I have a question about how desiccants work. Suppose I had a perfectly sealed container that would hold pieces of wood and a desiccant. Lets say the Desiccant was put in at less than 2% MC and the wood was around 12% MC. Would they eventually reach an equilibrium and both contain the same MC or would a recognized desiccant always hold more moisture?

Can a desiccant hold more moisture than the surrounding ambient atmosphere?

It’s not just an issue of moisture content – it’s how tightly the dessicant holds onto that moisture.

I once had a piece of lithium metal that I placed in a tightly sealed jar with dessicant. When I opened it later, I found that the lithium had saved the dessicant from that awful water by grabbing all the water and forming lithium hydroxide with it. That reaction wasn’t about to go back uphill.

(Yeah, I know you’re supposed to store the lithium under oil, like other alkali metals. But I’d cleaned off the oil with a heavy alcohol and didn’t want to go through al that again before I used it.)

Well, there’s always some equilibrium; but most commercial desiccants can, by the measure of grams/unit of volume, hold plenty more water than a piece of wood or air. In some cases (like P[sub]2[/sub]O[sub]5[/sub]), the reaction with water is practically irreversible, and hence the equilibrium approaches infinity…

It isn’t specifically Moisture Content that’s important, rather Equilibrium Relative Humidity. I’ll use this term sloppily, for which I apologise, but the ERH is the level of moisture in the surrounding air at which water is neither pushed into nor dragged out of the wood. That’s the ERH of the wood in this closed system.

So in your container full of wood, when the wood has reached equilibrium with the air in the container, the wood will neither gain nor lose moisture vs the air.

If you add the (activated) desiccant to your wood containing vessel and close it, then (presumably) the desiccant will have a much lower ERH. It will drag moisture out of the air. The air is then not in equilibrium with the wood, and will drag moisture out of the wood. Eventually a new equilibrium will be reached. At that equilibrium, there is no reason to assume that the moisture content of the desiccant and the wood will be the same – but the ERH will be. (Definition of equilibrium). It may be that, as it is designed to dry things, the desiccant can have a surprisingly high water content without elevating in ERH that much.

Desiccants are activated by heating them in an oven to drive the air off. When newly activated they have a very low ERH, though this does rise somewhat (as you would expect) when they absorb water.


This is all governed by Fick’s laws and by partition coefficients, which relate to the affinity the different types of molecules have for one another. The compounds in wood are generally not very aggressive at collecting water, whereas desiccants are chosen to be. There’s also a dependence on pore size distribution according to the Kelvin effect which governs the equilibrium between water and airborne water vapor according to the radius of curvature of the interface between air and water.

I am trying to figure out a way to control the moisture in my bows with a little more precision, 6% or 7% would be perfect. I find moisture meters to be unreliable and they lack precision. We have some less aggressive type desiccants such as rice and charcoal that I am thinking might be good for long term storage ( 3 or 4 months) to adjust.

Could you guys lighten up the dialog a bit? It’s kinda dry.
(I’m goin,’ I’m goin.’)

Well now, that’s a whole different question. How do you adjust the moisture content of a specific piece of valuable wood to a closely defined, narrow range. And maybe monitor to establish that you are within this tight range.

Going back to desiccants, which I do know a bit about (practical more than theoretical) you want one with an ERH which is the same as a bow with a moisture content of 6-7%. What ERH would that be? Answer that question and we can move on the the next step. (You would also have to consider storage temperature, because ERH of a desiccant (or a bow) will vary with that). But I would have thought that simple desiccants were a bit of a blunt instrument for what sounds like a specialised job. Also, as I noted above, desiccants pick up water, and when they do their ERH changes (BTW there was a miss-type - Desiccants are activated by heating them in an oven to drive the WATER off). So: you have to choose a desiccant with a water content adjusted to the right level to maintain your desired humidity range.

Take a look at figure 1 in this to see how ERH varies with moisture content.

I assume that the reason for asking this question is that people do it? If so, I would ask them how. They may use specific desiccants which are more user friendly for this application.


Hi Mutt. Who’d have thought that this was about a Bow. Wow.


(me too)

Actually, casting my mind way back, this is how we used to do it in the old days

It’s time consuming, messy and difficult, but it is in principle accurate. But you still need to know what ERH you’re aiming for.


I distinctly remember from my college years how I and my class mates distilled tetrahydrofurane from a molten potassium mixture in a large (perhaps five litre?) round flask, mounted above the usual sink under a fume hood. I prefer to think that the plumbing of that particular sink was disconnected from the “mainstream”; but you never knew…

If I found a desiccant that reached the same erh as the wood I could cook and weight the discussant before and after baking to see what moisture content it had, I think anyway.

We have charts that will tell us what wood should be at specific temps and humidity ranges. They are pretty close but I am looking for as accurate as I can get. Performance of the bows peeks at at 6%.

Lithium nitride too. It will react with nitrogen. Guy I worked with took it out of the argon glovebox and left it in the nitrogen glovebox and ruined it.

HoneyBadgerDC - always fun to read your posts.

I have good experience with the design of industrial driers for woody biomass and coals (low rank). Also have good experience with different dehydration systems including dessicants for the oil and gas industry.

Right off the bat; I think a change of strategy will benefit you a lot. You see you are trying to measure the water content in your bows - which is not an online measurement and it varies across the depth of the wood cross-section. It is also a finicky measurement.

A better measurement will be the following methods:

1> Strain gauges glued to the actual or a surrogate bow (of the same wood and approximately the same thickness) and watch the readings as the bow(s) dries from a display or your computer outside the chamber. See picture here :
2> Strain gauge glued to a piece which is stretched on the actual or surrogate bow’s string. See picture here :

You can also use actual tension sensors for method 2. See here :
These sensors come pre-assembled and you can wire them up to display the actual string tension outside your drying chamber.
Once you have a setup like above, you can have repeatability i.e. you can say I want to dry the bow (or the surrogate) till the tension reaches XX from a starting value of ZZ.

As to the dessicant itself, my recommendation is not to leave it in the chamber. This may result in overdrying the wood.

A simple drying schematic is shown here ( where the dessicant is located outside the drying chamber and the air circulation can be cut off at anytime by turning the inline fan/blower off.

This arrangement can also be coupled with the strain gauge measurements above i.e. the fan can be turned off automatically once the desired strain (or tension in the string) is achieved.

Although, it looks complicated, it really is not. All you need is a simple USB data acquisition system that will turn the fan on/off. The system will also give you a nice graph of how the wood dries over time.

And best of all, you don’t have to wait 6 months for the process to work. Good luck HoneyBadgerDC

In principle you could measure moisture content of the desiccant as you describe and infer an ERH; problem is you never know what the residual moisture content is after drying. Directly measuring ERH is, in principle, better (notwithstanding your comments about precision of meters). For a start, if you can accurately measure ERH, you never need to know the moisture content of the desiccant. (Those saturated salt solutions which I drew your attention to are primarily intended to calibrate meters, but they could equally well be used to adjust desiccants to match their ERH. They will act as desiccants themselves, of course, but they’re a pig to handle).

In practice you would be looking for a desiccant with a flat “response curve” at the ERH you desire - that is to say, if you need a ERH of 20%, you’re looking for a desiccant which has a ERH of ~20% over quite a large moisture content range, so it can perform as you wish despite drawing considerable amounts of water out of the bow (or losing considerable amounts to it). It also helps if the mass of desiccant is large compared to the mass of the bow.

All that said - we have a specialist wood drier on board now in am77494 and I’m happy to defer to him/her.


Treppenwitz, your method works too. If you were to use Treppenwitz’s method, you would select a saturated salt solution of Magnesium Chloride which will give you give you an ERH of 32.5% at 30 deg C (90 F). from the link Treppenwitz provided

This publication gives the ERH and temperature needed to for wood to get to a desired moisture content - ( See table X at the end of the paper). Using the table you can see that the above ERH and temperature conditions are just about right for the 6% moisture level in the wood.

The trouble I believe based on many years of posts from HoneyBadgerDC is that he is not specifically interested in the drying level. What he is more interested in is the mechanical properties (spring constant) of the bow. Therefore I suggest that he dries the wood and keeps measuring the mechanical properties (through the strain gauge) and stop when the maximum properties are achieved. This is easily automated too.

Great info, I will study up on that.

That is actually a better idea because not all wood behaves the same at different humidity levels. I had given this some thought. Most natural fibers tend to get weaker in tension below the 6% mark. It would be nice to know exactly where this took place. The compression features of the wood tend to get stronger as the wood dries more. So we tend to go as dry as we can without compromising the tension qualities.

I cooked 445 grams of rice for about 30 min shaking it about every 5 minutes. I just started to get the slightest color change on top and I took it out and weighed it. It lost exactly 10% of it’s weight. I have a feeling that wood and rice are about the same.