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).
Bergman R et al. (2010) Wood Handbook, Wood as an Engineering Material, http://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf.
Hoadley RB (2000) Understanding Wood, Taunton Press, Newtown CT.
These first two are fairly non-technical, but they focus on the what and have little to say about the how and why.
Baronas R et al. (2001) Modelling of Moisture Movement in Wood during Outdoor Storage, Nonlinear Anal. Modell. Control 6:3-14.
Includes an excellent comprehensive overview of the theory of moisture movement in wood; requires a working understanding of partial differential equations.
Simpson WT (1993) Determination and use of moisture diffusion coefficient to characterize drying of northern red oak (Quercus rubra), Wood Sci. Technol. 27:409-420.
Source for the data used in the computer simulations.