Managing Shrinkage in Log Slices
Advice for a woodworker who wants to make cutting boards out of cross-section pieces sawn from whole logs. January 29, 2009
Question
I will be drying cross section "slices" of a black walnut log for use as cutting boards. The round slices are 1.5" thick and 12" diameter. They will be sawed and planed to approximately 7/8" finished thickness. The log was harvested two months ago. The slices were cut immediately and have been immersed in water since then.
I won't be using a kiln. To minimize checking and cracking, I'm considering soaking the pieces in Pentacryl or polyethylene glycol (PEG). Which method is preferred? Or is there another better method? What steps are involved? Should final cutting/planing be done before drying, or after?
Forum Responses
(WOODnetWORK Forum)
From contributor J:
I guess it depends upon which chemical you desire to have your food come in contact with - your call. I would use neither, nor would I consider producing such a product for use by others.
From contributor S:
It doesn't really matter what you do, the log rounds are likely to split. These chemicals are basically water-miscible, slow evaporating solvents and have been used in car anti-freeze before now. They replace the water in the wood and retard the drying process, but do not stop it. In the end the tension of the wood shrinking will open up a radial split. What you can do is to cut fillets from one of the rounds to fill the splits in the others. One nice touch is to cut large sectors from two logs of different colors, then bond them together to give a dartboard effect.
From contributor D:
I think your project is doomed to cracking for the reasons above. While contributor S has a more evolved solution, the rounds will still open up. Changes in relative humidity and local extremes (cutting boards do get wet) will cause these to swell and shrink. The radial nature will not allow any movement, hence the compression as they swell, and then the inevitable crack when they shrink. Since they are all end grain, the MC exchange will be rapid and dramatic. There is a reason one doesn't see end grain rounds anywhere for long - they want to self-destruct. Then there is the issue of antifreeze type treatment being food safe. We now know that finishes are safe to eat once cured out, but the edibility (unintended) of the treatment?
From contributor S:
Once the timber had settled to an appropriate MC (10-12%) I would saturate it with a food grade oil to provide water repellency and advise the customer to keep up the oiling.
From contributor W:
Check the news - children have died from polyethylene glycol in toothpaste made in China. Don't use in where food will come in contact and check that your toothpaste wasn't made in China.
From the original questioner:
Thanks everyone for your advice. It looks like my plan needs a major course correction.
From Gene Wengert, forum technical advisor:
The most noteworthy publication is by Harold Mitchell "How PEG helps the hobbyist who works with wood." printed in 1972. USDA For. Serv., U.S. For. Prod. Lab.
First, PEG will eliminate 100% of the splitting is properly applied. Second, PEG-200 to 300 is used for a laxative and in toothpaste (and other products). In wood PEG-1000 is commonly used. Third, this is not anti-freeze, which is ethylene glycol. I encourage you to put your plans back.
Incidentally, various monomers are put into wood and then polymerized to form a hard composite used for knife handles, etc. Acrylic is one. Heat can be applied with radiation or hot air, etc.
Some so called "food grade oils" will turn rancid with heat and time. Choose an oil approved for cutting boards to avoid this problem.
From contributor W:
I noted at the FDA site a huge range of PEG variations and a large number of FDA documents. The problem for us little guys finding out what is what in what we use and taking the time to find out. I guess I would rather work out some other way to do this project and not make wood into a plastic.
From Gene Wengert, forum technical advisor:
PEG may not be the best for butcher blacks, but for occasional tables and similar, it is great.
Without PEG, you can expect a splat, so the best idea is to cut three discs and put one saw kerf (bark to center) in each disk and in a different place in each one (about 45 degrees from the adjacent one). Then dry all three. Then cut a pie shaped wedge from the 1st and 3rd disks; the wedge should include the split. Then go to the second disk and cut a wedge that will fit in the hole in the 1st and in the 3rd. By matching the grain and position exactly, the grain will match and 1st and 3rd will look perfect.
