Question (WOODWEB Member) :
I've been spending a lot of time, money and effort trying to dry all my lumber down to 7% in the core with probes and sample pieces. When it抯 done drying it goes in my big unconditioned pole barn (every lumber yard here in central Michigan has unconditioned lumber storage) and in a few weeks its reading 10-12%. Why bother to kiln dry to 7% in the first place? The only thing I can think of is sterilization, but I run the kiln at 140 for the last four days anyhow. The first 24 hours at 140 should do it.
Why fiddle so much with the final MC if it always sits a few months before the customer gets it anyhow. It seems like this is a lot of wasted effort. Am I missing something? My air dried lumber is close to 12% MC. Can't I just put in at 140 for a couple days and call it kiln dried and sterilized? Do most hardwood furniture grade lumber yards have conditioned storage of all their stock when it comes out of their kilns?
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
I doubt that the internal temperature of the wood will reach 133F when the air is 140F. I also doubt that you will increase the MC as much as you indicate in such a short time. You can wrap the lumber in plastic wrap and then store it for years. Air drying to 12% MC is excessively dry and is not usually done in air drying as degrade is high. A common value is 25% to 20% MC in air drying.
On the internal temps of 4/4 lumber reaching 133 degrees, do Nyle type dehumidification kilns not have the ability to sterilize then?, I've been told not to run them over 140 by Nyle. I've not thought about the idea that the lumber might not get to 133 even after a number of days at 140. This is a good question to figure out. Might have to call Nyle and see what they suggest.
Of course, if the grain is not straight, some warp can also be seen. Anyone delivering 10% or wetter lumber to a furniture, cabinet or flooring plant can expect to hear back about wet lumber. There are many cases where such wet wood has resulted in lawsuits with damages exceeding $1 million in some cases.
Bottom line: You need to dry to 7% MC for hardwoods and then provide storage facilities at 35% RH (7% EMC). You should also meter at least ten pieces just before you ship, to prove that the wood is still at the correct MC and that any problems later would be because of poor shipping or storage by the customer and not by you. In fact, it would be wise to specify that all your MC is measured with a Model 2000 Delmhorst insulated pin meter (or similar Lignomat), as some customers use an inferior meter.
Maybe summer is the wettest time and I can keep the lumber in a big tent in the barn and dehumidify, though our winters are quite humid too. It often takes six months or more to turn over our stock. I guess I抣l have to see how I can keep the lumber dryer in storage, but I抦 still stuck with the fact that the facility that retails our lumber is unconditioned and the lumber will simply go to 10-12% over the summer months. I did talk with the Nyle guys about the heat treating, and was told they normally suggest keeping at 140 for 24 hours.
I see moisture creep back in to dry lumber as quick as the original poster does. A stick dried to 8% will be about 11-12% in just a few weeks in unheated dry storage (metal roof). But bring it indoors, still unheated, and it will maintain the MC within an acceptable range.
Why is ADing to 12% too dry? Many woodworkers would kill for real seasoned AD material that was dry enough for use in furniture work. I try to dry to 15% or less before kilning (five months for 4/4 in Hawaii, most all species, 12-13% in six months).
I think harvesting, drying, storing, end use and the size of your operation all create unique circumstances that a strict scientific method cannot work with. I think many people especially in Northern N. America specify KD because of the obvious disadvantage of using wet lumber. Typical AD may only be 15% at best. Here in Hawaii many people will use 15% AD for furniture and cabinets depending on the climatic zone.
If there is no benefit of drying to target when the material will experience gain in storage, maybe the OP can only dry to 10% then custom dry for orders and get it out the door fast. It also sounds like if you抮e going to sell retail then you should invest in a show room that has some form of climate control. At least make it a bit tighter.
I have a friend that I dry small custom loads for. I dry to 8%. His storage and the duration it sits there yield him 12-14% final, but he still sells it as KD. I do not believe you can even call it KD after it gains so much. For personal use I only try to dry enough for a month's work at a time.
Once dried to 7% MC, the wood, when exposed to 12% EMC will not come to 12% MC, but more likely will reach 11% MC on the surface. The core will take many months to change from 7% MC to even 9% or 10% MC. I do wonder if the MC readings you give in your example might be taken with a pinless meter and not with an insulated pin meter. If the EMC is 12% then the surface might be 11% MC (due to hysteresis, it will not reach 12% MC) and the core will be 7% or maybe 8% MC in a month. So the average MC for the entire piece will be perhaps 9% MC, but probably even less after a month or so. We did a big study here in Wisconsin a few years ago monitoring the MC of lumber in unheated storage and found that the average MC gain was under 1% for the first month. Of course, the surface and ends did gain more.
As mentioned before, it takes about four gallons of water to change the MC of 1000 BF by 1% MC, so when 7% MC lumber is reported to be 12% MC, that is about 20 gallons of water that had to go into the wood. This is why using plastic will prevent virtually all moisture change, as there is little moisture that can enter through the wrapping. Also, when the EMC is 12% EMC, the average MC of the surface is likely only 11% and the interior of the wood would be even less. So, it would be hard to imagine that lumber dried to 7% MC can reach 12% MC when stored at 12% EMC (the conditions in most of the 48 states).
Incidentally, if the EMC is 12% EMC, it will be hard to dry a piece of lumber to 12% MC. The surface and ends might reach 12% MC, but the core will be wetter. Of course, in air drying where we have rain on the lumber, the lumber would be even wetter.
Generally, we do not want to air dry much under 25% MC as it takes too long; if the lumber gets wet, checking and warp will be worsen; and color loss is a risk. If we air dry under cover, the quality will be maintained, but the length of time makes air drying to a low MC too expensive. At some point (usually 25% MC), it is cheaper to kiln dry than to continue to air dry.
Note that in Hawaii we usually do not have central heating like we do in much of the 48. Central heating makes the interior EMC around 6% or 7% EMC. In Hawaii with open windows, your interior conditions are close to 10% EMC (50% RH) or a bit higher, so that 10% MC is going to work very well. No need to reach 7% MC unless the furniture is going to a dry interior location.
I guess my comment was smaller operations and individuals that are drying for instruments and artisan furniture works may not share the same philosophies on drying that larger commercial industry organism抯 do. Some woods retain a richer and more vibrant color when AD longer before kiln-drying the last bit. Some species and specimen are too precious to be regarded and processed as lumber. When AD to 15% or less I can dry to 8% for less than 12 cents a foot. The lumber is very stress free as well. Some of the kindest to re-saw.