking for casehardening
The purchaser of kiln-dried lumber can quickly check for the presence of unrelieved stress by cutting a stress sample. Transverse casehardening is measured by cutting a 1-inch, along the grain, full width and full thickness test section from a piece of lumber. Cut this test piece at least 12 inches from the end of the lumber. Next, on one of the freshly sawn faces, saw two prongs that are connected at the bottom, similar in shape to the letter "U." Note that the thickness of each prong is roughly 1/4 of the thickness of the lumber for 4/4 through 6/4 lumber. If these prongs remain straight or nearly so after cutting, then there is no stress. In simple terms, this test is trying to determine if warp will result; straight prongs mean no warp when processing the lumber. (Note: When such tests are made during kiln drying, it is necessary to heat the prongs for 15 seconds or so in a microwave oven and then wait several minutes after heating to read the prong position.)
Longitudinal casehardening is measured by cutting a test piece typically 8 inches wide, 24 inches long and full thickness. Then, rip this test piece in half, making two 4-inch-wide pieces. Put these two pieces back together in the same position that they had before sawing. There should be no gaps wider than the thickness of a dollar bill between them if there is no longitudinal stress.
These are very sensitive tests. Often it is not necessary that your lumber be totally free of stress. Therefore, it may be possible that some bending of the prongs or gaps between the pieces will be acceptable.
The presence or lack of casehardening is not included in the grading rules for lumber. However, when a cabinet or furniture shop purchases lumber, it is assumed that it is suitable for the intended use. When making cabinets, furniture and similar products, the presence of casehardening often makes the lumber unsuitable for use. Such lumber should be returned to the supplier for credit. To avoid any confusion between buyer and seller, it would be prudent to specify when writing a purchase order, to include that the lumber is without serious casehardening. (Specifying "No casehardening" is not reasonable or necessary in most cases.)
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