Self Air-Drying Versus Kiln-Drying

Carefully air-drying wood yourself may be preferable to trusting the load to a less-than-perfect kiln-drying operation. January 18, 2011

Suppose you had almost no experience drying wood, but had read a lot about it. Suppose you have about 900 feet of potentially very valuable freshly sawn figured hardwood that you wanted to build furniture with. It is wide and clear. Youíve exhausted all of your personal resources, and in the end you can only find a place from the internet that does kiln drying, but as a small part of their business. They give you a flat rate, without asking much about the wood. They tell you itís no problem to mix 8/4, 6/4, and 4/4. When you visit their facility, they have a load in the kiln consisting of about 5 different species.

Would you have them dry your wood? Or, if you have a pretty good setup, would you air-dry it (assuming you canít find anyone you personally know and trust to dry it)? In other words, do the risks of potentially improperly kiln dried wood outweigh the risks of air-drying? I can give a complete breakdown on the wood and the air-drying set-up if necessary. But kiln drying is a big unknown to me.

Forum Responses
(Sawing and Drying Forum)
From contributor M:
The kiln operator you visited is not drying properly (mixed species and thicknesses is not good practice for several reasons) and it only takes a few mistakes in the kiln schedule when the wood is above 20% MC to destroy the load.

Because of this, I would air dry properly to below 20% MC first, then you can safely kiln dry down to 8%. Another approach is to air dry to say 12-14% then move the stickered stack into a heated space to bring the MC down to 8% or so. The only drawback to this second approach is that you will not be able to kill any insects that may be in the wood. The standard kiln procedure is to raise the temperature in the kiln at the end of the drying schedule to kill any insects or their eggs and larvae. If your wood is oak, definitely air-dry it first to below 20%, then kiln dry. I would kiln dry oak from green only if the kiln operator specializes in drying oak and has references.

From the original questioner:
When you say that improper kiln drying can destroy the load it really puts me at ease that in my situation air drying is the way to go for now. At least it will give me more time to find somebody Iím comfortable kiln drying my wood with.

The wood is yellow birch with a flame or curly figure all the way through, sawn through and through (the logs are veneer grade). I have the wood pretty well stacked outside, off the ground, stuck properly, etc. There are a few pieces that I didnít have edged, because they have an interesting live-edge. The trees were cut in the winter, so the bark is pretty well stuck. Due to various time-constraints, I have the pieces on the pile, at the top. Will the bark attract bugs? Is it a bad idea to keep these pieces anywhere near the rest of the pile?

From contributor H:
Northern red oak dry schedule is 3.8% mc loss per day. How much mc loss is there on these nice warm windy days air drying? Youíre worried about some kiln jockey wrecking a load of figured hardwoods but letting nature do it for you is risky. I will somewhat air dry my woods but only the months when the humidity is the highest (slows down mc loss).

From contributor M:
The advantage to air drying is that the wood may dry quickly during the day, but will have a slower drying rate at night when the temperature drops and the humidity increases. In a kiln if you have the dry rate too fast and don't catch it the wood will honeycomb and be useless. If you dry too slow the wood can get fungal stain, warp, and bow. A kiln with the correct drying schedule for the species and thickness of wood will produce fewer defects, but as stated the only kiln available does not practice good drying methods.

I am on the west coast, so my experience is with oak is with Oregon white oak. I would never dry it at the listed maximum drying rate, it is not worth the risk. By the way, on the west coast we have high humidity in the winter months and low humidity in the summer. Most of our air drying occurs in heat of the dry warm summer months.

From contributor G:
My concerns are the same as Contributor S's. Air drying outside this time of the year can be very risky to a newbie, especially with thick lumber. I would be hesitant to kiln dry it for you once it is air dried. You will blame me for damage you did to the lumber by improper air drying. Air drying quickly during the day is not an advantage for all species. Damage can easily occur in less than a day if air dried too fast. The lumber doesn't care if it's being kiln or air dried. Too fast is too fast, too slow is too slow. I would recommend that you get the lumber in a shed ASAP that has good airflow through it that can be controlled.

From the original questioner:
I do realize it would be ideal to have this stuff properly kiln dried, but I just havenít been able to find anyone Iím comfortable with. I may end up ruining it, but Iím going to do my best not to.

Contributor G, the point is that I donít want to have to blame anybody at all, except for myself if the wood is ruined. If I do get it kiln dried, it will be by somebody whom Iíve built a relationship with. Iíd check for case hardening, uneven moisture, and other defects before I sent it off - not so I could blame him if something went wrong, but more so I could understand more about the process.

At bottom, there is a picture of the stack. I plan on weighing it down tomorrow, laying poly on the dirt underneath, and putting up shade cloth as soon as I can, though it is mostly out of the path of the sun this time of year anyhow. The narrower stuff on the top is birch I had sawn at the same time, but with some knots and no figure. Iím not too emotionally invested in that. If anybody has any advice on what else I can do to ensure that it doesnít dry too fast, I would be grateful.

By the way, I live in Maine - does that work to my advantage weather-wise? I plan on moving the 4/4 into my heated shop before winter, if itís dry enough. I plan to check it regularly with my pinless moisture meter.



Click here for higher quality, full size image

From contributor G:
Nice air drying set-up. Keeping it out of the sun as you are is a very good idea and so is the shade- dry. With the lumber that high off the ground poly probably isn't needed. It looks like you have done your homework. In the kiln operator's defense, I have always mixed 4/4-8/4 lumber in the same load with success. The lumber must be well air-dried before mixing thickness and certain species. I use a DH kiln that operates at a max temp of about 120 degrees.

From Gene Wengert, forum technical advisor:
Anyone who says they can mix species and thicknesses green is not the type of kiln operator that will produce quality lumber. The drying will be too fast for some wood and too slow for other. It just cannot be done safely. Air drying is fairly good if you can keep the rain off. Yellow birch dries well in air drying.

It is interesting to hear the comments about low humidity, etc. Certainly there are low humidity days every month, summer and winter. There are also high humidity days. In fact, the relative humidity for almost all of the USA does not vary much, summer to winter, on the average. In most of the USA, the average outside humidity is 65% RH, which is 12% EMC; exceptions are the humid coastal areas and the dry mountain west.

What this means is that there are some days throughout the year that might be too dry or too humid, but on the average, the wood will dry fairly well. Of course, with yellow birch, we do not have the high concerns that we would have with oak and checking. Nevertheless, end coat it; do not dry it too slowly (stain risk) or too fast (checking risk); protect it from the rain. Once you are down to 25% MC, then it can be kiln dried normally without much concern if conditions are typical and not way out of range. I assume you checked the list of kiln drying people here at WOODWEB.

From the original questioner:
A whole lot of my homework was done right here at WOODWEB. There is so much useful information here, and itís all people taking time out from their lives to help other people. Itís great, and it feels good to hear some approval of my air-drying set-up.

Gene, the whole RH thing confuses me. Iíve looked at charts listing the average RH in various places and on the whole there does seem to be little variance. Iíve thought RH as the most important factor in drying, with factors like sun, wind, and rain coming into play, but why do I read that drying almost stops during cold winters? Is this true? Is it because the water literally freezes or something? There is not any explanation given, just that drying slows way down.

We did put a call in to the one place listed at WOODWEB within my area, but didnít get a response. The whole thing was planned very poorly on my part. I am young and just getting started in a lot of ways, so the prospect of veneer grade flame birch caused me to bite off maybe more than I can chew. I donít regret it, but itís taught me to establish a plan well in advance.

The reason I started this thread was because I really just donít know a lot. I figure that thereís little incentive for somebody to kiln dry 900 bf of wood for someone else while following the ideal, perfect schedule, so I know concessions must be made. I just didnít know where the lines should be drawn, and this thread has helped me decide on that a little more.

Another question - as Iíve said, the birch is highly figured. It has the flame all the way through. It is almost totally clear, except around the pith. The logs were straight with a slight taper. Should the figure worry me, or should I take any extra steps? I believe the flame or curl is because of reversing grain, and I know that figured wood can and does go crazy, but is there anything I can do at this stage to help that? Weigh it down like crazy?

From Gene Wengert, forum technical advisor:
The flame means that you might have a little more warp, but it is not a significant factor - just stack the wood correctly. Pieces that contain the pith will often have a large split and can warp, but it is not an event related to the flame. The three factors influencing drying are temperature, RH and air flow. In your shed, air flow is probably low, which is not the best for a light colored wood. If there is any way to increase air flow, I suggest that you do it.

In the wintertime, drying does indeed go on and it is possible to check and end split lumber. The RH is 65% RH, and there is air flow. The cooler temperatures do mean slower drying. Certainly, the further north you are, the less sunlight and cooler the temperature, which does mean slower drying.

Finally, once you are under 25% MC for 4/4 and 5/4, there is little chance you can damage the lumber in drying (except over-drying, under-drying, poor stress relief). Also, well AD lumber can be damaged by letting it get re-wetted. Re-wetting and over-drying both encourage cupping. You may find that with the pith, cupping will be a problem, so do not encourage it by rewetting or over-drying.

From contributor C:
Geneís advice is spot on. I have been drying with a DH for six years and it is never advisable to mix thicknesses. I air dry oak to under 20% then KD with very little degrade. Keep talking to people till you find someone you trust to KD for you.

From the original questioner:
Iím definitely taking in Geneís advice. Iíll rig up a tarp system for when itís raining hard, and set up some strategic fans for when the air isnít moving. Iím going to set up for the best air-drying I can, and like you said Contributor C use that time to work something out with somebody with a kiln, for when itís 25-20%, to kill those bugs, even things out, and get closer to the day when I can start working it!

I feel a whole lot better than when it was dead stacked on the truck and calling frantically around everywhere to get it immediately and perfectly kiln dried. Iíve received a lot of great advice, and thank you all. All of this makes me appreciate the preciousness and awesomeness of wood more than ever, as well as advice from the experts.