Casehardening, Checking, and Warping

Dry lumber that warps when resawn offers a lesson about casehardening and its effects. May 27, 2008

I resawed some curly big leaf maple and it sprung open like a clamshell, becoming pretty badly bowed. The boards are roughly 7/8" thick and I was hoping to be able to get 3/4" once planed, but will be lucky to get 1/2" final thickness, probably a bit on the thin side for a dining table top. I am thinking of trying to wet the concave surfaces and pile a bunch of weight on to try and flatten them a bit. Anybody had any luck flattening bowed boards this way?

Long version... Awhile back I bought some rough 2" x 9 1/2 -11" x 8' planks of big leaf maple with the intention of resawing them and making a dining table. I cut them in half and built a sled so that I could use my planer as a pseudo jointer. To my dismay there were quite a number of checks below the surface, visible once I had planed the boards down. I was pretty disappointed but they have been sitting in my shop (flat with sticks in between) for about 2 years now and I am over the initial disappointment. It's beautiful wood and a few "stretch marks" aren't going to ruin it for me. Anyhow, as mentioned above, I guess there were major internal stresses which were released upon sawing them open. I guess the lesson in a case like this is to resaw right away and then sticker, especially with such (in retrospect) obvious signs of improper drying and internal stress.

Forum Responses
(Sawing and Drying Forum)
From contributor T:
In order to address your first question, we need some more info. Were the boards flat-sawn and ring-centered? Was the pith centered in the log? How far apart did you put your stickers and were they aligned vertically? Were there large knots in the boards?

From Professor Gene Wengert, forum technical advisor:
It sounds like when you resawed the lumber, the moisture gradient from shell to core was zero (as you stored them for two years). Did the two resawn pieces then warp immediately when resawing and warp so that they were "mirror images" of each other? If so, then you have drying stress, often called casehardening. Casehardening causes immediate cup when resawing and also immediate bow. Casehardening is nearly impossible to remove once the lumber has been resawn, but is easy to do with rough lumber.

It is common, when lumber is dried too quickly, for it to develop checks on the surface. These checks can go rather deep at times. Further, they are aggravated by wet/dry cycling, such as in air drying when exposed to rain. As the wood dries, the core will eventually dry and shrink and pull these checks closed on the surface. They are found only when the lumber is planed or machined.

It is worth noting that checking is very closely associated with casehardening. If interested in the technical details, read about the four stages of drying in 'Drying Hardwood Lumber.' So, the checks and your warping due to casehardening that was not relieved in the kiln go together. Sawing patterns, ring patterns, pith location, sticker alignment do not affect warp from casehardening when resawing to a great extent at all.

From the original questioner:
Thanks for all your input! This was store bought wood but I was directed to this forum because of your knowledge of wood drying and it seemed like a drying issue. It reacted just as you said, warping immediately in mirror images and, yes, it's pretty bone dry. I will try to straighten them but probably will end up with real thin wood and some sort of support structure for my dining table. I really can't afford to buy new wood at this point.

I bought this wood at a reputable hardwood store and the boards bore a stamp from a major wood company, nonetheless I got ripped off as I paid quite a lot for this wood. I wanted big leaf maple because I live in western Washington and there are a bunch of these trees around and several on my property. In retrospect I may have been much further along looking for a local mill. Thanks again for all your help and I will definitely study the art of drying wood so I can avoid this sort of disaster in the future.

From contributor J:
You should consider veneering it at this point. That way you can have the look you want with a nice thick, flat tabletop.

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

Comment from contributor A:
I would like to suggest seaming the wood (with an iron and wet cloths) and r-shaping. It is a tedious process that will produce results. You then might save enough wood for the top after the jointer. I spent $35 a bf for some rather remarkable big leaf (extreme quilt in a 14 by 60 board) and used this approach due to the cost and quality of the wood.