Estimating drying time for different woods

[Oskar Sedell]

Hi all,

when I´m buying wood, say from a lumber yard, it usually comes with a given (however precise) moisture content. Like 20 %. I'm looking for a way to estimate, or compute the needed time for the wood to reach a certain lower moisture level (for example 12%, or equilibrium) in my shop.

When searching for wood drying models I find a lot of estimates (like 1 year per inch) or drying curves for wet lumber, down to 20% or so. I'm looking for the other case, starting from lumber yard "dry" boards, and going down to furniture dry.

In the best of worlds, there would be a formula (probably some exponential) giving moisture level over time, and where the wood species, thickness, shop temperature, shop RH etc. all enter as coefficients/parameters. Maybe this is too much to ask for, and I'm interested in all the answers you have.
One more thing, I don´t have a moisture meter. Maybe stupid, but I´m thinking a good enough estimate/formula should exist somewhere.
How do you estimate the time for ´seasoning´ wood in your shop?

 

[AndyT]

I just keep the wood on a rack indoors for ages before I get round to using it.
I expect scientific graphs exist in technical books, but in practice, it's easier to get used to how dry something looks and feels.

If you want to be precise about it, you could weigh each piece every month and draw your own graph. You'll see the graph level off as the difference between the moisture in the wood and in the surrounding air reduces. So when it's stopped drying, start work.

Another practical thing you can do is to follow Richard Jones's advice in this useful article on his website and cut a sample to see if it moves.

http://richardjonesfurniture.com/Articl ... fault.html

 

[Oskar Sedell]

Andy, thanks for your answer. This is exactly what I do, I collect wood for different projects, and they usually lie around for a long time before I get to them. Still curious about if there is a way to get the drying time, given the moisture level.

 

[Woody2Shoes]

How does the RH vary in your shop daily/seasonally? - more than most people might imagine I suspect, this must be one of the key variables.

Ignoring variation between species (which perhaps could fall into four useful categories https://www.popularwoodworking.com/tech ... ture-types ), I think a key variable is the amount of endgrain per unit volume of the timber - given that moisture moves in/out predominantly via endgrain. Geometry matters.

I think that another question is, how much does it matter? - as long as your design and construction allow for (inevitable) movement.

Cheers, W2S

 

[AndyT]

If you want some proper timber science, stuffed full of formulas and graphs, you could try "Wood as an Engineering Material,
by the U.S. Department of Agriculture, 1999" available as a pdf at this site:

http://www.evenfallstudios.com/woodwork ... ing%20Wood

It's beyond me but it looks impressive!

 

[D_W]

Weigh and measure it. Too many variables to rely on general suggestions from elsewhere on the web, especially if your temperature or humidity is different than the suggestions'.

I've found most lumber to dry fine (for use) faster than the estimates would suggest in a partially heated shop that is of higher than average humidity in the US in the spring and summer.

 

[Oskar Sedell]

How does the RH vary in your shop daily/seasonally? - more than most people might imagine I suspect, this must be one of the key variables.

Ignoring variation between species (which perhaps could fall into four useful categories https://www.popularwoodworking.com/tech ... ture-types ), I think a key variable is the amount of endgrain per unit volume of the timber - given that moisture moves in/out predominantly via endgrain. Geometry matters.

I think that another question is, how much does it matter? - as long as your design and construction allow for (inevitable) movement.

Cheers, W2S

Not much at all. My shop is a closed room in the cellar, with dehumidification. Pretty much 20 degrees Celsius and 62% RH all the time.

Agree on construction that allow for Wood movement. Then normal dry/seasoned works fine. But for some projects, like tool making, knowing that the wood is close to equilibrium is good to know.

 

[Oskar Sedell]

Weigh and measure it. Too many variables to rely on general suggestions from elsewhere on the web, especially if your temperature or humidity is different than the suggestions'.

I've found most lumber to dry fine (for use) faster than the estimates would suggest in a partially heated shop that is of higher than average humidity in the US in the spring and summer.

Thanks David, I guess you are right. Maybe drying time is too dependent on air circulation, variation among the same wood species, etc. for a simple formula to be trustable.

 

[Oskar Sedell]

If you want some proper timber science, stuffed full of formulas and graphs, you could try "Wood as an Engineering Material,
by the U.S. Department of Agriculture, 1999" available as a pdf at this site:

http://www.evenfallstudios.com/woodwork ... ing%20Wood

It's beyond me but it looks impressive!

Thanks Andy, this is a very significant contribution to todays "wasting time at the office"

Jokes aside, I´ll have a look and see if something resembles what I´m looking for.

 

[Sgian Dubh]

In the best of worlds, there would be a formula (probably some exponential) giving moisture level over time, and where the wood species, thickness, shop temperature, shop RH etc. all enter as coefficients/parameters. Maybe this is too much to ask for, and I'm interested in all the answers you have.

There are formulae for calculating the time required to bring wood down from a higher MC to a lower known MC, usually, for example, from green or FSP to a target MC, e.g., 20% (construction wood target MC), ~12% (European furniture target MC) or in North America, 7% (US furniture target MC) in each case ±2% MC. In addition, such formulae exist for different wood species and for different thicknesses of boards of the same species. Unfortunately, none of these formulae are likely to be of much help for the circumstances you cite, i.e., partially dried wood purchased or acquired and then set up to acclimatise in such places as your workshop, a shed, a room in your house, or some other place. The reason for this likely lack of assistance to you is because these formulae are kilning schedules designed to bring wood down to a particular target MC within a set period. They rely on controlling conditions within a wood drying chamber (the kiln) through the application of heat and steam in a series of controlled and scheduled steps.

The obvious difference between what you want and a kiln is that you don't have the same kind of environmental control in which to acclimatise your wood. If you can control the environment for moisture loss to a specific MC you could use existing formulae, or make your own to account for different wood species and thickness - effectively, you'd create something similar to a wood drying kiln. Slainte.

 

[Oskar Sedell]

Hi Richard, thanks for taking the time to answer my question. Your answer makes me curious about the general shape of these formuli. Maybe they can be modified, or simplified for the simple conditions in my shop (no heating or steaming cycles or schedules). If you have a reference for these (different species, different thicknesses) I would be interested to have a look.

There is an example on Wikipedia https://en.wikipedia.org/wiki/Wood_drying (under the section "Model") where such a formula is given for red oak. Sadly I have no intuition how much the model parameters for red oak change when considering for example beech, apple or hornbeam.

 

[Sgian Dubh]

I can't tell you the general shape of the formulae, nor how they're put together, i.e., the factors that are allowed for in the formulae. A good source for kilning schedules is GH Pratt's Timber drying manual which has been published in various editions since 1974. Even here, the derivation of the formulae aren't provided, just the schedules. Slainte.

 

[AndyT]

I suspect that although Richard is too modest to mention it, a more useful reference book for the practical woodworker is due out later this year!

a-new-timber-tech-book-would-you-buy-it-t75332.html

 

[custard]

when I´m buying wood, say from a lumber yard, it usually comes with a given (however precise) moisture content. Like 20 %. I'm looking for a way to estimate, or compute the needed time for the wood to reach a certain lower moisture level (for example 12%, or equilibrium) in my shop.

When searching for wood drying models I find a lot of estimates (like 1 year per inch) or drying curves for wet lumber, down to 20% or so. I'm looking for the other case, starting from lumber yard "dry" boards, and going down to furniture dry.

I suspect you're over-thinking this Oskar. Unless you're buying air dried, waney edged boards from the one of the few remaining yards that stock them, then the distinction you're drawing between "timber yard dry" and "furniture dry" isn't particularly relevant. The majority of boards will show some further shrinkage once a finished piece is moved from the workshop into a home, but good cabinet making practise can easily accommodate this small level of movement without problems.

The more significant question to ask, instead of "how dry is this board?", is "how stable is this board?".

Pretty much all kilned timber that's been stored under cover is dry enough for furniture making purposes, but you'll regularly encounter boards that refuse to behave themselves no matter what their moisture content.

 

[patrickjchase]

Ignoring variation between species (which perhaps could fall into four useful categories https://www.popularwoodworking.com/tech ... ture-types ), I think a key variable is the amount of endgrain per unit volume of the timber - given that moisture moves in/out predominantly via endgrain. Geometry matters.

There are two big problems with this logic: First, if you want your lumber to dry uniformly and without checking then the very first thing you do is to seal the endgrain, so that it behaves more as might an infinitely long log.

Second, inter-species variations are very large even within those structure types. For example Beech and Maple are both "diffuse-porous" hardwoods, but their moisture transfer characteristics are radically different. The very high fraction of medullary rays in Beech leads to faster radial moisture transmission.

 

[patrickjchase]

I suspect that although Richard is too modest to mention it, a more useful reference book for the practical woodworker is due out later this year!

a-new-timber-tech-book-would-you-buy-it-t75332.html

I expect that Richard's book will be fantastic. There is already a reasonably practical reference for some people in Hoadley's "Understanding Wood".

I say "some people" because while Hoadley's book seems very practical to me (speaking as a mechnical engineer) I've heard several WWers that I respect a great deal describe it as overly technical and impenetrable. I expect that Richard's book will prove very helpful for them.

 

[patrickjchase]

If you want some proper timber science, stuffed full of formulas and graphs, you could try "Wood as an Engineering Material,
by the U.S. Department of Agriculture, 1999" available as a pdf at this site:

http://www.evenfallstudios.com/woodwork ... ing%20Wood

It's beyond me but it looks impressive!

I've read that book, though I'm not sure I'd wish it on any but a fellow engineer.

 

[Sgian Dubh]

Both Andy and Patrick are being generous. I wasn't indulging in modesty, just responding to a topic raised by Oskar that tickled my interest. It's true that a book I have written on timber technology should be available from about early April, 2018. How it compares to other books on the subject will have to be judged by readers after its release.

I'll say no more on the subject now because I don't wish to be accused of inappropriate self promotion, although there is information 'out there' about the book if it's looked for. Slainte.