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.
Chris gives you geometry homework; I give you partial differential equations:
This is the diffusion equation. It’s often called the heat equation, because it also models the diffusion of heat through a solid. And you’ll sometimes see it as Fick’s Second Law, after Adolf Fick, who first formulated it around 150 years ago. (The Wikipedia article on Fick’s Laws is a good introduction to diffusion from a physical, rather than abstract math, point of view.)
In our case, the quantity of interest, symbolized by u in the equation, is the moisture content of the wood. While it looks formidable, the interpretation of the diffusion equation is actually fairly simple:
Gradient: You have more “stuff” in one place than another, and stuff tends to move from high concentration to low concentration; the gradient measures the steepness of the slope.
Diffusivity: Also known as the diffusion coefficient, this is a property of the material and represents a sort of speed limit, controlling how fast stuff can move.
Divergence: This is a little trickier to wrap your head around: It measures the three-dimensional shape of the gradient. Imagine that you’re in a boat, bobbing up and down on a choppy sea. At a specific moment, you look around you and see that you’re on the crest of a swell, and the water all around you is lower than where you are. What’s going to happen next? The water underneath you is going to flow away in all directions; you’re in a spot with high divergence. Conversely, if you’re at the bottom of a swell, all of the water is going to flow towards you. That’s also a large divergence, but of the opposite sign. But if you’re on the side of a swell, even though there’s a gradient, you’re not going to move up or down because the water is going to flow right past you, from the high side to the low side. That’s a spot with very low divergence.
So now you know the key to surfing: Find a spot along the leading edge of the wave that has zero divergence, and just stay on that spot as the wave moves towards shore. What could be easier?
If the diffusivity of wood were a simple number, we’d be able to very quickly solve the diffusion equation for any specific set of conditions. Unfortunately, it is anything but. It seems to depend on just about everything, short of the phase of the moon:
Temperature: Diffusivity exhibits Arrhenius behavior, increasing rapidly as temperature goes up. (This is why kiln drying works.)
Density: Diffusivity is higher in low-density, lightweight woods than it is in high-density woods, so low-density woods dry faster than high-density woods. In fact, density is a pretty good proxy for species as far as drying is concerned: Woods of the same density tend to dry at the same rate (there are prominent exceptions, however; oaks dry more slowly than most other woods that are comparably dense). This also explains why extremely dense tropical hardwoods seem to never dry at all.
Orientation: Diffusivity is anisotropic, meaning that it has different values in different directions (the technical term is symmetric rank-two tensor). In wet wood, at moisture contents of 25% or higher, diffusivity along the grain is roughly 2-3 times as large as it is across the grain.
Moisture: Diffusivity is also a function of the moisture content of the wood. But it’s complicated: As the wood dries, the diffusivity along the grain increases (as long as the wood isn’t extremely dense), but the diffusivity across the grain decreases. At a moisture content of 5%, the disparity in along-the-grain vs. across-the-grain diffusivity in a medium-dense wood is a factor of 100 or so.
We’re going to use a technique called finite element analysis (FEA) to model the moisture flow through wood. (I’d give you a link to a good layperson’s description of FEA, but I have yet to find one that doesn’t immediately descend into esoteric math.) Suffice to say that FEA works as follows:
Chop the problem up into a very large number of tiny little pieces,
make some straightforward assumptions that simplify the equations within each piece,
solve the simplified equations for each piece, and
stitch the gazillion solutions all back together.
FEA is a technique that could not have existed were it not for powerful computers.
I’ve set up a web site at https://github.com/Steve-OH/lap_water where you can download all of the stuff that I’ve put together for this post, and run your own simulations, tweaking the parameters as much as you want to. I’ll leave the hard-core technical discussion to the documentation there, and just focus on the results of some simulations here.
The first problem that we’re going to look at is a simple workbench slab of white oak, 18″ wide, 96″ long and 6″ thick. The initial moisture content is 30%, and the equilibrium moisture content is 10.3%. Here are the results after a simulated 6 months (click to enlarge):
(The three panels represent cross sections of the slab cut through the center along each of the three principal planes, laid out like a conventional three-view mechanical drawing.) Note that the end grain is drying noticeably faster than the faces and edges. After one year:
Here, we’re starting to see the moisture-dependent change in diffusivity really take effect, as the end grain drying starts to take off. After two years:
And after five years:
At this point, the slab has lost most of its moisture, so the rate of moisture loss is slowing down considerably. Even though it’s not fully dry yet (the center is at about 13.5%), the slab is nevertheless “settling down” and further changes will happen slowly. The ends are much drier than the center, so they’ve probably cracked quite a bit from differential shrinkage.
What about coating the end grain to slow down moisture loss and avoid major end cracks? I decided to explore that idea, but hit a road block trying to find published data on the diffusivity of water in wax (or any other candidate for end coating). So I used the best data I could find, and adjusted the thickness of the coating so that moisture loss through the ends was negligible. If you look at the simulation parameters, my model wax accomplished this at a thickness of 0.010”, but I think in real life you will probably need a thicker coating to get the same effect (i.e., my model wax is more impermeable than real wax). Anyway, here’s what it looks like at one year:
You can clearly see that we’ve stopped moisture loss through the ends. At five years:
What’s most striking here is that although we’ve completely sealed the ends, the peak moisture content in the center is almost exactly the same as it is in the uncoated case. So in the long run, the ends aren’t as important for drying a large slab as one might think; the take-home lesson here is that it probably always pays to end-coat a thick slab to prevent (or at least reduce) cracking, even if you’re in a hurry to dry the slab.
What about drilling holes in the bench to speed up drying? I modeled the simplest variation of this by means of a 1”-square hole, cut all the way through, smack dab in the middle of the slab. After one year:
And five years:
As you can see, the hole definitely works, but it’s also obvious that the effect doesn’t extend very far, so you’re going to need a lot of holes to adequately ventilate the whole slab. And, you’re probably going to get some end-grain cracks inside the holes, which certainly isn’t a good thing.
But even if you could do it, is it a good idea? Consider a simple thought experiment: Take a board, flat sawn and about 1/2″ thick and 8″ wide, and plane it nice and flat. Do the same with a 2″-thick board. Now wait until the weather turns rainy for a few days. What will happen to the two boards? The thin one will have an obvious cup in it, maybe as much as 1/16″. The thick board, on the other hand, will hardly have budged. The reason, of course, is that the thick board has more “moisture inertia.” In order to get it to move, you need to add a lot more water, over a longer period of time, than you do to the thin board.
And that’s the real advantage, from the point of view of stability, of a thick workbench slab: It has a lot of moisture inertia. So while it may take several years to fully settle down, once it has, it’s going to stay settled down. By drilling the slab full of holes, you eliminate that inertia. While you do speed up the initial drying, you also speed up its response to day-to-day changes in the weather. So it’s going to dry quickly, and then keep getting wetter and drier and wetter and drier forever.
Some of you may have noticed that the drying times in the simulations above are shorter than in the graphs I presented in the first installment of this series. Part of that is the result of what I mentioned above: the model is based on density, and oaks dry more slowly than other woods of the same density. But that doesn’t seem to be enough to explain all of the difference. I think the rest of the story involves free water, which, as I mentioned last time, is supposed to move freely through the wood, but might not in at least some species. Unfortunately, there appears to be zero published data on free water movement through thick oak slabs, so I may have to do the experiment myself: The next time I find myself having to fell an oak, I’ll sacrifice a few large, thick pieces in the name of science. Of course, it will probably take the rest of my life to amass enough data to be able to come to any conclusions, so don’t hold your breath.
One final caveat: As we all were reminded a few posts back, the map is not the territory. Models are useful for gaining an understanding of what’s happening in the real world, but they are simplifications. Wood is a complex, highly variable material. Don’t just blindly follow formulas when drying wood; use them as a baseline—a way to intuit what’s going on—but don’t forget to listen to what the wood is telling you.
In man there is what may be termed a “making instinct,” and our houses, garments, ships, machinery, and in fact, everything we use, are the practical results of this instinct. How important then that this faculty be cultivated, and that the idea be at once and forever abandoned that none but mechanics require this great element of usefulness and happiness.
Whatever a man’s occupation, whether he be a farmer, a merchant, an artist or a mechanic, there are hourly occasions for its practical application. Being thus general in its usefulness, the cultivation of this constructive faculty should be a primary consideration with parents. Skill in the use of tools is of incalculable advantage. It gives useful employment to many an otherwise idle hour. It prompts one to add a thousand little conveniences to the house, which but for this skill would never be made.
In a word, it is the carrying out, in a fuller sense, of the design of the Creator, when he implanted this faculty of constructiveness within us. Let it then be cultivated in children. Indulge the propensity to make waterwheels and miniature wagons, kites and toy boats, sleds and houses, anything in fact which will serve to develop it and render it practically useful.
Give the boys good pocket knives, and what is better, give them a good workshop. Employed in it, they will not only be kept out of mischief, but they will be strengthening their muscles, exercising their mental powers, and fitting themselves for greater usefulness, when they shall be called upon to take their place in the ranks of men.
Planing a board of any width and length by hand, so that it will be true, need not call for any special talent on the part of a patternmaker, yet tradition and bigotry are responsible for the survival of the belief that expert handling of “winding strips” furnishes the only correct means of planing a board surface out of wind.
As this apparently hard-and-fast shibboleth has still many adherents, an attempt to shatter the long established belief and practice is at best but a thankless task. The use of “winding strips” is so manifestly unnecessary that the chief wonder to any thinking man is, that the use of them didn’t “die abornin’.” (more…)
I took a vacation day on Friday, ostensibly to work on my kitchen rehab – but my lunch “meeting” and shopping trip were a great deal more fun than installing a sub-floor and leveling cabinets. Chris and I met at Tom+Chee for grilled cheese and tomato soup, then ambled the few blocks down to one of my favorite places in Cincinnati: Ohio Book Store.
With five floors of used books, it’s the perfect place to search for 1930s novels to use as exemplars for the design and production choices for Roy Underhill’s forthcoming book, “Calvin Cobb – Radio Woodworker! A Novel with Measured Drawings.”
Because Roy’s book will feature illustrations, we spent a most of our limited time (we were both parked at 1-hour meters) perusing the shelves in the “juvenile” section, thinking books therein would be most like the look we want…not that we yet know exactly what that look is.
The content of the volumes didn’t matter to us at all; they just had to look good. But rest assured – we weren’t just judging the books by their covers – we also judged their bindings, typography and layout. (Don’t worry; we’ll skip the self-destructing acid paper common in the period – that’s a verisimilitudinal* step too far).
I think we got four books for less than $20, each of which had things we liked and things we didn’t – or things we loved but are likely impractical (inked edges on the pages, for example). We’ll pass these along to the designer for inspiration…a hand-off I’m hopeful will come soon, so she or he can get started on the layout and font selection.
I mentioned last week that I’ve the updated manuscript in hand, and I’ve now completed a full read-through and have just a couple of questions for Roy. The next step is to talk with Roy about my notes and any additional edits he wants to make, then fact-check all dates, building names, numbers etc., and finalize the illustrator. We’ll be working on the drawings alongside layout, leaving blank pages where the art will later be dropped in.
Right now, we’re on schedule to have it edited, illustrated, printed, bound and on its way to you well before the end of the year.
