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Wood Shrinkage/Expansion

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Blog entry by John8059 posted 03-14-2009 05:17 PM 1195 reads 1 time favorited 7 comments Add to Favorites Watch

After seeing a few blogs about wood shrinkage, I thought I would throw in my 2 cents. I was considering building a 44”-wide solid maple countertop in my non-air-conditioned shop. It had to fit snuggly over an existing island countertop made of granite. I was concerned with what would happen when the wood acclimated to its new air-conditioned environment.

I needed a ballpark answer to what I thought was a relatively simple question. How much shrinkage did I need to account for? I found there is a lot of information available; ranging from peer-reviewed papers to opinionated forums. But none of them answered my questions and most of them just raised more questions. Frustrated, I devised a small experiment of my own to find the answer I needed and to see first-hand what I was reading about.

See data here: http://www.acuteimagination.com/WoodShrink.htm

I assembled a number of samples of wood acclimated to my non-air-conditioned shop including plain and quarter-sawn walnut, mahogany, and maple (OK so they are not exactly described correctly, but you can see from the grain pattern that some of them have grain almost parallel to the top surface while in others it is at least 45 degrees if not perpendicular to the surface.) I planed the pieces to an identical thickness of 0.185” and cut the pieces to a width of 2.297-2.302”. I then further divided each of the samples into 3 pieces and labeled them “C”, “W”, and “D”.

Pieces labeled “C” were taken into an air-conditioned environment and after acclimating, measured for changes in the 2.297-2.302” width. This would be the basis for the ballpark answer I was looking for.

Pieces labeled “W” were submerged in distilled water for 3 hours, and then placed in a zip-loc plastic bag (without drying excess water) to prevent the loss of and to allow for equalization of any water already absorbed. Any changes in the 2.297-2.302” width from this treatment (approaching 100% moisture content) would be purely a curiosity.

Pieces labeled “D” were placed in an oven for 3 hours at 250 degrees Fahrenheit. After removal, they were immediately placed in a zip-lock plastic bag to prevent them from re-absorbing ambient moisture as they cooled. By allowing the samples to cool before measuring, any changes in the 2.297-2.302” width from heat expansion would be eliminated. Any changes in the 2.297-2.302” width from this treatment (approaching 0% moisture content) would be purely a curiosity.

CONCLUSIONS:
Considering that this experiment has a very small sample size and only a single repetition, one should not read too much into the data; however:
1) I would be comfortable thinking that I could expect somewhere between 0.35 and 0.61% shrinkage. Over a 44” surface, that translates into 0.15 to 0.27 inches of shrinkage. I would probably build a little extra fudge into it.
2) It appears that 2/3 of the shrinkage that occurs after 3 weeks of adjusting to an air-conditioned environment can be measured after just 3 days.
3) It appears that “quarter-sawn” wood shrinks less than “plain-sawn” wood of the same species.
4) It appears that once a wood is soaked, it does not shrink back to its original size.
5) It appears that once a wood is dried to 0% moisture, it does not expand back to its original size.

SUMMARY:
If you are concerned about wood shrinkage in a project you are working on, you might be able to predict the shrinkage by preparing a sample as described above and moving into its new environment for as little as 3 days. Multiplying the result by 1.5 might give a good estimate of the shrinkage in 3 weeks. Assuming the thin sample can acclimate in just three weeks, one might infer that thicker, full-size samples of the same wood would over longer periods shrink accordingly.

This was done in the Tampa bay area of Florida during July of 2007. Of course results will be different for your location and time of year. Give it a try and let us know how it works out for you.

-- Cuz



7 comments so far

View gizmodyne's profile

gizmodyne

1763 posts in 2808 days


#1 posted 03-15-2009 03:49 PM

Interesting.

The quarter sawn should have increased in thickness rather than width. Did it?

For conclusions 4 and 5 did you ever remove from the bags and expose to a regular climate?

-- -John "Do I have to keep typing a smiley? Just assume it's a joke." www.flickr.com/photos/gizmodyne

View John8059's profile

John8059

52 posts in 2135 days


#2 posted 03-15-2009 06:23 PM

I’m glad at least one person read this after all the time I put into polishing up the results for LJs. This was just a quick experiment I threw together. After I had the wood cut, I happened to notice that some of the samples looked more “quarter-sawn” than others and began to keep track of it. To test your hypothesis and if I were going to do it again, I would be more selective about the wood I was testing; however I would still probably only measure the width and I would probably increase the width so I would have more change to measure. I tried to keep the wood thin so it might acclimate more quickly, but that is an assumption I made. The thickness of the wood would add another variable to test and one could make a career out of this.

Anyway to answer your questions:
I did not measure changes in thickness as they would probably have been too small to detect, but good question. Measurements were taken at 3 days and 3 weeks after the treatements. No measurements were taken in bags. After 3 days, the wet and dry samples were removed from the bags and measured before they were allowed to lose or gain any moisture content – that is what you see in the picture above. If you follow the link above, you can see it graphically. After the initial measurements, all samples were left exposed to air-conditioning for the remainder of the 3 weeks – without any containment such as the plastic bags. The measurements are shown graphically for after 3 weeks also. You can see that 2 of the wet samples did shrink back to smaller than when they were first cut, but they never shrank back to as small as the control samples did.

It was probably retarded to even bother with heating and soaking the samples because no one would ever do that on purpose, I don’t think so anyway. But it did reveal a curiosity that occured under the conditions of my experiment. I would have expected ALL samples to return to the same width as the control, but even after months they did not. Apparently some stress occurs at the extremes of wet and dry that permanantly alters the wood’s structure – maybe more for some woods than others.

-- Cuz

View DaleM's profile

DaleM

923 posts in 2102 days


#3 posted 03-15-2009 07:32 PM

John8059, It looks like 90 people read this but some are probably just like me, when I read it yesterday, I just hadn’t commented even though I found it interesting. I should have commented though, to thank you. The amount of expansion in some of those boards is just amazing, also, I never really considered contraction much when drying it more although as you pointed out, they really don’t contract back much after expanding. Once again, thanks for doing all the hard work on this.

-- Dale Manning, Carthage, NY

View jimp's profile

jimp

207 posts in 2479 days


#4 posted 03-16-2009 05:30 AM

Very Interesting. Thanks for taking the time to do this little study. Did your sample pieces stay flat or did they cup? I would think that the wet and dry samples might cup due to the internal stresses changing.

-- - Jim, Carroll, OH

View John8059's profile

John8059

52 posts in 2135 days


#5 posted 03-16-2009 09:50 PM

Hi Jim. All the pieces stayed very flat. Some (including those that were not wet or heated) warped ever-so-slightly that I had to apply an ounce or two of pressure to flatten them out before I measured them – not that it would have made that much difference. Perhaps because even the heated and wet samples were uniformly treated, they did not warp.

-- Cuz

View Karson's profile

Karson

34901 posts in 3119 days


#6 posted 04-06-2009 04:18 PM

This was a great experment. The federal government has published a Wood Manual from their Forestry Division that has tables for many different species and the amount of wood movement.

I’ve got a little calculator on my computer that will calculate the movement for about 50 species.

Here is my blog on wood movement and links to the wood handbook and the calculator.

It looks like the calculator has disappeared on the web. Here are other wood calculators If you want a copy of the one that I have send me a PM with your real e-mail address.

I found it again here

-- I've been blessed with a father who liked to tinker in wood, and a wife who lets me tinker in wood. Southern Delaware karson_morrison@bigfoot.com †

View John8059's profile

John8059

52 posts in 2135 days


#7 posted 04-11-2009 03:17 PM

Can’t wait to check it out. Thanks Karson.

-- Cuz

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