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.