Lumber Conditioning Advice

Info on mixed loads, re-wetting dried lumber, and related topics. September 25, 2014

Question (WOODWEB Member) :
I am nearly done building a kiln and have been reading up on its use. I've snooped around the Knowledge Base and have been reading the "How To Dry Hardwood Lumber" book and have a couple of questions. First, I read that rewetting wood will swell the potential surface cracks to the point of their actually pushing the cracks deeper into the wood, then that a good conditioning will rewet the surface to relieve surface tension and make things better. I am guessing there is a point of wetness I'm not picking up on and would appreciate any advice or opinions.

Second, I'm picking up mixed reviews on mixing thicknesses in a single charge; understanding things are best to use the same thickness. Is it taboo to mix 4/4 and 8/4? Is it that the thinner wood will absolutely dry too much and be destroyed, or is it more a matter of economics in needing to slow the drying down to protect the thinner wood while finishing off the thicker? I will be drying the same wood (big leaf maple or black walnut mostly) in a single charge, and not in much of a hurry at it.

Forum Responses
(Sawing and Drying Forum)
From contributor K:
You can mix thicknesses but what you will need to do is load the kiln such that the thinner stuff is on top. I've had luck just opening the kiln and pulling the stuff as its done. You won't have luck making it all finish at the same time as 8/4 lumber takes more than two times as long to dry as 4/4, especially if you are drying from green. I would recommend air drying everything down below 20% MC before drying as it saves a bundle in time and energy. 4/4 walnut that is 20% or less MC will dry in a dehumidification kiln in less than one week, 8/4 walnut air dried will take a little less than two weeks. The one big thing that will help you that most people elect not to do is buy the Lignomat meter and probe system for a couple hundred bucks. You get little probes on wires that plug into their meter. You tap say six different samples in your kiln and run the wires to the outside. To check the core MC, which pinless meters aren't good at telling so well, you just plug each wire into the hand held Lignomat meter. It saves tons of time, is pretty accurate and gives a good sample of the different thickness and types of lumber in the kiln.

From the original questioner:
Thans. Our Nyle unit has such pins (it's in transit as I write this), and from the literature it seems that the pins provided will do this (I have lots of learning to do). Im going to keep a lot of records and check everything along the way with an alternative tool just for comparison purposes. I have read a thread of putting the thinner wood on top to take it out early and had that in mind, but experience is a better teacher than a book. We do have plenty of woods cut at varying thicknesses, but like you said we're not likely to have tons of 4/4 stuff every time. Everything we have cut has been under cover with open air flow for at least a year, should be close anyway.

From Gene Wengert, forum technical advisor:
When mixing thicknesses, you have the risk that if the thin material is much drier (from short air drying perhaps) than the thick stock, that you will have some moisture regain, as the humid conditions required to protect the thick stock are too humid for the thin, drier stock. If the MC is the same at the start, then your only risk when mixing is that the thin stock will get too dry. To avoid that, avoid the very low RHs. In other words, the kiln will be on an equalization setting for many days (6% or 5% EMC for hardwoods). It may not be the most economical operating condition, but it is the correct and safe way to mix species and thicknesses, so that makes it reasonable. When conditioning, the added moisture, so long as it is vapor and not liquid when it hits the wood, will not damage the wood as the wood is twice as strong. But, just to be safe try and avoid checks during the early stages of drying. Then things cannot get worse.

From contributor Y:
Gene, would you please expand on your comment about the wood being "twice as strong"? I'm not sure I follow.

From Gene Wengert, forum technical advisor:
In round numbers, dry wood is twice as strong as wet wood. So, it is perhaps twice, and even more as difficult to damage the dry wood.

From the original questioner:
Thanks for the help, I missed the "vapor only" part of it - makes better sense now. Economy isn't the greatest of my concerns (though of course I care) and lightening up on the EMC does make good sense. Once we get it up and running I'm going to dry up some stickers and get at it. Time to read some more!