Using a Corn Drier as a Kiln

Lumber drying is a controlled process, that an agricultural corn dryer probably cannot accomplish correctly. February 15, 2009

A corn drier is, in most cases, a small cylindrical structure with a false floor so that the burner may pass heat. This structure is much like a kiln and localized in many hardwood areas. In the corn drier, augers churn the corn so that it dries evenly. These can be removed easily.

I have some questions: can I dry hardwood in the corn drier? Has anyone attempted this before? Will it contaminate the corn drier? Its used for pig and cattle feed. Black Walnut is poisonous in some ways.

The hardwoods I need to dry is:

1. Black Walnut (4000 BF, various quarters)
2. Q-Sawn red Oak, (2000 BF)
3. White Walnut (butternut)

Thanks for your thoughts.

Forum Responses
(Sawing and Drying Forum)
From contributor B:
It sounds as if this setup may dry wood too quickly. I'd be concerned about degrade and introducing stress to the lumber. If you could figure out a way to regulate the rate of drying then you may have some luck.

From contributor S:
I'm skeptical about using a corn dryer drying lumber. I think you'd be better off looking for some ideas about solar lumber kilns on the internet, then scrounge up some materials and put in a 1,000 bd/ft unit. You'll have much better results.

The other option is finding someone in your area to dry it for you. Around here the going rate is about $.30 a bd/ft. The corn dryers I'm familiar with use propane and are designed to dry the corn pretty fast. Your lumber will be worthless if you take that approach.

From Gene Wengert, forum technical advisor:
Lumber drying requires more than just applying hot air to the wood. We need to supply the correct air flow and the correct humidity, especially at higher MCs. We start, with a wood like oak, at around 110 F and 87% RH with about 250 fpm air flow past the lumber's surfaces. For hard maple, maybe 100 F, 65% RH and 600 fpm. For pine, 130 F, 60% RH and 1000 fpm to 240 F at times. Basically, the answer is that you have a heat source that is not suitable for drying lumber. As you probably already have heard, some species, like walnut, have sawdust that is not healthy to animals.

From the original questioner:
I have read up on the solar dryer. My experience with drying is very limited. I have only dried outdoors and in my basement. In the basement, I dried black walnut very evenly over the course of a dry winter in Minnesota. This time, there is more, as I mentioned - too much for my space.

There is an old-school corn dryer in the field - a non-powered version with louvers and grates. Its more of a shelter than anything. We cut 4x4 walnut posts and spanned the dryer turning it into a huge stacking shelter that captures the breeze. Will air dried hardwood be sufficient for furniture production?

From contributor G:
Air dried wood would be good for outdoor furniture, but I wouldn't use it for indoor stuff. Then again, it depends on how you build things. If built in a way to allow for contraction, then maybe. The finish would also have a lot to do with it.

From contributor B:
Maybe put some fans in there and paint the outside black. I have a customer who has built fine furniture for years using nothing but air-dried wood with good results. You'd want to be careful about panels in doors and things of this nature. Oil finishes are his specialty.

From Gene Wengert, forum technical advisor:
In a home in MN, you will oftentimes see 6% MC in the wood, even with a finish. (The finish slows the MC change but does not stop it). So, if you air dry, you may achieve a low of 12% MC in your unit that you described. The wood will therefore shrink about 1-1/2% more after you build something. You can use a solar heated shed (paint black, etc.) or you can dry the wood further in your heated shop before use, both of which will get it to a lower MC and therefore have less shrinkage in use. (It is impossible to make good furniture at 12% MC for indoor use).

Note that in the old days before central heating, the interior climates were not as dry. This is why in Colonial times they were able to avoid kiln drying the wood. However, if you go to Colonial Williamsburg, you will see that wood is kept in the loft above the shop before use, which was hot and dry location and provided the dry wood. Further, the wood stays in the heated shop during manufacturing for a long time as production speed is rather slow compared to today.

If someone says they use air-dried wood, a check will indicate that they dry the wood lower than the 12% MC (the value that is the lowest achieved in air drying in most of North America). Usually they do this by bringing the wood into their heated shop area just before they use it.

As has been discussed here many times, air dried wood has a risk of insects (lyctid powderpost beetle especially). For softwoods, a risk of resin bleeding for years is high. Temperatures over 150 F are commonly used to provide insect-free, resin-free wood. Outside air drying can also result in a lot of quality loss. Your corn drier should keep the sun and rain off the wood which will greatly improve quality over outside air drying.

From the original questioner:
It may be the case that the old corn drier may be turned into a solar drier with some modifications. However, I think that the insurance that commercial drying offers is paramount due to the quality of these products.

From contributor R:
I put some freshly cut black walnut lumber in a Butler grain bin a while back. My bin is 24' diameter and 18' at the eaves, with a perforated floor. It has a ten horse centrifugal fan, which is started would likely unstack the lumber rather quickly. I covered the inlet on it and installed a one horse axil flow fan designed for ventilation, not drying, that I bought at a farm sale. It gets pretty warm in these bins without any additional heat. My guess is 140 degrees or so. I did a check of the walnut in a while back and it was at about 12%. I have no warping or splitting to speak of.

Anyway, I would certainly give it a try. Many will suggest that you have someone dry it for you. Where I live you're about as likely to find a custom kiln operator as you are a combine in Manhattan.

From Gene Wengert, forum technical advisor:
I have worked with Amish that have kilns. I have worked with Amish that dry their lumber after air drying in their heated shops.

From contributor G:

There are many Amish communities. Many of them are excellent woodworkers, but some are not. I have seen staircases built by them that after a few months started twisting bowing and cracking. Kiln dried lumber would not do this. Sounds like to me that a corn drier is a good way to get to 12%, and then finish it off in a nice heated shop. Having air conditioning in the summer also helps if you live in a high humidity area.

From Gene Wengert, forum technical advisor:
When drying oak, we normally start at 110 F and 87% RH and hold that for a week or more. Could you do that in a corn dryer? If not, then forget the idea of drying green to 12% MC; instead, just air dry to 25% MC or so.

Black walnut, especially if thick, also needs special control of conditions. The sawdust residue left from drying certainly would be a hazard to some animals. You would really have to clean the dryer after drying lumber.

From contributor G:
We must consider the fact that drying stickered lumber in a corn drier pulls air around the stack and not through the stack. Therefore, the drying rate would not be consistent with accepted air drying techniques. You would have to watch the rate of moisture removal per day so as not to damage the wood in the process. I would think the rate of drying would be less than conventional air drying. The big guys just sticker and stack it outside. When air drying, you should avoid exposure to wind, rain, and sun. Sounds like to me that you meet all these requirements in your drier-you might need to run the fan on a low speed.