Category: Uncategorized

By now, most people in the world should be viewing the Lost Art Press web site we launched this afternoon. As with every new web site, there are plusses and minuses (mostly plusses, we hope). Here are some changes you should be aware of.

1. We now take American Express and Discover Card.

2. We now offer a discounted price when you purchase both a hardbound book and the pdf of that book.

3. We have switched over entirely to pdfs for all our digital books. Sorry, ePub and mobi, you aren’t suited for books that are graphics-heavy. All pdfs will be served directly by our new store (no more two-step download processes). If you ever have problems with a pdf, ePub or mobi file you purchased in the past, just e-mail us and we’ll help.

4. No more Captcha code crap-ola to contact us. You can get to us directly here. This means more spam for John and me, but I grew up eating spam (actually Underwood deviled ham).

5. You won’t have to create an account to buy something. However, if you do want an account to make future checkouts easier, you will need to create a new one in our new store. Sorry about that.

There are lots more little changes, but those are the highlights. The biggest change is one you won’t see for a while. This new store will allow us to have your orders filled by a local fulfillment house, which will be faster. And, if all goes well, the new store will pave the way for us to offer international shipping. (Though I promise you that buying our books from our international sellers will always cost less than getting it from us.)

Huge thanks to Ben Lowery, who helped us realize our vision – both graphical and functional. If you need some spot-on web site work from a guy who is also a woodworker, we cannot recommend anyone more highly.

— Christopher Schwarz

I recently returned from a week and a half in Colombia. While there, I got to experience some of the temperature and humidity extremes that we subject our furniture to: We traveled from the warm, humid llanos at Villavicencio, to the cool, wet cloud forests near Bogotá and in the Santa Marta range, to the hot, near-desert thorn forests of La Guajira. Fortunately, I personally contain very little cross-grain construction, so I emerged without any significant structural damage, save for a few mosquito and chigger bites.

Water exists in wood in two forms: Free water is water that occupies the voids in the wood, and generally behaves like a liquid. Bound water is water contained within the solid structure of the wood itself, and behaves more like a vapor “dissolved” in the wood. For our purposes, free water isn’t very interesting, as it moves freely (duh) via capillary action, and doesn’t contribute significantly to moisture-related wood movement. There is, however, some evidence that movement of free water is hindered when the surface of a piece of wood is very dry, which may be another contributor to the why-won’t-my-thick-slab-dry? phenomenon. As Chris noted previously, thick slabs often act as if the dry surface wood somehow seals in the moisture in the bulk of the wood, and it may be the case that free water is being trapped.

Bound water moves via diffusion. Imagine a large room containing 500 drunken woodworkers (e.g., the banquet hall at Woodworking in America). They’re all in there, randomly staggering around. We draw an imaginary line through the middle of the room, and discover that there are 450 woodworkers on one side of the line, and only 50 on the other side. Why? Guess which side of the room contains the open bar?

When the bar closes, the distribution begins to even out. This doesn’t happen because the woodworkers are purposefully trying to move away from the bar; it’s just that anything moving around randomly is more likely to move from an area of high density to an area of low density than the other way around, simply because there’s more “stuff” in the high-density area.

If the room has open doors, then some of the woodworkers will occasionally exit and end up in the hallway. As long as there aren’t very many woodworkers out there already, there will be more woodworkers exiting than coming back in. Eventually, a point will be reached where there are as many woodworkers randomly coming back in as randomly leaving, and the system will be in equilibrium. This is diffusion in a nutshell.

In a future installment, we’ll get into the nuts and bolts of diffusion, but for now we’ll skip all that and look at some wood, namely three boards, each 8′ long and 20″ wide, and 1″, 2″ and 4″ thick, respectively:

Imagine that you’re a molecule of bound water, located smack dab in the center of a board. Your quickest escape is through the face of the board, as the edges and ends are too far away. If you’re in the 4″ board, you have to travel four times as far (2″) to get out than you would if you were in the 1″ board. Remember from last time that the drying rate in this case goes as the square of the board’s thickness, meaning that it’s going to take you 16 times as long to go four times as far.

Now imagine that you’re a molecule of bound water very close to the end of a board. Your quickest escape is through that end. As we go from the 1″ board to the 2″ board to the 4″ board, your local environment doesn’t change; you’re still very close to the end of the board. In other words, the rate at which you leave the board is independent of the board’s thickness. And this is what gives dry kiln operators nightmares (and why many of them won’t touch thick slabs): The disparity between drying rates at the center and ends of a board increases dramatically as the board gets thicker, and along with that disparity comes increased stress in the board. The rapidly drying ends shrink faster than the center, with end splits being the inevitable result, as in the photo above.

This also applies to the faces and edges of the board, but as we will see next time, it’s usually less of an issue there, except for certain species, such as oaks, that have a strong tendency to develop surface checks, especially on tangential (flat grain) faces.

Even though some of the boards in the photo aren’t cracked at the ends, they nevertheless contain a substantial amount of built-in stress, leading to an increased likelihood of cracking after the board is placed into service. Knowing how the board’s neighbors behaved, and depending on how it will be used, I might inset a butterfly key on the underside of such a board to help restrain it. If I’m really nervous about it, I might even cut the board in half lengthwise, plane the cut edges and then glue it back together, just to be on the safe side.

We can reduce the drying rate disparity by covering the ends of the board with a substance that retards diffusion. (Again, slowing moisture loss from the end grain is usually more important than slowing loss from face or edge grain.) People have used a lot of different materials for this. In my own experience, latex paint offers some protection, but seems to be more or less worthless in the long term. I currently use a commercial water-based wax emulsion product, Anchorseal from UC Coatings, with good results. The absolute best product for eliminating moisture loss that I’ve tried is Leak Stopper aerosol roof sealant, but it’s a bit messy and kind of on the expensive side. I’ve also heard that oil-based aluminum paint works well, but I haven’t tried it.

Next time: The scary-looking diffusion equation! (You may want to ask small children and sensitive adults to leave the room before reading.) Also, we’ll answer the question of whether or not it makes sense to drill holes in a thick slab to hasten drying.

 –Steve Schafer

Freshly-cut red oak, destined to become Roubo workbench legs

A few days ago, Chris broke one of the cardinal rules of this blog: he began a sentence with, “You should…” He said to me, “You should be writing about this stuff…as a full-on blogger.” I agreed, then explained that the reason I don’t is that I find writing to be painful. So what does he do? He offers to let me write on his blog. I suppose I should be thankful that he didn’t kill me outright, like poor Raney. On the other hand, Raney’s death was quick and presumably fairly painless. Mine will more likely be slow and lingering.

Wood and water. From the woodworker’s point of view, not exactly a match made in heaven. Why is it that thick slabs of wood take so %$#@! long to dry? You may have heard the old air-drying rule of thumb, “one year per inch of thickness.” I remember reading it for the first time many years ago and thinking, That can’t be right. From basic geometry, doubling the thickness of a piece of wood should quadruple, not double, the drying time, and my own experience with drying wood since then has at least approximately confirmed that. There are two factors that cause thicker pieces of wood to dry more slowly: one, there is simply more water to remove through the same amount of surface area, and two, the water has to travel further, on average. Both of these factors are proportional to the wood’s thickness, and they’re multiplicative, so in the simplest approximation the drying time goes as the square of the thickness.

I’ll skip the physics lecture (for now, but there might be an exam later) and show the results of some computer simulations that I ran for water loss in wood, from fully saturated to near equilibrium, using a few assumptions (physicists are all about making simplifying assumptions). The assumptions I made are:

• the board is wide and very long compared to its thickness, so I only have to worry about water movement through the faces of the board, and not the edges or ends,
• the temperature is a constant 52°F (the year-round average for southeastern Ohio, where I live), and
• the equilibrium moisture content is 12% (a typical outdoor value for this climate).

Here are two graphs (click to enlarge), showing the computed change in moisture content over time in red oak, in a 1″-thick board (left) and 6″-thick board (right), subject to the above assumptions and using published data for the various water transport parameters:

As the graphs show, the 1″-thick board settles down in a couple of years, while the 6″-thick board takes over 70 years to equilibrate to the same degree. Apart from the 36-fold difference in time scales, the graphs are nearly indistinguishable. In other words, there’s no significant qualitative difference between thin and thick boards; they both behave more or less the same, just on very different time scales.

So that’s at least a partial answer to the puzzle. In part 2, we’ll look at the role that end grain plays in drying (there’s good news, and there’s bad news), and examine Chris’s hypothesis about how “being surrounded by so much dry wood keeps the moisture in” (hint: he’s at least partly correct).