But is it dry enough?

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:

Image

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

 

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13 Responses to But is it dry enough?

  1. Steve, maybe that’s why the Nicholson and later benches all have thinner tops?

  2. wortheffort says:

    I always though that was a woodturners rule for wall thickness’s of bowls green bowls that just got propagated elsewhere.

  3. Luke Barnett says:

    how about thin wood, lets say half an inch. would the drying time be 3 months??? also what about a piece of wood that is small?? say 1.5 inches thick by 4″ by 20″.

  4. Luke Barnett says:

    I apologize. I just read the other posts, and I see I am way behind. sorry for being that guy.
    but how does length affect drying time. the reason im asking is because I am fighting with some 20″ long chunks of red oak right now. I need them like yesterday. I only need two, but I cut five chucks, because I anticipated drying problems. I put one in the kiln and it pretty much exploded like that fellas bench. luckily no one got hurt.

  5. Also, from where I,m from anyway(Quebec/St-Laurence valley), They use to harvest wood for sawing only in the winter(september to april) months so the amount of moisture was quite lower to start with and then probably that the 1 year/ inch was quite accurate?!?!?! But it is sure not the case any more, so when trees are harvested in the late spring and summer, when the sap is running at its peak, you end pup with wood that has more water within its body, and takes more time to dry and hand up moving more to.

    That is what the old timer back home told us!!

    Cheers
    David

  6. dzj9 says:

    The rule might stem from the Old Continent. In most places in Europe, the minimum EMC is around 12%.
    This shortens the drying time (according to your calculations by about 25%) and makes the curve less steep, giving it a more linear appearance.
    Looking at the amount of variables governing the process, I doubt that it can be described precisely by the usual analytic geometry repertoire anyway.
    It’ll always be in the ‘close enough’ ballpark.

    +1 on context.

    (There is raw data concerning air dried slabs?)

  7. These have been great articles, thanks Steve. I also think the question was “Is it dry enough?” Also, as a practical matter, I can’t really think of much in the way of a use for such thick slabs traditionally. Timber framing, as you noted, you can use pretty green wood. If a woodworker’s benchtop moves a bit, he gets out his plane and makes it flat again. On the old wooden ships, they routinely worked with planks 6-8″ or more thick, but that is pretty specialized work. In terms of joinery and cabinet making, using wood more that an inch or so thick would really have been kind of unusual. dzj9 also has a point about the normal ambient humidity in Northern Europe, and probably it is pretty similar in New England, where the first settlers came…

  8. Sean Hughto says:

    Will you be adding in the further complication of the structural differences among wood species as far as how they react to drying. Some species just sort of get lighter and don’t move much or crack, while others twist and curl and crack at the slightest glance. In short, “is it dry enough?” doesn’t just change with time or with intended use, it also changes among wood species and even sometimes, in my experience, between examples of the same species.

    • steveschafer says:

      I’d love to, but there just isn’t much data to draw from. The information available is mostly just descriptive and anecdotal; there’s been very little investigation on how those kinds of properties correlate. There are some general principles, such as the fact that the most ornery woods also tend to be very hard, but it doesn’ really go beyond that.

  9. Jay C. White Cloud says:

    It is funny how perspective run and how a profession or hobby can have a sub culture of a sort. As a traditional Timberwright and general “green woodworking” practitioner it has always been either, “is it too dry?” or “thats dry enough for our needs.” Coming to woodworking from mainly a logging (no winter cut wood actually has more sap and was cut in wither for logistical reasons not moisture content), timber framing, and green woodworking background, I (we) have made everything from tables to armoires with what most wood call…”green wood.”

    The question should be…”is it dry enough.”

    Regards,

    j

  10. jacon4 says:

    Interesting subject, is it dry enough? As above posts note, it all depends on what you are using timber for. Several years ago, i needed a slab of walnut 27 inches wide X 5 feet long X 1 inch thick for a table top, found one at a boutique saw mill for $500. This board had been air dried for 4 years at which point MC was 20%, kiln dried for final MC of 11%.
    It has not split, warped,,checked,cupped or any other problems, it just sits atop table in all it’s glory, perfectly flat. Did this board really need to be air dried for 4 years? I don’t know but i do know that it adds to the cost quite considerably.

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