I’ve expended quite a few electrons recently, demonstrating that the “one year per inch of thickness” drying rule-of-thumb doesn’t work with thick slabs, both in terms of actual experience and in theoretical models of how wood dries. But that begs the question: Why, then, does the rule even exist? I haven’t been able to dig up any real evidence, but I can think of a few possibilities:
It’s close enough: For relatively thin boards (up to about 2″ or a bit more), it could be that our predecessors just figured that the rule was close enough. After all, 8 months is sort of a year, and 30 months really isn’t that much longer than two years, right?
We’ve got really wet wood: Some woods contain a huge amount of water when green. Such a wood, especially if it’s fairly low density, contains so much free water that getting rid of the free water can have a significant impact on the drying times. An example of such a wood is American chestnut—a species favored by our predecessors—whose green moisture content is a whopping 120%. Free water removal can make the initial stages of drying look more linear:
However, as the graph shows, the rule still fails with thick slabs.
We’re asking the wrong question: What if the answer is correct, and it’s the question that’s wrong?
“Alex, I’ll take ‘Woodworking Maxims’ for $600.”
– Jen Kennings
What if rather than, “How long will it take my wood to dry?” the question to the answer were actually, “How long until my wood is dry enough to use?” Let’s say that you have a 6″-thick slab of white oak that’s been drying for six years. Is it dry enough to use?
- If you’re going to use it to timber-frame a barn, it’s more than dry enough.
- If you’re going to use it as a cabinetmaker’s workbench, it’s probably dry enough, although it will continue to move a bit for the next several years.
- If you’re going to make a Mid-Century Modern slab coffee table out of it, it’s probably not dry enough, since your customer is going to be upset a few years down the road, when it warps to the point that the ends have cracked and stuff starts rolling off the top.
- If you’re going to cut it up into cabinet parts, it’s definitely not dry enough. In that case, you pretty much have to restart the drying clock from zero once the wood has been cut up.
So, context matters.
– Steve Schafer