Air-Drying and Kiln-Drying 8/4 Oak
Question (WOODWEB Member) :
From Contributor A:
I built a small kiln that is sprayed with insulation, with bottom to top air flow. It seems to be working out so far at seven months in. This type of insulation is air tight but worth the expense. We have 15 8/4 slabs of oak and pecan rotate every month.
From the original questioner:
So Iím going to put the 8/4 red oak in the kiln at 70 percent moisture content. Iím not going to put any heat in the kiln and have the fans go ten percent with the door closed. No sprays on and see what the kiln does. Does anyone think this will be ok?
From Gene Wengert, forum technical advisor:
If the humidity in the kiln gets too high, then that is not a good thing. I suggest that you follow the standard kiln drying procedures for 8/4 oak that are given in DRYING HARDWOOD LUMBER.
From contributor L:
That depends on the size of the fan motors and the size of the chambers. If you don't get to 100% humidity, that means your kiln leaks too much, either air leaks or heat leaks. If you do get to 100% humidity, then you have problems with the lumber as Gene said. The air velocity measured on the leaving air side of the stack should be checked but there is no hard number that can be recommended for that unless you know how many feet deep the charge is. I agree with Gene, follow the normal steps but the big unknown is how tight and well insulated the kilns are. The way to test that is to warm it up to about 90F, turn off the heat, set the fans to about 100 FPM on the leaving side and see if the chamber holds temp just from the heat from the fans. If it cools off then the kiln leaks and that will affect drying a lot.
I'm with Contributor L on this one. Get it in a good kiln (air tight with accurate controllers able to hold a depression of a couple degrees) fresh off the saw. I realize this isn't everyone's philosophy. I realize that thick stock oak put directly in a kiln takes weeks or maybe months longer to dry depending on how thick it is. I realize this increases your kiln costs and decreases your kiln production. Why wouldn't we want to get a high dollar, prone to defects species directly into a tightly controlled kiln environment vs. sitting in a pre-dryer at best or sitting in an air-dry yard at worst with little protection from the elements (sun, rain, snow, wind, varying temps/EMC throughout the day and night)? I am not the expert on drying thick stock lumber by any means and only hope that my comments are food for thought. It has always intrigued me that we take our thickest, most valuable products and just leave them in a semi-controlled environment for so long, put them in a kiln for as little time as possible and then wonder why we see the defects we do. I would venture to say that you have answered some of your own questions and are on the right track. Proper sampling is extremely vital with any thickness/species but especially with thick oak. My guess from your post is that your samples were dryer than most of the lumber in your kiln and you advanced your charge too quickly (higher temps/bigger depressions) causing your honeycomb. I wouldn't be afraid to start my kiln with a low temp/low depression (85 or 90 degrees/2 degree depression), and watch it. Drying too fast equals checking. Drying too slow equals mold.
I don't want to high jack this thread but you guys are using the wrong equipment. If you were using vacuum kilns, you wouldn't even consider 8/4 thick. True you do want to go straight from saw to kiln but even 12/4 will dry with zero defects in 14 days. So, if you have a market for heavy lumber, get the proper equipment. Otherwise, you are wasting a lot of time and material, which reminds me of some heavily figured 8/4 hard maple I recently dried. The owner had paid $10/BF and the 10' pieces were cut 128".
From Gene Wengert, forum technical advisor:
I agree with Contributor O's approach. But, in this case with the need to know immediately I suggest drying in an open shed (to maintain the highest quality, compared to air drying outside) and then going into the kiln at a fairly low MC (under 30% MC). I do not like to go into the kiln with slow drying lumber at a higher MC. The reason is that it ties up the kiln, so after 120 days you have added perhaps $300 to $400 per MBF to price of the lumber. This means that the 2000 BF kiln with three loads a year is generating about $1000 net profit a year. With air dried, and a 30 day cycle (and maybe you could dry ten loads and add ten times more profit, even with a small additional quality loss).
From Contributor I:
My experience is very limited, but I've been studying it and analyzing it and I think my theory is sound. I agree with Gene. The literature says that shed-drying (not yard drying) of green oak produces just as good quality oak as going straight to kiln. As long as RH is below 70% in a shed, biological degrade is mitigated. Slow drying doesn't necessarily cause mold. Specifically, mold grows when the environment is both above 70% RH *and* below 95 deg.F.
Theoretically, there is less stress on the oak in air drying than in kiln drying. So using Contributor Z's logic, why not take the safe route, be patient and get high dollar? I suppose that a climate-controlled environment would only be preferable to a shed, if the shed environment had RH above 70%, or it was windy or it was really hot.
If checking is caused when green-wood is dried too fast, and the purpose of a kiln is to dry faster, then why would you want to put a green check-prone species into a kiln? Isn't that nuts? Why even take that risk, if air drying is safer? It seems to me the best approach is to air dry your oak down to 30% MC, then kiln dry. Best not to air dry below 30% MC. If you really want top quality, use a climate-controlled environment at around 70% RH and 70 deg.F, or whatever the optimum is, where the RH high enough to slow drying, but low enough to inhibit biologicals and where the temperature is high enough that the wood will dry, but low enough to minimize checking.
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