Practical/real world rate of re-absorbtion of moisture by kiln dried timber?

[ian maybury]

Pardon the length, but i'm hoping for some fact/experience based input here - based on stuff like meter readings and wood behaviour. In the context of mainstream hardwoods like walnut, oak, maple, beech and the like. So far I've tended to take supplier claims on trust, but after some dodgy experience and conversations (again) today with two separate (local and fairly large) hardwood merchants who either didn't have a clue or were likely not letting on or both thought it might be worth trying to tie the facts down a bit better.

The theory suggests that commercially sold woods like the above are typically kiln dried to 8 - 10% moisture content by commercial producers, and then shipped on in containers and the like to merchants like our local guys who unload it in bundles and stack it in their warehouses which are typically open to the atmosphere. I'm not aware of it being shrink wrapped or anything like that in the way that e.g. solid flooring is. Our climate being fairly damp it then presumably starts to accumulate moisture again - to head back to the 17 - 20% equilibrium moisture content that's apparently fairly typical year round here. The rule of thumb being that it supposedly equilibriates at the rate of about 1 year per inch of thickness.

The above gentlemen both claimed that the most anybody need worry about by way of an increase in moisture content was a couple of percent, and that this would be towards the surface and primarily only the boards to the outside of the bale. That I would need to allow it to settle for a 'day or two' before using it. I asked about how fast they turn it over, and was told that loads would normally go though in a month or two - although my guess is that that's far faster than reality in the present market. That you could easily get a piece that is a year or more old. The commercial makers of kitchens and the like i've spoken to meanwhile seem to just take it as it comes.

The merchants don't claim to consider moisture content to be even a minor issue, although they grudgingly would accept a meter being used on the wood. A surface reading should at least be worst case I guess.

There are a few small producers kiln drying local wood, but it tends to be a bit variable and wild in quality - and the supply irregular in given varieties as there's only two or three, and all are pretty small. (the total market here is tiny)

My personal instinct and longer term plan presumes that getting control of the situation requires installing a small kiln to condition wood before use - or at least moving it into a similar environment to where it will eventually be used for maybe several months - but you wouldn't think so to talk to these guys.

Am I being overly cautious?

Experience suggests maybe not - i bought a large load of supposedly kiln dried 2in beech from a well regarded UK supplier a couple of years ago . This despite promises to the contrary turned out depending on the board and part of it checked to be between about 13% and 16% moisture content - which settled to around 12% in my moderately heated and very dry shop over about a year. What's been used has seemed OK.

Is much more moisture likely to be acceptable given the above and our fairly cool and damp climate? My gut tells me that these guys left to their own devices will simply say what they think is needed to make sales, and after that will without differentiation between boards on the basis of MC simply move them through their operation.

So what's the deal for a small guy that doesn't buy enough to have much clout? Is it just a case of keeping an eye out for fresh deliveries, bringing a meter and refusing boards above say 10/12%. Plus allowing them some time to equilibriate in the shop and after that taking care while working. e.g. process in stages, checking in between.

Does the 1in per year rule in your view stand up in practice for both drying and re-absorbtion of moisture? For both small (few %) and larger moisture changes?

Is there any convenient means of getting a core moisture reading? (damp in the centre is a much bigger deal than just through a thin outer layer given the additional drying time and the risk of warping it entails)

Any key questions/bits of information/good practices a decent timber merchant should have to hand. i.e. how do the good ones do it?

Anything else?

Thanks

ian

[steve joly]

Also very interested to hear some input, sorry I have nothing to contribute but I've been wondering about this also

[Mel Fulks]

I think the most important thing to understand is that kiln drying is a permanent change to wood ,not just a reduction in moisture . Kiln dried wood can re absorb water but will not move as much as air dried . I have an old post about running a test and I still have the wood I used .Both pieces were adjacent in a board ,one piece was KD the other AD.

[Bob Lang]

Ian,

Here in the USA, there are charts that give the average equilibrium moisture content for different areas. The E.M.C. is the key. Kilns here typically dry to 6% or so, and once the wood is out of the kiln the wood begins to take on or lose moisture until it reaches the local E.M.C. In my neighborhood, E.M.C. is generally between 8-10%, dry winter to humid summer. The "one inch per year" thing refers to freshly cut logs (soaking wet) to reach E.M.C.

There isn't anything you (or anyone in the lumber business) can do to change the nature of how wood reacts to it's environment. The best thing I can recommend is to get a meter, and before you go to the lumber yard, check some pieces of wood that have been in your shop a while, they should be at your local E.M.C. The lumber available is what it is. It might be fresh out of the kiln and drier than your shop, or it may have been sitting in a damp yard and wetter than your shop. Either way you ought to wait anywhere between a few days to a few weeks, depending on the difference.

If you don't let new wood reach equilibrium with your shop, it will be moving while you work on it. If the environment in your shop is different from where the finished piece will live, it will be moving until it reaches E.M.C. with its new home.

Bob Lang

[Mel Fulks]

I agree it is good to aclimate when possible,especially with AD wood. The architectural specs that govern most trade work call for KD wood proscribe AD and don't much get into moisture percentages. In commercial work it is common practice to start working wood right away ,regardless of weather or where lumber was stored.

[Joe Hillmann]

In my experience in steam bending kiln dried wood it only take days for the wood to dry back out after steaming it. I don't have any type of moisture meter so my results aren't scientific.

[ian maybury]

Thanks very much guys. For sure the $1M question Mel is how much difference there may be between drying and re-absorbtion. Some digging since posting (the term it seems is 'moisture re-adsorbtion after kiln drying') suggests the following: (it's not a topic that's dealt with in a lot of detail anywhere i could find)

The big guys (e.g. USDA/p. 114 Bruce Hoadley's book - Understanding Wood) seem to suggest that wood regains moisture after kiln drying in a similar manner to that in which it drys in the first place. (the rate is determined by relative humidity, air temperature and air circulation) He reckons that while as you say Mel that it doesn't quite get up to the same moisture content (actually equilibrium moisture content for a given relative humidity - see fig 6.3 p. 113 - the effect is called hysteresis) that kiln drying (as we know) doesn't leave wood in any way irreversibly dry or dimensionally stable.

The 1in per year to equilibrium mositure content rule of thumb doesn't get much credit - it seems it can be quite a long way off depending on the wood species, conditions and shape of the piece.

They don't give much theoretical information about re-absorbtion of moisture, but there's some good info here: http://www.conradlumberco.com/pdfs/c...f_Moisture.pdf - page 12-14 Moisture Control During Transit and Storage - Forest Products Laboratory. 1999. Wood handbook—Wood as an Engineering Material. Gen. Tech. Rep. FPL–GTR–113. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 463 p.

They say: 'When standard 19-mm (nominal 1-in.) softwood lumber, kiln dried to 8% or less, is piled solid under a good pile roof in a yard in humid weather, average moisture content of a pile can increase at the rate of about 2% per month during the first 45 days. An absorption rate of about 1% per month can then be sustained throughout a humid season. Comparable initial and sustaining absorption rates are about 1% per month in open (roofed) sheds and 0.3% per month in closed sheds. Bales on deck on a ship can pick up as much as 7% during a voyage.

Your weather and/or shop conditions will influence or more to the point stop any further moisture loss or gain or beyond the EMC determined by your %RH - the above is probably worst case seeing as it relates to humid weather.

Which bears out what you say Bob. So it is possible to get very wet timber from a supplier, but it may be a bigger issue over here on the other side of the Atlantic than with your US suppliers who presumably may move more wood sooner after kilning(?).

One basic strategy to avoid this is probably to get it freshly kiln dried - testing with a meter to confirm. I guess unless the wood was stored in exactly the same conditions as your shop/the end use of the wood that there's always going to be a moisture gradient (as in it's wetter or drier near the surface than at the core) - that as you say it needs to be held in the shop for long enough to equilibriate or the result will be movement upon machining/cutting open....

More thoughts anybody?

ian

[michael osadchuk]

I agree with Bob and others that MEC is the key factor to pay attention to in knowing when to use wood, whether kiln or air dried.

Letting new bought kiln or air dried timber 'acclimatize' to the relative humidity in the setting in which you will work iy (and preferably also place the finished piece) by passive storage (no special home made kiln drying efforts) is typically a matter of a week or so rather than months for me. My practical guide is comparison of the moisture content reading of the new stufff vs. wood of same species (or other species corrected for density) which has lived in my workshop/home for years. And I'll use the new stuff once it's within a couple percentage points of moisture content vs existing wood known to be at MEC.
There are also charts/tables in books giving average MEC by region by seasonal relative humidity.

Seasonal MEC shifts for me - just north of Toronto, heated, well insulated basement workshop but with no active control of relative humidity, which varies seasonally on average inside my shop/home from around 35 to 75% - is only about 2 or 3% percentage points of moisture content; example: maple mec ranges 8 to 11% in my situation

The 1" per year for drying guide - R. Bruce Hoadley, author of Understanding Wood, says is unreliable.... in the first edition of his book on page 103 he has a table, titled appropriamate time to air-dry 4/4 lumber to 20% MC and the table gives wide ranges, typically 50 to 200 days or more, for a variety species; he states that the minimum number of days number refers to drying lumber in good drying weather, generally spring and summer... lumber stacked late in this period will ....usually not reach 20% until the next spring.....
... and my experience above with kiln and air dried lumber in my workshop says that the reabsorption of of moisture by such wood is 'not much'.

good luck

michael

[Jeff Duncan]

There's a timely article by Gene Wengert in Cabinetmaker FDM magazine this month. Not sure if you can find it on their website (www.cabinetmakerFDM.com) or not, but it provides a little insight to this question. Actually it more specifically deals with the delay of wood to expand after being dried and then re-absorbing moisture, but is closely tied to your question.

JeffD

[Mel Fulks]

I guess there isn't a way of making wood "irreversibly dry" but kiln dried wood does move LESS than air dried. Test it. Somehow using a moisture meter on wood that is saturated with water seems silly when it has clearly changed dimension much less than an air dried piece also saturated. Some explanations of the difference between AD and KD say that KD removes the moisture in the cells while AD removes moisture between the cells...I don't know. I do know they produce different products.

[John TenEyck]

This might be the article Jeff was referring to: http://www.cabinetmakerfdm.com/8931.html. After I had a couple of bad experiences with "kiln dried" lumber I wised up and bought a moisture meter. I use it with near religious fervor now, not only when buying wood but also to measure how long it takes to come to equilibrium with my shop. I want to know what the MC is of the wood I buy because I don't want to pay KD wood prices if it's really AD wood. But I don't really care if it's 6% or 12%, because I know with almost certainty that it won't be the same as my shop, and that I will have to let it acclimate to my shop for a period of time until it has reached the EMC. Buying and, worse yet, using wood w/o knowing what the MC is or if it's at the EMC of your shop is just asking for trouble - I know this from experience. The rate at which KD wood that has a higher MC than my shop comes to the EMC is surprisingly fast. I recently bought some KD 4/4 maple and poplar that was stored in a closed but unheated building. It measured about 11 - 12% when I bought it. It was down to the 6 - 7% of the other wood in my shop at 35% RH in about 7 - 10 days, at which time I used it w/o any issues.

Moisture meters are pretty cheap compared to just one ruined project.

John

[Mel Fulks]

The samples I referred to were run thru two soakings and dryings about four years ago.Got them out today for another test .Looks like they are now moving at the same rate. That means that I was wrong in saying the difference between KD and AD is permanent. More accurate to say that ,in practical use ,the difference is long lasting to permanent in most uses.I have observed that when KD lumber is checked with moisture meter on arrival and allowed to acclimate ,there is no change in width of wide boards after moisture readings are deemed low enough to begin work;not so with AD. The samples are 13 inches wide and only 3 inches long in order to make soakings more thorough .

[ian maybury]

Aha! Thanks again guys. I missed your point Mel - the moisture content does increase as kiln dried wood re-adsorbs moisture in a similar manner to during drying, but it produces quite a bit less movement - because judging by one of the Gene Wengert pieces mentioned http://www.cabinetmakerfdm.com/88415.html nothing much happens by way of movement immediately following the initial increase in moisture content. (another aspect of the hysteresis effect) Which is why he says it's better to start with wood that's a percent or two dryer than the likely EMC in the end user's house. I may have it wrong, but this effect is likely swamped by large changes in EMC. You're also saying that kiln dried wood moves less than the air dried variety in this situation - maybe to do with something like case hardening? This might explain the wearing off of the effect you've seen - the wood eventually relaxes.

I'd not seen Cabinet maker FDM before Jeff, it reads like really good resource. Great to see a fact based/professionally oriented journal. Thanks for that.

Agree 100% John on using a moisture meter and getting the situation under control. I bought a Mini Ligno more than a year ago, and a copy of Bruce Hoadley's Understanding Wood - and am looking at other options too. I have minimal epxerience with it so far though as most of the past year has gone into workshop set up. Our climate is pretty damp, but one positive is that we don't get a great deal of variation in average % RH through the year - so the EMC is pretty steady compared to the dry Winters and humid Summers you get in parts of the US.


ian

[Andy Pratt]

I'll put in my two cents in the following areas:

- Kiln dried wood is cool/popular right now. As a result (sometimes) uneducated suppliers or kiln operators have increased their rhetoric on why it is superior to air dried wood because it helps them with sales. Some of the stuff you hear is truly nonsense and makes you wonder if the kiln operator really understands what he is doing, and some of it happens to be spot on. I'm not delving into that argument but wanted to point out that there is a lot of misinformation out there as a result of this (not referring to this thread) and you are right to ask for data as opposed to just opinions because many people have bought their suppliers claims without doing the research on their own.
- I'll therefore keep my opinion on how much kiln dried wood moves vs air dried out of the equation and offer you my advice on the situation in general. It seems like you are looking for a "what should I do here" sort of answer so I hope I'm not off base giving one as opposed to providing data.

I want to understand every aspect of wood movement that I can, but that is an academic pursuit. For the effectiveness of my work, I only care that nothing I send out has a defect that is bothersome to the end user or advertises my work in a negative way. Toward that end I usually find myself planning for the "worst reasonable case" scenario with each board I use, and find that the kiln dried/vs air dried argument becomes irrelevant at the point where you do that.

The truth of the matter is that properly kiln dried and properly air dried woods are all going to be easy to work with and react in predictable manners that normal woodworking practices account for, and woods of either variety that are improperly dried are going to cause unpredictable results that are hard to detect before using them. So, if you are looking to do high-quality work, I wouldn't worry about how fast kiln dried wood reabsorbs moisture. Excess wood movement is easy to account for if you use high-quality construction techniques, and if you do that you are prepared to be wrong on a variety of assumptions you might make about the wood you are using, including the affects of kiln drying.

In other words, you will probably (and should) build everything to a tolerance that exceeds the claimed benefit of kiln dried wood anyway to account for other possible issues, so the disputed answer to this question is not something to let affect your work or how you deal with your suppliers. I am as curious to know as you but it just affects my mindset, not the way I will actually do my work. I would take the importance of wood acclimation as far, far more important. That is where you can really go wrong on a project.

[ian maybury]

I guess in the end Andy wood is wood - whether kiln dried or not. Which leads to the risk that if an unthinking supplier hands you a piece that was kiln dried but which has been around for long enough to recover all that lost moisture it's possible to run into trouble. Which suggests that it make sense to decide on moisture content limits for sensitive jobs.

'Trouble' of course is as you suggest relative too. Joinery methods allow in large part for movement - but against that it's got to be hard to sidestep the visual effects of movement in through tenons or breadboard ends or whatever. So it gets down to your design, and what you regard as acceptable. Movement in a rustic piece isn't the same as movement in a highly finished contemporary item.

Digging around in the numbers is no subsitute for a practical understanding of what's going on, or for having a practical working regime - but seems to have its place too. In that it tends to ground or set limits on the fairy stories if the right data can be found and used properly.

This to try to collect some snippets from this discusssion and the related digging in the topic:

1. Use of the term 'kiln dried' does not of itself define/commit wood as being at a given % moisture content.
2. Timber merchants do oversell - or at least often don't overtly manage the moisture issue in kiln dried wood.
3. Kiln dried wood gains and loses moisture in response to atmospheric humidity much like air dryed wood.
4. This gain or loss of moisture leads to the same rates of movement (up and down) as air dried wood. (Hoadley's Understanding Wood sets out how to calculate the movement - how fast it happens depends on the humidity of the surrounding air, temperature, coatings, species, air circulation rates and the like is is much harder to predict)
5. It as a result pays to check the moisture content before buying kiln dried wood.
6. The good news is that properly stored kiln dried wood (i.e. wood down around 8% EMC) exposed to the atmosphere seems to pick up moisture fairly slowly - at the rate of at worst about 2% increase in EMC (on average - it'll be wetter to the outside in practice, and less towards the core) in the first month, and 1% per month thereafter. (presumably lots less for hardwood in dryer weather) Wrapping in polythene etc slows this right down, exposure to liquid water massively increases it.
7. This still seems to suggest that it's best to get recently kiln dried wood if you need it down around 7/8% - for example for furniture going into low relative humidity conditions.
8. Kiln drying done well is kept slow enough and includes a finishing step to reduce the moisture gradients that arise during the drying process, and hence minimise any residual stresses. Poor drying can leave high stress levels in the wood (case hardening and the like), or even honeycombing or cracking
9. Despite this it's still advisable to allow some time for kiln dried wood to equilibriate in your shop before working it, or after a piece is cut open. That is to allow the moisture content to become more or less uniform through the pieces.
10. One issue with this is that the wood can't ever fully equilibriate if the humidity in your shop keeps on changing (as it may naturally do). This suggests staged machining etc where possible - but it clearly often doesn't become a practical issue.
11. Notwithstanding (3) it seems that hysteresis means that kiln dried wood hardly moves (expands) during the first 1 - 2% increase in EMC when it starts to re-adsorb moisture. (over the first month or more)
12. One result of this it seems is that you can minimise movement in use by starting with kiln dried material at about 2% below the EMC you want the wood to equilibriate to when the work is installed. (i.e. arrange for it to return the target EMC from having last been dried to a point just below it. e.g. see the link to Gene Wenegert's piece in CabinetMakerFDM above)
13. This (if we're running a super precise humidity controlled set up) seems to determine the % relative humidity a shop needs ideally be held to. i.e. if it's air conditioned. (there's a direct relationship between the EMC wood settles to and %RH - refer to the usual % RH/% moisture content (EMC) graph that's published fairly frequently e.g. in Hoadley again)

Please shout if any of the above makes no sense...

ian