Air-Drying Versus Kiln-Drying for High-Value Wood

Done right, kiln-drying gives you better control over quality. June 9, 2007

I am a full time sawyer looking for a kiln that is right for me. I am very confused about what kind of kiln is good for what application, and just how they work. I have read that kilns need a heat source and a dehumidifier. I have heard of guys making kilns with a woodstove and a room that holds the heat. That sounds wrong, like they are asking for extreme amounts of defect, but I may be wrong.

I am currently sitting on about 40,000 board feet of wood in the process of air drying. Majority of it is rare and unusual wood like curly, burls, very large slabs, and other figured woods. I'm interested in air drying my wood for about a year, and then putting it in the kiln. Is this going to decrease kiln time dramatically vs. putting wood into the kiln within a week after sawing?

I need a kiln that can handle only 2,000 - 3,000 board feet at a time. I'm interested in Wood-Mizer's solar version. Is this or any other kiln something that has to be monitored every day? I already own a Wagner moisture meter, and I monitor my wood once a month or so.

Forum Responses
(Sawing and Drying Forum)
From contributor D:
It is a scary thought to be air drying that kind of material. You are risking tens of thousands of dollars. Kiln drying is not tremendously difficult. There is a lot of information available and some good suppliers. (Though as Gene said in a recent post, not quite as good as me :-) [Nyle]

From the original questioner:
You said "a scary thought to be air drying that kind of material".

What do you mean? Are these the words of a salesman? This is the exact opposite of what I have been told for the last 10 years. Are you saying that kiln drying correctly will yield less defect? If so, that is also the opposite of what I have heard and read.

Not like tens of thousands of dollars, but roughly $150,000. I want to move this material quickly so I don't have to sit on so much money in inventory for such long periods of time. So what do you supply, and what reasons should make me believe that your statements are true? I am hoping to purchase a kiln within the next 3-6 months.

From contributor W:
With the type wood you handle, I sure hope you have it under a roof. I would look at either DH or vacuum for your kind of wood, because you sure aren't drying cheap wood like pine. You need not only dry it, you also need to kill the bugs - they love them green slabs.

From contributor B:
I'm curious about the concern over AD lumber too. Seems from previous posts, that's at least somewhat common prior to kilning lumber. Sounds like it's usually done to cut down on the drying time in the kiln.

Not to change the subject, but... what are your thoughts about your Wagner moisture meter? Have you had it long - used other brands in the past? Good luck with the inventory - not a half bad problem to have.

From contributor W:
I have no problem with air dried wood as long as it's low value or outside use. But with air dry, you have no control of the final product. Until you have it dried, defect and insect free, you just have so much fire wood. But that's just my opinion, and I've thrown a lot a boards in the burn pile.

I have a Wagner and love it.

From contributor V:
I have never had any trouble air drying oak, but on light woods like maple, air drying has left me with deep sticker stain that won't plane out. Air drying past 2 or 3 months doesn't really gain anything and could cause problems. In the summer I have had poplar go from green to 20% in a couple weeks by air drying.

From contributor I:
"Are you saying that kiln drying correctly will yield less defect?"

I think that's basically correct. Kiln drying wrong will yield more defect, of course, and sometimes companies are under commercial pressure to put more wood through their kilns than they should.

Kilns give you control over the drying. With air drying, you are at the mercy of the weather. Some species are pretty forgiving. I air dry my Monterey cypress with very good results. But other woods are not so easy. Get a couple of weeks of warm wet weather and your maple or pine will be stained. Get a couple of weeks of hot dry wind and your oak will develop checking.

A kiln run at the proper schedule will a) get the wood dry faster, and b) remove the weather risk. You have control over the temp, humidity and airflow.

Air drying then finishing in a kiln is a valid drying method. It allows much more wood to be dried if 90% of the drying is done outside the kiln. So it can make economic sense, but it does have the possible problems with the air drying.

From contributor D:
Generally you can get the best results drying lumber under controlled conditions. Air drying is not controlled, though you can do things that help, like being sure the lumber is stacked properly, covered, protected from excessive or inadequate air flow, etc. But you can control conditions in a kiln much easier. Certainly if you have a poor kiln or run it incorrectly, you can ruin lumber. But if you run it correctly, which is not brain surgery, you will get better product by putting lumber in a kiln as soon as possible after sawing. The economics of kiln drying are good too. The cost of running a kiln is usually less than a dime a board foot while the added value is much more. The degrade losses from air drying due to stain, checking, bugs, etc. as well as tying up money for a long time, are much higher than the costs of running a kiln.

From the original questioner:
So what about these guys using woodstoves to dry 1,000 or more bf at a clip? Where does one learn about running a kiln properly? Experience in time? Book/manual with the kiln?

I shouldn't have said unusual woods like just burls and other figured woods, because that probably only makes up 4,000 or 5,000 BF worth of wood in my inventory. Most of the wood is unusual, though, because it is wide. Average boards are anywhere from 15"- 55" wide. Does board width make any difference with kiln drying?

I am very happy with my Wagner moisture meter. I haven't owned any other meters, but have used pin meters, but never like the thought of leaving holes that may screw up the woodworker in the future of the board. It is a MC220.

I don't understand it completely. For example… Some reference tables say walnut has a SG of .49. Others say walnut has a SG of .57. Most say it has a SG of .55. Should I pick the average SG or what? Another is osage orange. I've seen SG's ranging from .72 - .81. If you plug in each number, it makes quite a difference in determining the moisture content. I am determined to develop a high end sawing and drying company, so would this mean I ultimately have to test the SG for each tree? Doesn't seem practical.

From contributor K:
I too dry several thousand board feet of curly maple a year. I use a solar kiln, but am now building a kiln chamber for a Nyle, and here is why. Air drying is the worst thing you can do to white curly maple. I had over 600bd ft of 5A 8/4 figured curly maple last year air drying, on stickers, and under cover, and through the winter, no less, go completely bad as it was waiting its turn into the solar kiln. The problem is with blue stain. It warmed here last Jan into the 40's and 50's for a couple weeks, and that is all it took for the mold to grow. The worst part was that it looked fine until I planed it. In those couple of warm weeks, the mold grew right through the planks. I tried resawing them, but it was all the way through. I paid the mill $900 for these boards, that I later converted into BTU's with a very heavy heart. I cried 3 times in my adult life - when Dad died, when the Buckeyes won it all, and when I burned $900 worth of curly maple that I already had buyers for at $3600. Now when I get curly maple, the solar kiln is emptied of cherry, walnut, or whatever else is in there to make room for the maple. A DH kiln is the best way to go, and those Nyle guys have answered a thousand questions for me on the chamber construction. The L200 is the one for me, and probably for you too, and no, I am not one of their salesmen. I live in OH, they are in Maine, and so far I have been impressed greatly with their service.