Drying burly slabs

Keeping oak slabs from cracking. January 16, 2002

We've just milled a 200 year old, extremely figured white oak, standing dead for 3 years. We cut off half a dozen 80 pound burls from two sides and slabbed the rest: 16 pieces, average 2 3/4" x 40" x 12'. The burl in the slabs penetrates way back in time, near the pith. The wood is all very sound and incredibly beautiful, with some insect life (pin holes, a couple of tunnels and one 3/8" white worm), but I can see all of it ruined if not treated ideally. What should we do?

Forum Responses
Drying white oak slabs is somewhat of a problem as you cannot impregnate white oak with any helpful chemicals. Putting them all in a freezer would work, although it would take a lot of time. Otherwise, very slow drying...very slow. (Air drying in a shed, for example.) Yet not too slow, as then mold and mildew will develop. In short, this is tough and requires good luck. The cool weather now will help.

If the slabs develop a large crack, perhaps you can use the wood from an adjacent piece for repair...cut a pie-shaped (wedge-shaped) piece out of the good slab, with the wedge-shaped piece having the crack in it. Then cut an identical sized piece from the adjacent slab and glue this second wedge in the opening in the good slab. The grain will match nearly perfectly.

Gene Wengert, forum technical advisor

White oak burls crack, but don't worry--this is part of the rustic character of white oak. When there is a possibility of someone getting hurt from a table with a crack in it, e.g., a child getting a finger caught, I put filler in it or cut a piece of walnut and glue it in with a dark stained epoxy filler.

The above is true, but if you can dry them without cracks, the turners will love you to death. A heavy coat of Johnson's paste wax on exposed areas of the burl will slow the drying (a trick used by green turners I know).

The paste wax sounds like a good idea, or you can use end grain sealer (also wax-based). The best, I think, is sold by Baileys. Make sure to get your goop of choice into those tunnels, too. Cool dark places are also key.

From contributor J:
The critical time is right after you cut the wood. The surface wants to shrink. The rays form weak points where the wood will readily crack. The trick is to have the wood dry slowly at first, so cracks will not develop. Then after the surface has dried somewhat, speed up the drying to reduce the warpage.

This is accomplished in a few ways. For boards, you can drape burlap or mesh over the stack to reduce the airflow through the stack of wood. For chunks, use paste wax or wrap it in newspaper or encase it in clean sawdust in an onion bag. Within the newspaper or sawdust, a moisture gradient will develop starting at about 12% on the surface (EMC of air) to about the moisture content of the chunks. The moisture gradient in the chunk will be very small and therefore generate very little stress to cause cracks. As the drying progresses, the rate of drying can be safely increased by removing some newspaper or sawdust.

One point on your side is that the wood may have dried some since it was standing for a few years, reducing the chance of high stresses forming. If you know what you will make out of it, rough shape it to the desired size. Leave it heavy to allow for shrinkage and warp. Then put it in newspaper, sawdust or cover with wax. As Gene said, the weather turning cold now is also on your side, reducing the drying rate.

Lee Valley sells two products that might interest you. Pentacryl Wood Stabilizer and PEG.

The chemicals mentioned above will not work. The white oak pores or vessels are plugged with tyloses.

When drying a circular or oval cross-section, one of the problems is that the wood wants to shrink about 4% in diameter. But, where can the wood go? (Drilling a large hole in the center is one solution.) Further, the outside circumference wants to shrink (with white oak) as much as 10%. With the outside shrinking that much, it is nearly impossible to dry it without cracking. What contributor J said is fine (you can have some success) for some species that do not shrink so much, but not for white oak.

Wood can absorb some shrinkage, but drying it fast or slow, the wood will be trying to shrink up to 10% and that is way too much for the wood to absorb without cracking. So, that is why some people will make a kerf with a saw where they want the crack to be and then the stress will all show up at this spot. It is then easy to repair with a wedge piece from an adjacent slab, as I mentioned.

Gene Wengert, forum technical advisor

I received several e-mails that asked for clarification. The tangential shrinkage, which is the shrinkage tangent to the rings, or the circumference, for white oak is about 8% or larger. Using the math, the circumference is pi times diameter. This means that a 20" diameter disk would shrink (or try to shrink) 8% of 20" or 1.6" in diameter! This is certainly quite a bit of shrinkage and hopefully, because it is hard to compress wood 1.6", you can understand why a crack develops. Slow drying gives the wood time to relax the stress a little, so very slow drying (much slower than contributor J implies) can be helpful. Putting the wood in a freezer is one way to get slow drying without mold growth, fungal stain, and so on, which would be a risk in the method contributor J mentioned. But even then, expect a crack with oak.

Gene Wengert, forum technical advisor

If the tree has been dead for 3 years, wouldn't the majority of shrinkage have already have happened?

From the original questioner:
Although the tree was standing straight, it's mighty wet. The wood is extremely dense and complex. I've retained parts of the outermost squaring slabs intact in relatively thin sections, including some smaller burl structures, for less than two weeks. Lying face down on an irregular wood platform equivalent to 1/4" stickers, in a closed dirt floor shed, it's doing fine. Face up in a dry basement, a piece with just a 3"-4" radius burl is cracking up. All the typical problems seem exaggerated, so tight control is the intuitive response (but the wife nixes the truck box freezer in the yard).

What is tyloses? Would water penetrate the burl?

The cells in the tree that are vertically aligned typically have the major role in life of carrying liquids from the roots to the leaves. As such, they must be wide open--very permeable to liquids. However, as these cells age, there are chemicals that are deposited into the void space. In the case of white oak, these chemicals are very effective in plugging the cells to the flow of liquids. These "plugging chemicals" are called tyloses.

Hence, we use white oak for barrels that have liquid in them (wine, whiskey, etc.) and the wood does not leak. (Interestingly, not all white oak is plugged--for example, chestnut oak is not.)

Gene Wengert, forum technical advisor

But the white oak has to be quarter sawn for barrel staves, not flat sawn, or the barrel will leak for sure.

The barrel may leak at the joints, but the wood will not leak! With red oak (and a few white oaks), the wood will leak!

The reason we like quartersawn is that the radial shrinkage (the width shrinkage in a q-sawn piece) is fairly low, while the tangential shrinkage (which is the width shrinkage in a flatsawn piece) is quite high.

Gene Wengert, forum technical advisor

Tyloses are also in black locust. This is why black locust and white oak are good for posts. Moisture and bacteria, etc. can't get in the plugged holes.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
We have been drying oak for several years. After cutting the tree in slabs, we apply a paste made of 3# table salt/1 gal water. Bring to a boil and mix in corn starch and water until it thickens to the consistency of batter. When cool, add one cup of borax to inhibit bacterial growth, then three or four egg whites to keep the paste from flaking. Cover both sides of the wood and stack on stickers or spacers to dry.

Comment from contributor B:
I get the same results with the old timer's whitewash: quick lime and salt. It inhibits mold and fungus and, I believe, actually draws moisture to the surface. Whatever it does, I get far less drying defects when it is used.