A Word on Wood Movement

I’ve been sitting near the old gas heater all day, doing quotes and various bookwork I rarely make time for. The table saw was a little hungry yesterday, and took part of my finger. It wasn’t terrible, just a deep cut that required some stitches, but Hannah’s ordered me indoors today, which is probably for the best. On the upside, I’ve gotten quite a bit done today, including updating this poor website, which I’ve neglected for some time. In any case, be careful out there, all ye who would be crafty.

I thought I’d put a few words down about wood movement, since in this week alone I’ve had to deal with two different pieces that required some level of rebuilding because of issues caused by wood movement. As one who attempts to build furniture that is long-lasting and durable, wood movement is ever on my mind during both the design and building stages of any given piece. Wood is an organic material, and for those of you who don’t know, it expands and contracts with the fluctuating level of ambient humidity. That is, when the humidity rises, a board will expand across its grain - for instance, a board that is 4.5” wide under one condition, might expand to 4.6” in a more humid condition. This elasticity creates all kinds of problems for the woodworker. Drawers that once fit become stuck in the case; gaps show up between boards that once were non-existent, etc. Some people believe that if you coat a board in enough finish it will prevent this movement. Maybe this is true of a thick epoxy finish, but in all practical cases, if the air can hit it, so can moisture.

I’ve always designed my work for wood movement, but have worried less about the wood itself. That is, I use air-dried lumber from time-to-time, specifically walnut (Kiln-dried wood is brought down to a lower MC, or moisture-content, which is closer to the ambient MC of a home with air-conditioning). This is partly due to cost, and partly due to the fact that much of the kiln-dried walnut one finds on the market these days has been steamed, which is a real bummer to look at. As far as I understand it, the lumber industry started steaming walnut to intentionally blend the stark color differences in the sapwood and heartwood, to give it a more uniform color, and to allow the industry to cut down younger trees, with a higher sapwood to heartwood ratio. It looks bad though, when set beside the real thing. It’s fairly obvious to my eyes, and I’m surprised they do it at all to such a valued lumber. Anyway, so I use air-dried, in cases when wood movement shouldn’t be too big an issue, especially since walnut is fairly stable. However, this recent experience has got me thinking a little differently on the matter. I think that if you’re out there wondering if using air-dried could give you problems down the line, just get the kiln-dried stuff. It’s not worth it. No amount of conscientious design can predict the sort of backflips a stick of air-dried will go through once it enters a home.

The second piece which gave me problems was a vanity with a bunch of drawers. I wanted to use poplar for the bottoms, rather than plywood, which doesn’t expand and contract like hardwood does. Because my shop is open air, and will be until I can air-condition it, I have to try to predict what my timber will do once it leaves my shop and enters a home. Usually, I'm expecting everything to permanently shrink, but what I didn’t account for on these drawers was the possibility that, while still in my shop during a dramatic humidity spike, it could expand considerably. The bottoms of my drawers busted out, and every one of them were ruined. That may be the last time I use anything other than plywood for my drawer bottoms. It’s really the best option, since the difference between a racking drawer, and one that isn’t, is often just a fraction of an inch. This is all to say that if you’re not worried about wood movement, you should be.

So, we all learn our lessons…I try to keep in mind something I read once about how they used to use wooden wedges at the quarries: They’d hammer a wedge into a crack in the rock face, and then pour water on it. The expanding wedge could not even be stopped by rock, which would break under the force. So, be careful out there folks, both with your fingers and your wood movement!