Kiln-Drying Reclaimed Heart Pine

A few simple tips for a new kiln operator. October 2, 2005

After several months, a lot of reading, and a good bit of trial and error, I have finished my kiln. It is electric, heated by what was once heat strips out of a commercial air handler unit. The fan is a large squirrel cage fan out of an industrial size air conditioner. It is belt driven, with the motor outside the kiln. The kiln is about 14ft long, 8ft high, and 7 ft wide.

During my research, I found that the kiln should be well insulated and as air-tight as possible. We obtained very good results with minimal exchange of air from outside, even with the driving wind inside. I am wondering if maybe I have it too air-tight? I am not drying green wood, just reclaimed heart pine. The air inside is so humid it drips down the walls and steams. Seems like this steaming air should be vented pretty quick to get rid of the moisture, not sealed up inside, right? How will the wood dry if it's in a sauna? Should I let it build up, then shut the kiln down and open the door to let the air escape periodically? Should I install a sort of vent? What size vent would be recommended for this kiln, if one should be used?

Speed is issue #1, economy is #2. Electric heat is not the economical way, I suppose, but it is the most practical for me at this time. I don't think I have to worry about much checking/splitting, since this wood is old as the hills anyway, but I could be wrong. I'd like to dry it in a matter of a day or two if possible, without damage. How could this be done? Are my expectations too high? I haven't had a lot of luck finding info about drying reclaimed wood.

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
Someone has not given you very good advice on the design of an electric kiln for pine. (Air tight is not a good property.) You need heat of at least 160 F and you should have a vent to exhaust moisture when the RH in the kiln is too high. To give you a good set of operating conditions, we would have to know starting and final MCs at least.

From the original questioner:
Thanks, Gene. I wasn't very confident about the info I was getting. But, as I said, I had a hard time finding any info at all about drying reclaimed wood.

I guess we do need to vent the kiln. I am starting with wood that will probably average somewhere around 12% to 18% MC, and want to finish it to 8% MC or thereabouts. No wetter than 10%. This wood is to be made into flooring, and most will be sold in my area for now. Most homes in the deep south where I am seem to run somewhere around 9% inside, so this would get me as close as I can get to a "ready to install" product (although I still recommend acclimation in the home).

The wood is always going to be 1" thick or less and usually 6" wide or less, if that gives you an idea of what I need to do to dry this as quickly and efficiently as I can. If I may ask, why 160 degrees? I can do that easy enough if I need to. Is that temp for setting the pitch? Or is it needed to get the moisture out of the wood? Thank you for your time and knowledge. I wish I had talked to you sooner!

From Professor Gene Wengert, forum technical advisor:
At the low MC, you have no great concern about drying conditions being too severe. You might start at 130 F and 8% EMC and then gradually increase to 160 F and 7% EMC. This would get some pieces a little drier than needed, but using a higher EMC would lead to longer drying times. I prefer 160 F to set the pitch and to make the process more quickly. It does help set the pitch. At least use 130 F to assure that insects are no longer active.

Be very careful about selling this wood, as there are many restorations that are willing to pay good prices for "old" heart pine wood. You might contact local museums and maybe even run an ad in a museum or antique magazine. Over the years, I know of several museums that wanted such wood for authenticity, and price was not an issue. This also means that you should consider leaving some of the wood in a rough state so it can be manufactured to a required size. Good luck.

From the original questioner:
Thank you for the advice. I ran the temp up to 160 and let it run a while this afternoon. I also let the door open just a crack. By the time I got back to check on the wood, I had already overshot my mark a bit. The meter shows the wood to be in the 6 to 8 MC range overall (after making temp and species corrections). It dried much faster with the door cracked than it did with it shut tight.

Looks like I'll be venting it for sure. I spent about 300 KWH drying this charge. I can live with that, no problem, but I think now I can reduce that a fair amount. This was my first load through the kiln. I am proud of the job it did, and the time and energy consumed. Thank you again!