Will kiln dried lumber return to normal % moisture?

[Andy Pratt]

I was recently watching a woodworking video where an experienced woodworker inferred that kiln dried lumber will never get back to the moisture level of air dried lumber, even after decades of sitting around. He mentioned that it stays in the 6-8% range instead of 12% like air dried wood.

This doesn't seem to make sense. I would think wood at 6% moisture, placed in an area with 12% moisture, would get back up to 12% over the years. Does kiln drying chemically alter the wood and prevent this from happening, or was this individual incorrect?

Along these lines, it was also mentioned that air dried wood "moves more" and is less suitable for furniture in today's modern, humidity controlled homes (inference was that there is a lower moisture content in the air in the house, presumably more toward 6% than 12%). All this sounded a little sketchy to me, more like marketing hype than reality, so I wanted to get some real-world opinions.

Thanks in advance for the replies,
Andy

[Howard Acheson]

Wood, whether initially air dried or kiln dried, will always move to it equilibrium moisture content (EMC) relative to the relative humidity. In other words, wood at 4-6% EMC will move to 8-10% EMC if it is moved to an environment of 50% relative humidity.

He didn't know what he was talking about.

[Chris Padilla]

Andy,

Maybe you heard him incorrectly? I have a hard time believing that an experienced woodworker would say that. All that kiln drying does is speed up the time of getting the wood down to a usuable range for building furniture with. Yes, it has other affects like changing colors (esp. in walnut) but it doesn't alter the wood in a way that it won't absorb moiture agian and therefore stop moving! If it did, it would be a revolution and a boon to woodworking all over the world to know that your solid wood projects no longer move!!

Who said this? What video?

[Richard Wolf]

I would believe that once it is Kiln dried the cell walls are no longer as elastic as they are when air dried. They then lack the ability to reabsorb moisture. This is why air dried lumber is much easier to steam bend then kiln dried, the air dried lumber absorbs the steam moisture much better. MHO.

Richard

[Frank Drew]

The fellow mispoke.

I agree with Richard, though, that wood's ability to absorb moisture diminishes over time with the effects of age and cellular collapse. Kiln drying might speed that up, but I think the point was overstated in the video. If you put a piece of kiln-dried lumber in a rain forest it will pick up moisture.

[Jim Becker]

This is why air dried lumber is much easier to steam bend then kiln dried, the air dried lumber absorbs the steam moisture much better.

A lot of this is relative to the lignum in the wood...kiln drying "sets" it and it becomes somewhat inelastic even when steamed. This damage is not present in air-dried or green wood.

[Steve Schoene]

Apparently, hysteresis in moisture content levels is a reality in kiln dried woods, so that in fact they do not absorb moisture to the same degree than air dried wood at the same humidity levels. Apparently the magnitude of the effect depends, among other things, on kiln temperatures during the drying. There is some research that shows this effect based on experiments with spruce.

http://www.skogoglandskap.no/filearc...helsinki07.pdf

This does not mean that we can ignore wood movement in construction design, but it is interesting and suggests. that the video mentioned by the o.p. has a kernel of truth in it, though I doubt it is to the same degree as mentioned. Note however that even if the hysteresis is a "temporary" a significant enough lag in the return to Moisture Content that would have been achieved air dried wood, given a fluctuating environment (as seasons fluctuate) could actually prevent the wood from ever really achieving the EMC that it would have achieved without the kiln drying.

In a sense, it is similar to the effects of finishes on furniture. A finish that resists moisture vapor transfer keeps the equilibrium level considerably lower, than bare wood. Hoadley shows this in his chapter 5, figure 2, in my 1980 edition.

I googled hysteresis wood moisture to get this info. I had to look up hysteresis-- here is the most accessible definition I found in a brief search:

Hysteresis is a physics term that means, literally, to be late. It describes systems that do not directly follow the forces applied to them, but react slowly, or don't return completely to their original state: that is, systems whose states depend on their immediate history. For instance if you push on a piece of putty it will assume a new shape, and when you remove your hand it will not return to its original shape, or at least not entirely. Source: knowledgerush.com

[Tony Joyce]

Yes, it has other affects like changing colors (esp. in walnut) but it doesn't alter the wood in a way that it won't absorb moisture again and therefore stop moving!

Chris,
What is the source for this information? I've never seen that stated before. Steaming walnut(also sometimes cherry) is a - separate process - that must be done prior to drying. Or so I've been told by the people that do it. If the logs lay too long before processing(sawing and/or steaming), steaming is not as successful. This would seem to be true based on what I've seen.

Unsteamed kiln-dried walnut looks just like air-dried. This I have observed for myself by handling both types of lumber on an almost daily basis.

Tony Joyce

[Tony Bilello]

......."was recently watching a woodworking video where an experienced woodworker inferred that kiln dried lumber will never get back to the moisture level of air dried lumber, even after decades of sitting around. He mentioned that it stays in the 6-8% range instead of 12% like air dried wood. "

There are two layer of water in wood. Think of the cells in wood as a soda straw. Air drying dries out the moisture in the cells or soda straw. This is called "free water". There is also moisture trapped in the cell walls themselves. This is called "bound water". This is the extra moisture that kiln drying will get to and air drying wont. A kiln is not just an oven. It provides regulated air flow and a vacuum of sorts. I dont know if the wood would remain in the 6-8% range because it will absorb a certain amount of water back into the soda straw cells, but I dont think it would get back up to 12% except near the surface exposed areas.

I build high end furniture on commission and I would never use lumber that was not kiln dried. Most antiques will eventually split and dry out when put in a modern heated and air conditioned home but most modern wood furnitue will not be affected by moisture if put in the garage. If there is any hype at all about kiln drying, its by the local sawyers that dont own kilns.

Logic dictates that if kiln drying wasnt necessary, furniture manufacturers wouldnt go for the extra cost.

[Jack Briggs]

I would believe that once it is Kiln dried the cell walls are no longer as elastic as they are when air dried. They then lack the ability to reabsorb moisture. This is why air dried lumber is much easier to steam bend then kiln dried, the air dried lumber absorbs the steam moisture much better. MHO.

Richard

Bingo! This is the correct answer.

Kiln drying causes a collapse of the cell walls, inhibiting the rehydration of the cells at later time. Air dried lumber's cell walls are intact, retaining it's hygroscopic nature.


Cheers,

[Larry Edgerton]

I too will not waste my time or take a chance with my professional reputation by using air dried lumber.

Another point not brought up is that a kiln kills all organisms living in the wood such as powder post beetles, etc.

Kiln drying is a science, each species requiring a slightly different approach. Do some research and it will help you better understand wood.

The man's statement was correct.

[Jim Becker]

Larry, lots of very famous folks have been building masterpieces out of air dried lumber for thousands of years...

That said, kiln dried lumber is great to work with and yes, depending on the method used, it can also help deal with critters.

[Clifford Mescher]

......."was recently watching a woodworking video where an experienced woodworker inferred that kiln dried lumber will never get back to the moisture level of air dried lumber, even after decades of sitting around. He mentioned that it stays in the 6-8% range instead of 12% like air dried wood. "

There are two layer of water in wood. Think of the cells in wood as a soda straw. Air drying dries out the moisture in the cells or soda straw. This is called "free water". There is also moisture trapped in the cell walls themselves. This is called "bound water". This is the extra moisture that kiln drying will get to and air drying wont. A kiln is not just an oven. It provides regulated air flow and a vacuum of sorts. I dont know if the wood would remain in the 6-8% range because it will absorb a certain amount of water back into the soda straw cells, but I dont think it would get back up to 12% except near the surface exposed areas.

I build high end furniture on commission and I would never use lumber that was not kiln dried. Most antiques will eventually split and dry out when put in a modern heated and air conditioned home but most modern wood furnitue will not be affected by moisture if put in the garage. If there is any hype at all about kiln drying, its by the local sawyers that dont own kilns.

Logic dictates that if kiln drying wasnt necessary, furniture manufacturers wouldnt go for the extra cost.

Interesting topic. If memory serves me properly, Tony's explanation sure rings a bell. Clifford.

[Tony Joyce]

.......
Logic dictates that if kiln drying wasn't necessary, furniture manufacturers wouldn't go for the extra cost.

Yes but, economics would make up for the cost of drying. Meaning, sitting on inventory for the time required to air-dry. 30 days versus 1-4 years. Time is money in manufacturing.

Tony Joyce

[Chuck Tringo]

Bingo! This is the correct answer.

Kiln drying causes a collapse of the cell walls, inhibiting the rehydration of the cells at later time. Air dried lumber's cell walls are intact, retaining it's hygroscopic nature.


Cheers,

I'm not sure I agree with this statement 100%...if this were the case then you would assume that everyone who used kiln dried lumber wouldn't have to worry about wood movement when planning/building their projects, but we know that no matter how the drying is done, wood will still move. I do have to say that I like the idea of killing off all of the critters, but not sure I buy the lack of re-hydration...just my 2 cents