I suspect that just about everyone in woodworking has been approached by someone with the question that goes something like, "A tree Grandpa planted 100 years ago just fell, and we want to make a few tabletops from this wood as a family remembrance. We would like to cut a disk (circular or oval), dry it without any large cracks, and then finish it with polyurethane. Can you do this for me?" Or maybe a library or school has come to you with a question about preparing a disk from a very old tree that will then be marked with pins showing various historic events that occurred while it was growing.
Certainly, we would all like to help, but anyone who understands drying knows that a disk will develop at least one major, unsightly crack extending from the outer perimeter to the center during drying.
Here is why a crack will always develop. Consider a green, circular disk that is 24 inches in diameter. As this disk dries, its circumference will want to shrink about 8 percent. So, the original circumference of (24 x pi = ) 75.4 inches wants to shrink 8 percent to 69.4 inches. Of course, this cannot happen unless the diameter is reduced to 22.1 inches (that is, 22.1 x pi = 69.4 circumference). But, the bad news is that the diameter will naturally shrink only about 4 percent during drying. (Technically speaking, the circumference shrinkage is called tangential shrinkage. The radius shrinkage is called radial shrinkage.) So, the radial shrinkage is not enough to prevent the development of some substantial stress.
To avoid stress development and cracking, we have several options:
* A hole could be cut in the center to allow the radius to shrink more.
* Several disks could be cut and then one used for repairing the others.
* A salt paste could be applied to restrict shrinkage (see sidebar).
* For thin, porous wood, the disk can be dried in alcohol.
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Gene Wengert, aka The Wood Doctor, troubleshoots wood related problems, and explores lumber and veneer qualities and performance, species by species, in Wood Explorer, inside FDMC's Knowledge Center.
Cutting multiple disks
Cut three disks of the size and shape required. These three must be adjacent to each other (minus the sawdust created when sawing the disks from the log) and of the same thickness. The disks are numbered No. 1, No. 2 and No. 3 in order from the bottom of the log upward. Then, for each disk, before drying begins, saw a slot, full thickness, from the bark to the center.
It is important that the saw slot be located at least 45 degrees from the location of the slot in adjacent disks. By creating this "crack" before drying, almost all the shrinkage stress in drying will occur by opening this crack further rather than creating new cracks. At the end of drying, each of the three disks will have a large crack in a different location.
Now, after drying, take disk No.1 and cut a wedge-shaped piece (like a piece of pie) out of the disk, from outside to the center, that includes the crack itself. Make sure the sides are straight and flat. Then, go to disk No. 2, and after aligning the two disks rotationally as they were in the tree, cut an identical pie shaped wedge from disk No. 2 (but it will not contain the crack, of course) that will be a patch for the hole in No.1. Then, glue this piece from No. 2 into the hole in No. 1. (In most cases, I think that Titebond II would be the perfect adhesive, but PURs would also be OK.)
The grain, color and character of the patch from No. 2 will match the characteristics of No. 1 very closely as the two were actually adjacent when the tree was growing. Only an expert will note the subtle differences. Then, repeat the process cutting out the crack from disk No. 3 and getting a patch from No. 2. (If you understand this process, it should be clear why the man-made cracks are at least 45 degrees from the adjacent disk ...we need uncracked repair material.)
Will the bark stay on? Unfortunately, we cannot guarantee that the bark will stay fastened.
Sometimes the bark shrinks different amounts than the wood. Sometimes (early to mid-spring) the bark is not well attached and slips right off the wood. But, with a little care, it will be possible to refasten the bark using disk No. 2 for repair pieces of bark.