Drying Wood: Humidity Explored

The following article attempts to help determine the appropriate temperature to set a drying box to achieve a given moisture level in wood.  I hope it contains useful information for you.  I've also created a spreadsheet (described in the article below).  It was done in Microsoft Excel, and may be downloaded for personal use.  This article appeared in the January 2001 issue of More Woodturning...


While woodworkers generally have the luxury of kiln dried wood, turners often use green timber.  Much has been written elsewhere about how to go about drying a blank: seal the end grain, rough turn to about an inch thick & let it sit for several months, store it in a bag of shavings, etc.  Nevertheless, the question arises; how do you know when it's done and how do you get it there efficiently?

Knowing when its done is really the easy part.  You may either buy a moisture meter, or weigh the piece and when it stops losing weight it has pretty much reached it's equilibrium moisture content (EMC).  That's not quite all there is to it however.  The EMC of the wood will change as the humidity changes.  The moisture content of wood is the ratio of the weight of the water in the wood to the weight of the completely dry wood (referred to as oven-dry wood).  As the humidity goes up, wood will re-absorb water.  As it drops, the wood will again dry, hence the term equilibrium moisture content.  At a given temperature and humidity the wood will reach a state of equilibrium, neither absorbing nor losing water.

In his book Understanding Wood, Bruce Hoadley notes that a typical fiber saturation point of wood is around 30%.  This will vary somewhat with different species, but it's a good working number.  (The fiber saturation point is the point at which the cells cannot absorb any more water.  Any water contained in the wood after that point is called free water - moisture in the cells is referred to as bound water.)  He further notes that at 50% relative humidity (RH) wood will have approximately 9% EMC.  At 25% RH, wood will have about 5% EMC, while at 75% RH, wood will exhibit around 14% EMC.  If you can remember these three numbers, it's pretty easy to extrapolate a ballpark guess as to what the moisture content of a piece of wood is.  If you want to be a bit more precise, the following table should help:
Relative Humidity Equilibrium Moisture Content
19 - 25 5
36 - 32 6
33 - 39 7
40 - 46 8
47 - 52 9

Generally, over the course of a year the EMC of dried wood may be between six to twelve percent depending on the ambient temperature, the relative humidity, the finish on the wood, etc.  This is important to remember, because as the moisture content of wood changes, the wood will move.  If it moves very much, it may warp significantly, or crack.  The tricky part is getting the wood to a state where the movement is minimal.  It's important to remember too, that the temperature and humidity inside a home is often significantly different than that of a shop or woodshed.  Therefore, we need to get the wood to an EMC that approximates the inside of a home since that is where most turned work will be.

Commercial lumber mills kiln dry large quantities of wood to get it to a stable point in the shortest amount of time.  This is probably beyond the scope of most of us, however building a small drying cabinet isn't.  Most of use won't want to deal with complex drying formulas but fortunately, it's pretty easy to determine what temperature to set the drying cabinet to in order to reach a desired humidity level.

First however, we have to understand what humidity actually is.  Meteorologists speak of the amount of moisture in the air as the mixing ratio.  The Saturation Mixing Ratio is the maximum amount of moisture the air can hold at a given temperature.  As the temperature rises, air can hold more water.  As it decreases, it can hold less.  If the temperature drops below the Saturation Mixing Ratio, the excess water will precipitate out.  The point at which it does is called the dew point.  We're all familiar with the process; rain, or fogged up bathroom mirror after a shower are common examples.

Relative humidity is the Actual Mixing Ratio divided by the Saturation Mixing Ratio.  As noted above, if the RH drops, the EMC also drops.  Thus, if we want to dry our wood below the EMC we simply need to lower the relative humidity.  The easiest way to do that is by simply raising the temperature, because warmer air can hold more moisture, however simply raising the temperature won't automatically increase the amount of moisture in the air.  Thus, the relative humidity falls, and the wood can dry further.

For instance, if at temp X the air can hold 2 grams/kg^3 and the absolute humidity is 1 gram, then we have a 50% humidity (1/2).  Now, if we raise the temperature appropriately, until the air can hold 3 grams/kg^3 the RH drops to 33%.  The actual amount of moisture present remains the same but the air is "drier" because it can hold more before precipitation.

Now the question becomes "by how much should we raise the temperature?".  To figure that out, we need to do a bit of math.  Don't worry that we're suddenly dealing with saturation pressure rather than mixing ratios.  It all comes out in the wash.  We will need to know a couple of things first however, such as the current temperature, the current relative humidity (RH) and have decided how dry we want our wood which will determine our target relative humidity.

OK, first, we determine the saturation pressure at the atmospheric temperature using the formula:

 P = exp(54.6329 - 12301.686/T - 5.16923ln(T))

where the saturation pressure(P) is in psi and temperature(T) is in Rankin.  ln(T) is the natural log of (T) for temperatures above 32F over water.

The actual pressure (Pa) of the moisture in the atmosphere is given by:

 Pa = P * RH/100

where RH is the relative humidity.  The pressure of moisture (Ptar) at your target relative humidity (RHtar) is then given by:

 Ptar = Pa * 100/Rhtar

Below is an explicit equation for the relationship between temperature and saturation pressure. This makes the final calculation of setting temperature (Ts) easy:

 Ts = A*Ptar^B + C*ln(Ptar) + D

where the constants A = 93.016617, B = 0.22179961, C = 12.818107 and D = 8.7034683.  This will give the required temperature in Fahrenheit.

All this may look quite confusing, and I wouldn't want to have to do the work by hand, however it's fairly simple to implement in a spreadsheet on the computer.  To this end, I created a spreadsheet to do the calculations for a range of temperatures.  By inputting the current relative humidity and the target relative humidity, the target temperature is calculated.  Remember our rule of thumb above that 25% RH = approximately 5% EMC, 50% RH =  approximately 9% EMC, and  75% RH = approximately 14% EMC.  From there we can see that as a rule we probably want the target RH to be somewhere around 25% to 30%.  After entering the current and target humidities, we merely have to find the current temperature on the chart, and look for the corresponding value in the "Target Temp" column.

Having derived the desired temperature, we need a way to actually dry the wood.  A simple drying chamber is neither hard nor expensive to construct.  An excellent example may be found in American Woodturner, Vol. 10, No. 2, page 23:  Mini Kiln, by Robert Rosand.  In general, an insulated box with a light, and inexpensive fan will suffice.  If you want to get a bit fancier you can experiment with a timer, or thermostat.  It's not rocket science however, so a simple vent hole may suffice for all  the adjustment necessary.  Small items may be dried quite effectively in a matter of weeks.  For hobbyists, that's fun; for pros, that's money in the bank!

I would like to thank Bill Lamond, of Edinburgh Scotland for his help by providing me with the equations.  Without his expertise, I would have been lost.  Bill is a retired crop drier and woodworking enthusiast.

I'd also like to thank my friend (and local meteorologist) Jim Truitt of Juneau, Alaska, who also assisted me in understanding the process and providing me with some nifty skew-t adiabatic diagrams.
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