Kiln dried wood characteristics

Paul Brubacher

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outside of Toronto, Ont
During recent family visits I discovered that 2 of my cousins operate sawmills separately. One of them also has a kiln, I think he said that 7% moisture is the usual target.
What happens to wood when it is kiln dried?
He said that the windows in the original farm house, 100 plus years old are still good, but the ones in an addition that is only 60 years old had to be replaced. He blamed the early demise of the new windows on them being made with kiln dried wood.
 
Paul, I use a kiln that I built for drying all of my roughed out bowls and so on. I don't use a moisture meter as I seem to always manage to ruin them somehow. But, what I do is before I put something in the kiln with pencil I write on it somewhere the wood, date, and weight. The weight is the key thing for me. I usually see them lose about a third of their original weight. Also, to reduce checking drastically in the kiln I oil the piece down with walnut oil, the same I use the seal the finished product before finishing. I use 5 125 watt brood lamps for my heating. Depending on the weather determines how many I have turned on. I like to keep it at about 110 degrees. Some run theirs kilns hotter than that but I don't find it necessary. I know two very successful furniture builders and they use kiln dried wood for all of their work from tables to chairs. I don't think the early demise of the wood you mentioned is from them being kiln dried. I could be wrong. But to me if you want any wood to last it has to be maintained. Anyway, I don't know if that helps you or not.
 
Paul, Wood will acclimate to the environment in which it resides. If 7% kiln dried wood goes into a house in which everything is at 11%, then that wood will eventually stabilize at 11%. I'd be more inclined to look at the quality of wood in the 100 plus year old windows versus the quality of wood in the addition. Perhaps old growth tight grained, properly oriented and possibly hand crafted material lasted longer than mass produced stuff made with any old quality wood?
 
My cousin said that an old timer told him that kiln drying the wood causes some cellular changes, but I can't remember the details. These changes don't happen if the wood drys slowly over a number of years.
 
From what all I know about drying wood is this. Whether you do it in a kiln or let it dry out over the years, there are cellular changes going on regardless. Anytime the moisture in the wood's cells is moved or dried out it is a cellular change. Again, makes no difference how you dry it. Kiln or time. I have been using a kiln for my roughed out pieces for about 8 years and have no problems. Kilns offer wood turners and wood workers a viable option for preparing wood to work with that has been used for a long time. Some folks don't like them because they think the process is not natural or "pure". Anyway, I use one, and in fact I am working up the plans to build me a new and better one. I have a friend who is very successful in furniture building and his kiln is a 40' long metal box trailer modified with a propane heater rigged in it with rolling racks for the boards. I don't need anything like that.
 
My cousin said that an old timer told him that kiln drying the wood causes some cellular changes, but I can't remember the details. These changes don't happen if the wood drys slowly over a number of years.

He was probably talking about something like this - "Quality Control in Lumber Purchasing: Lumber Stress/Casehardening"
https://www.extension.purdue.edu/extmedia/FNR/FNR-132.pdf

This can happen with air drying (for example if you put wet wood in a really arid and windy place), but its less common.

I'm dubious that the window failure was due to kiln drying as well. My guess would be that the older windows were more heart wood.
Compare the longevity of heartwood vs sapwood in the long term post durability study here, its definitely significant:
http://www.fs.fed.us/eng/bridges/documents/tdbp/decayres.pdf

Its also possible:
  • That the old windows weren't pine at all but were cypress or something similar (or a different species of pine) that has much better durability.
  • That the old windows were better made and the new ones or the casement for the new ones didn't account for water movement as well. Sometimes there are devils in the details here.
  • There was some other cause we're not considering and it would be hard to know without doing a full forensic analysis and even then many things we'd be guessing on.
 
Thanks Ryan, those are interesting articles.
Unfortunately they don't mention in the paper on wood durability how the wood was prepared.

What was the moisture content, was it air dried or kiln dried?

Did they use galvanized nails?
 
I don't think that initial moisture content probably mattered to much as pretty much all of the specimens would have reached equilibrium within a couple of years given thier size. It's possible that if some were overly dry it might have extended the time it took to uptake moisture so that could have had some impact I suppose. The nails question would be interesting, without asking the original authors I doubt we'll ever know for sure there.

Over other point is that kilned dried wood is usually raised to a temperature that kills most microorganisms in it so it should generally be less decay prone. If there is subsequent moisture intrusion that benefit is of course lost. I see a couple of studies that appear to mostly support this, but can't find any to the contrary.

My best guess is still that there was some sort of moisture problem on the new windows that the old ones didn't suffer from or it was heartwood versus sapwood or more likely both.
 
I do know kiln dried walnut will not reach the dark brown color of air dried walnut. I also know that properly kiln dried wood does kill any inhabitants in that piece of wood whereas air dried wood to insure no live woodworkers within require a chemical treatment.
 
I wonder too if the speed at which the wood changes moisture content has anything to do with the composition of the wood.

It's possible that kiln dried wood, being that is went through the process so fast has somehow changed the composition of the wood. The air dries wood went through a much slower process.

I know there air dried wood "looks" different than kiln dries wood.

Steels definately are affected by time in a heat treat process

Food cooked slowly is different that food cooked fast.
 
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