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).
–Steve Schafer
Further reading:
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.
Glad to see another intelligent poster added to the lost art bloggers. Great post. So the “rule” of working with thick timbers is to do it now and plan for movement because you won’t be able to wait it out. (I guess that means I should start using those 4″+ slabs I cut 2 summers ago for a Roubo)
5 years ago I bought, from a local saw mill, some 8 X 8 and 6 X 6 wood beams for a timberframe project. They have been living in one of those tent garage things ever since waiting for me to finish the foundation. Certainly they have lost a lot of water but the checking has been minimal and they are still quite wet when I cut a mortise.
Or wait 70 years.
Or buy some reclaimed timbers from a century old barn….
BTW excellent post, more please.
But Raney is still dead, right?
Great post. Regarding your assumption about moisture movement through the ends, I always paint the ends of my freshly cut or split slabs and logs with a waxy paint to seal the end grain and hopefully eliminate, or at least reduce the checking while they dry. So far I’ve been successful, or at least lucky with most of my wood. My assumption was that by trying to make the moisture movement more uniform throughout the wood I was reducing the localized stresses that cause the cracks. I don’t know if that is an accurate assumption or not. I look forward to future posts about this subject so I can learn more about this very interesting topic. Thanks.
Jim
This is awesome. I’d be interested to see how changes in your temp & EMC assumptions affect the time scale. For example, at a constant EMC, would every 10 degree increase in temperature double the rate of drying? Does wood dry faster in the cold dry winter, or the hot wet summer? Better yet, can your computer simulation be boiled down to a simple equation for moisture content as a function of time, thickness, temp, and EMC?
Unfortunately, the answer to the last question is no. The reason for making assumptions is that otherwise, the problem is mathematically intractable—computational methods are currently not powerful enough to obtain answers in a reasonable amount of time. But each assumption you make limits the range of applicability of the computations, so you need to make different assumptions to cover different scenarios. There’s no one-size-fits-all formula that works in all cases.
There wasn’t enough math!
Be careful what you wish for…
The x-axis is in months in one graph and years in the other?
Yeah, something’s fishy there.
Sorry, it seemed like a good idea at the time (to emphasize the vastly different time scales), but I now realize that it just adds confusion. Replace the “12” and “24” in the first graph with “1” and “2,” and now the two graphs have the same time units (years), and the only difference is that the left one has been stretched horizontally so that you can see the details of what’s happening during the first couple of years.
Doesn’t the Baronas article only discuss the dynamics of moisture content of seasoned wood?
I find it painful to read. Blog on tape?
I have a side interest in paper restoration, I place wet books in a vacuum chamber to dry them – works well if done carefully. Could wood be dried in a vacuum chamber to turn the water to vapour? Many woodworkers have vacuum bags.
Wood can be dried with vacuum (I seem to recall it being done on at least a small scale commercially), but there are no real advantages to doing it that way. It will speed up the process a little bit, but so will just warming the wood up a few degrees.
When I dry paper or books the vacuum chamber is also heated and I raise the temperature slowly. The vacuum is by a continuously running vacuum pump. With 26 inches of vacuum and temp at 140 F the water vapourizes and is pumped out very quickly. A pound of water in a soaked book is gone in an hour or so. The insurance companies think we do magic.
I should add that during the drying I open a small valve to “leak” air into the chamber to maintain an airflow moving the water vapour out of the chamber. The vacuum is dropped to about 14 inches. The valve is opened every 5 minutes for about 1 minute.
Capital Lumber out west is using these vacuum kilns. The big difference is that they pepper the wood with an RF signal to “excite” the molecules and increase diffusion of the bound water. I have bought a fair of amount of this material in 6×6 and 8×8 Fir Timbers for customers using them for interior timber framing work and it is an impressive product that is KD’d 100% through the large piece. So I agree that it will speed up the process but I found the key difference at least with an RF kiln was the 100% dry, isothermic result. I assuming the key difference is the RF part of this equation.
Perhaps moisture in wood as sap reacts different than moisture in wood as water. I know in Europe they steam beech to force the sap out, allowing the wood to dry in a fraction of the time any other method would do. It also gives the beech that distinctive red coloring.
Or, perhaps I am talking about something completely unrelated.
Sap is 99+% water, and a lot of the non-water component stays in the wood during drying, so for our purposes, “sap” and “water” are essentially the same thing.
Steaming is just another step in the kiln-drying process of certain species. With beech, my understanding is that the main purpose of steaming is to improve the working properties of the wood. The heat from steaming softens the wood structure (hence, steam bending) and allows built-in stresses to relax more readily.
Other woods, like black walnut and pear, are steamed to improve their appearance.
The Forest Products Laboratory has answered many of these questions already. They are a good place to start with any research on wood. Here is a link to a study on drying wood.
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr121.pdf
They show 4/4, 5/4, 6/4 and 8/4 thickness in different types of wood air dried in different areas of the country , starting the drying in monthly intervals to 3 different moisture contents.
So, my take aways are:
that the rule of thumb of one inch per year is only good if the wood is as thick as your thumb.
that waiting a long time for thick wood to dry is not realistically viable, so just use it.
that simulations still suck for predicting weather and wood drying. 😉
Thanks for good info.