Read Part 1 here.
When predicting what a thick workbench top will do, I don’t consult the tables in the Forest Product Lab’s “Wood Handbook: Wood as an Engineering Material.” Instead, I prefer to think that building a thick benchtop is like moving into a haunted house.
Yes, the haunted benchtop is going to give you trouble at first. It’s going to scare the plop out of you when you stare into a gaping crack that seems to open up more each day. Your tools will behave erratically on the new benchtop, like they also have been possessed.
But if you are patient and observant, you will come to an understanding. Soon the ghost will be serving you tea.
Does that magic moment occur when it dries out and reaches “equilibrium moisture content?” No. I don’t think that thick benchtops follow the same rules of moisture loss that we expect from thin stock.
Benchmaker Richard Maguire recently told me about a slab-top workbench that he built from 50-year-old stock. After 50-plus years of drying, it still gave him fits.
“After all those years I imagined it was as dry as it was ever likely to get, and yet when I cut or drilled in to it it was apparent that there was too much moisture still in the middle,” Maguire wrote. “Thinking to the original French benches with these thick slab tops, I’ve begun to feel that these must have been very wet when built. Just based on the amount of time it takes for this thickness of timber to dry, I don’t imagine they had a good 50 years or more to leave it lying around.”
His experience lines up exactly with mine. When dealing with thick stock, getting the wood spirits out of the middle are next to impossible – and probably not necessary.
After observing how these benchtops behave, I have a theory for you to consider. I hope to ask the scientists at the Forest Product Labs about it someday. Here it is:
When you cut open these thick slabs, they dry from the outside-in and from the ends primarily. (That’s not the theory; that’s well-established). As the outsides dry, they move as they start to reach equilibrium moisture content. The middle stays pretty wet. Why? Don’t know. Perhaps something about being surrounded by so much dry wood keeps the moisture in.
But here’s the important part: I’ve found that the outside gets dry and hard – like the shell of a lobster – and is less susceptible to gross movements by further drying from the inside.
So waiting for a thick slab to reach equilibrium might never happen. And it might not be necessary.
A good example of this is the all-handtool Roubo bench I built for Popular Woodworking almost four years ago. The top had been in Ron Herman’s log yard for a long time. It was wetter than the French oak we used in Barnesville, Ga., this summer. And it was punky. But it was what I had to work with, so I built the bench with it.
The first year with the bench was rough. I flattened the top twice. It shrank around the legs, leaving the end grain proud. It didn’t, however, distort all that much. I thought of the top like a slat-bending frame you use for chairmaking. You put a wet slat in a bending frame and it stays that shape. You put a wet benchtop in a frame of legs and it is somewhat restrained from distortion by the legs.
It has been three years since I messed with the top of that cherry-top Roubo, and it is still functioning quite well. After my oak bench went wonkers on me, I decided to see how out the cherry bench was doing. Three passes with a jack plane brought it back to perfect.
So I wouldn’t be afraid of slab benchtops – even fairly fresh ones. Just remember my theory, which I’m sure is as accurate as my theory that a gnome or small dwarf in my stomach makes me sick each winter.
— Christopher Schwarz