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Author Topic: Equilibrium Moisture Content of Wood  (Read 2997 times)

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Kirk_Allen

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Equilibrium Moisture Content of Wood
« on: January 25, 2004, 12:16:39 PM »
This link has an international chart from the Forest department.  It is a list of wood moisture equilibrium for all over the world.

http://www.fpl.fs.fed.us/documnts/fplrn/fplrn268.pdf

I recently took a furniture building course from Graham Blackburn and I found it interesting that he uses woods with a moisture count of 8-12%.  

Now I know all the advantages to Kiln drying and do plan on building a solar kiln this summer but I was impressed with his comments.  

Wood that has been dried to 6% then used in furniture MUST be designed for proper expansion as that material is placed into service some where in this country.  When wood has that low of a moisture count it will definitely expand in every place in the world with only a handful of exceptions.

I have found this quite interesting in the fact that to date, all the wood I have used in my own projects were air dried to about 11% (which is below what the chart says it should be in my area).  I then bring the wood into my shop and after a couple of weeks or a month it is down to 9.5-10%.  I have not had any problems with any of my construction when it comes to expansion or contraction.  Now keep in mind I do utilize woodworking techniques that allow the wood to do what it wants to do without creating problems in the piece I built.
 
Just thought this information might be interesting and valuable for your customers.

By the way, I think the above link came from the FF on another post.  Who ever posted it first, THANKS!



Offline Neil_B

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Re: Equilibrium Moisture Content of Wood
« Reply #1 on: January 25, 2004, 01:37:19 PM »
Great info Kirk,
I've read before that any wood you use should aclimatize (?) to the final resting area that the finished project will be used. I have lumber in my heated basement that will generally not go below 10% and most projects I've built were fine because that is the same as the house.
I did see somewhere recently that 6% wood should sit in the house or shop to equalize which would more than likely bring it up to 10%+ anyway. Makes you wonder why you would want to waste money bringing wood down to 6% to begin with.
There is a chart somewhere that tells you how to compare the humidity in the air to the MC of wood and to keep wood at 6% would require a pretty dry house.

I think I'm remembering that right  ;)
Timberwolf / TimberPro sawmill, Woodmizer edger, both with Kubota diesels. '92 Massey Ferguson 50H backhoe, '92 Ford F450 with 14' dump/ flatbed and of course an '88 GMC 3500 pickup.

Offline Den Socling

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Re: Equilibrium Moisture Content of Wood
« Reply #2 on: January 25, 2004, 01:57:29 PM »
I bet that you're right about 6% being too low. The 'experts' call for 6% in climate controlled homes. This doesn't describe my home lately!
I wonder, also, why companies insist on 6% when the MC isn't going to stay there because of the EMC in warehouses and factories.
The information from FPL is interesting but note that it's for wood in outdoor locations.

Offline Ianab

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Re: Equilibrium Moisture Content of Wood
« Reply #3 on: January 26, 2004, 03:55:49 AM »
By way of a rough and ready experiment I microwave oven dried a lump of 'air dried' pine. By my calcs it was 21% mc, after 3 months in the drying shed. For the last 2 months its been sitting on the end of my desk and I weigh it occasionally to check it's mc. It is now stable at 10%.
Now this indicates to me that drying wood in a kiln to below 10% would be a bad idea.. well for my house and climate anyway.
I believe our outdoor emc is around 12-14%, but I run a small d/h unit in the house to reduce condensation and mould that an 80% humidity climate allways seems to produce.

Anyway.. it seems a simple enough way to calculate your house emc... totally dry board, and let it drift up to equilibrium point.

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Offline beenthere

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Re: Equilibrium Moisture Content of Wood
« Reply #4 on: January 26, 2004, 08:43:48 AM »
The report referred to is for EMC outdoors, but when I read the introduction, it (to me) explains quite well what the intent of the report is about, and how to use the information. Seems to me, it also relates to the above comments about wood kiln-dried to 6% and used in an environment that is not always at the EMC of 6%.  Seems that when drying and using wood, many conditions have to be accounted for, and there isn't one rule that is going to fit all the conditions.

If this drying section is going to work for the benefit of the members, I would suggest trying to understand what the "experts" are trying to say.  ;D  Sometimes it is not what we want to hear.  8)

I believe the idea of drying wood to a lower MC than what the EMC conditions are going to be, has to do with the fact (hysterisis) that the wood does not swell back to the same dimension at that EMC which it was when it was shrinking. So in a sense, it is 'more stable'.  At least this is what I have used as an explanation for many years. If one knows that the wood will be used at a higher EMC (say 12%), then one would be correct in not drying it to 6%. Industry standard, I think, is a more universal 6% kiln-dry for hardwoods.
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