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Humidity Control, Warping, and Case-Hardening

Maintaining the right drying conditions is a tricky problem, and improper control can lead to defects in the lumber. January 2, 2012

I finally decided it wasn't my logs, but my kiln drying that is wrecking all my super nice cherry. I have a Nyle 200 kiln with about 1500 bd ft of space. Is a twist indicative of any particular drying problem - too slow, too fast, bad conditioning? Too dry? I often sneak different species in the same load, and nothing twists like the cherry, so it's not the piles or the kiln carts.

Maybe part of the problem is the fact I got too dry, down to 3% on the cherry. Does this wreck the lumber or does it bounce back to normal when acclimated to 6-8%? This was the only time I got it so dry, but the twist is common. Does cherry just do this? I saw heavy to 1 3/16" to give room to joint, but my nice clear 14" wide boards get trashed every time!

My biggest problem in general is conditioning, as I can't simply throw the bucket of water on the floor, as my kiln is on a wood floor in my barn (covered by plastic). Is there a specific humidity to keep it at for a certain time? My Nyle kiln manual isn't so specific, though it's really old. I've been winging it and have lots of tension in all my lumber. (Pinching on the tablesaw blade is common.)

On the cherry problem, I did notice more cupping in my red oak than normal. This might be drying too slowly. The schedule seemed pretty speedy till the end.

Forum Responses
(Sawing and Drying Forum)
From contributor A:
You are drying too fast.

From the original questioner:
Thanks. Is the clue that cherry twists when dried too fast? I thought I read in Gene's articles somewhere that oak cupping can happen when drying too slowly. This sure is an art form.

From Professor Gene Wengert, forum technical advisor:
The faster you dry (low RH), the stronger the outer layers of wood fibers, which means that the wood will be flatter. The lower the final MC, the more warp of any kind, and raising the MC afterwards will not remove the warp.

How did you measure 3% MC? It is virtually impossible to get that dry even in a steam kiln. You would have to run under 10% RH or 1% EMC. DH kilns cannot get that dry.

Twist is caused because the fibers are at an angle within the wood due to spiral grain (growth factor) or diagonal grain (sawing effect). Due to the low RH in air drying, compared to some kiln schedules (remember that it is not the schedule, but the actual conditions, that are the key), air dried lumber will usually be flatter.

Cup is also a natural defect, but it is greatly accentuated by adding moisture back to the dry surface fibers (starting the kiln at too high an RH) above 20% MC.

So, in your situation, what is the initial, actual RH for cherry and for oak and what is the initial MC?

Here is an article that I wrote which should help your understanding.
Warp in Drying

From the original questioner:
Thanks. I used a Wagner L606 pinless meter that seems to be quite accurate. Actually there are no numbers under 6%, just says "dry," but I was halfway from 6% to 0 so I thought it would be 3%. I'll oven dry and check.

This might have to do with my schedule, as my kiln unit is quite old and I think it doesn't pull enough water when very dry. I talked with the Nyle guys and they had me turn off the compressor and turn up temps at the end, so maybe 5 days at 150 degrees with fans on to dry the core. I will read the article and see if I can glean more info.

Normally I air dry to 12-15% before putting in the kiln, but I still notice twisting in the cherry (only about 10-20% of boards do this). This load was wetter than usual, with cherry and oak around 30-40%. I used oven dry method, and didn't feel like my samples lost too much water too quickly, but I'm a hack, so possible. Think I was at 2-3% MC per day at most, as I was careful of the oak.

I can understand a log that might have some lean to it, but these were quite straight and forest grown. Seems to happen quite a bit in the wider planks (ones I was hoping to sell for a premium). I'm also curious if finishing my loads at 150 might impart more tension in the lumber, as when I'm machining it often seems to pinch the chop saw blade and the tablesaw blades.

From contributor D:
In your kiln, what is your system to put weight pressure on your stack? Is it the same when you air dry outside? For the stress or tension in your lumber, the best thing is to use steam for conditioning, about 10 gal/1000bf.

From contributor T:
I'm with contributor A. I think it's drying too fast, causing tension, or not stickered close enough. I don't own a kiln but understand tension that can be created from drying too fast. Does the warp seem to appear in the beginning or towards the end of your process? This could give the others clues on what may be happening, such as pulling too much moisture too quick or too much heat at the wrong time. Wider boards seem to have more issues according to my readings. Gene could probably advise you if something is done different in starting the kiln for the wider boards.

From the original questioner:
Well, I called Nyle to see what they might suggest. I oven tested the cherry and Gene was right, even though the meter showed 3%, it is 5%, so that is better. Below are some pics to show my troubles. All the wood is very case hardened, as you can see from prong test. I use the same setup to weight the kiln as the air dried stacks, so I don't think that's the problem.

After talking with Nyle, my theory is that my kiln chamber is probably leaky enough that all the water I added escaped rather quickly, and I continued to simply dry out the jacket, if I even got it to equalize at all. I bought a cheapo hygrometer today, and will restack the 1500 bd ft back in the kiln, and try to get it balanced while checking RH.

I do have a steam generator for a steam box, though it only holds about 2 gallons and is propane fired, so leaving it running scares me in the old barn. Thinking of adding that to my setup. Now that lumber is cool, it's supposed to equalize easier.

I need to figure out what RH to hold the lumber at what temp for how long to condition. In some of the archives 4-10 hours was mentioned for steam injection.

I'm worried I've been selling bad lumber for quite awhile. I think I've been helped by the fact that it's mostly air dried and not green. Green seems to case harden the worst in my setup, and this load was pretty wet. Right now I'm putting 6 five gallon buckets with 2 gallons in each with a towel for a wick. They dry up, so I thought it was going into the wood. I also wet the sides of the chamber down with a deck sprayer, as my floor can't have water dumped on it per instructions.

I know these crazy systems are terrible to figure out, as there are so many variables, and few actual meters to tell what is going on inside, but there isn't profit enough in what I'm doing to do it right. I'll keep trying through trial and error and hope my customers don't give up on me. I've sure noticed that when I've had to buy hardwood from the local retail store, their lumber is dead flat, and neither springs apart nor pinches the tablesaw blade. It's almost likes it's plasticated - they sure have it down to an art. My 2" stock is even worse!

Click here for higher quality, full size image

Click here for higher quality, full size image

From Professor Gene Wengert, forum technical advisor:
Faster drying has less warp. With high RH and slow drying, the fibers are not strong enough to resist the natural tendency to warp. That is also what happens when air dried lumber is put into the kiln and the humidity is too high initially... The outer fibers are wetted and become weaker, so the wood warps, especially cups.

Your most recent post points to excessively high RH when starting the kiln, as well as a bit of over-drying. You need to start the kiln and have conditions under 10 % EMC at all times. You need to realize that over half of the shrinkage occurs above 18% MC. As casehardening, a shrinkage defect, occurs above 45% MC, but is relieved gradually in air drying, the longer AD occurs, the less casehardening.

From the original questioner:
I finally got a RH meter in the kiln and my system of conditioning with buckets and deck sprayer is the problem. I have too low of RH for conditioning. Thought, if I'm evaporating 5 gallons of water, how is it different than dumping it on the kiln chamber floor? Well, guess it is!

Curious on your comment, Gene, of having too high of a RH to begin with when drying. What RH is recommended to start with? I use the Nyle schedule of 70% compressor at 90 degrees until under 35-40% MC, I think, though I don't know what RH I was at actually. Like you said, it's actual conditions that matter. Hate to think of all the case hardened lumber I've been making.

Is it not possible to case harden air dried lumber then? We dry down to about 15% MC usually, so unless it happened to the lumber while air drying, it won't happen in the kiln at that point? You mentioned it occurs above 46% MC. Aren't some species like ash only around 40% MC when green? Can't case harden ash then?

From Professor Gene Wengert, forum technical advisor:
Casehardening occurs when the outer fibers try to shrink but the wet core prevents them from shrinking fully. So, they dry in an enlarged shape, which is called tension set or casehardening. It occurs during the loss of the first 1/3 of the moisture. So, for ash which has an initial MC in the 40s, the casehardening occurs from 45 to 39% MC. See Drying Hardwood Lumber for more details.

Regarding warp, the outer fibers after air drying are around 12% MC in most of the USA. We need to avoid adding moisture back to avoid increasing warp. Therefore, the EMC at startup should not exceed 10%. This includes heating up too.

From the original questioner:
Thanks. I think I see what happened. The high initial RH probably is warping the cherry. I'll have to see what my actual RH is when I start the next load. And the lack of RH at the end is making it so I can't condition the case hardening.

I bought a turkey fryer and have it pumping steam into the kiln and now the RH has come up to 55% at 115, which should give me 9% on the lumber jacket. Can't remember the EMC there... but this is 2-3% over target MC of lumber. I'll look this up in my manual when I get out there. Hard to get the RH up high enough with this goofy rig, but better than my old system.

Thanks for the all the input!

From Professor Gene Wengert, forum technical advisor:
You can buy small boilers, or you might consider renting one to see how well it works before investing.

The key for conditioning is rapid addition of moisture to the shell at as high a temperature as possible. Also, the wood cannot have a shell to core MC gradient and all pieces should be within 2% MC to assure uniform relief. In many countries, conditioning is done in a separate chamber.

From the original questioner:
Where would you go to rent something like this? Regular tool rental company? Not sure what one of these would look like? I thought of a steam wallpaper remover, but they aren't very large volume.

From Professor Gene Wengert, forum technical advisor:
There are boiler rental companies. Probably 20 HP is the smallest. You might be better served if you rented a steam cleaner for car engines. Many industrial rental places will have them.

From the original questioner:
In relation to this issue, would this mean that if you air dried lumber to 12-15% MC you would most likely not have to bother conditioning the lumber? I guess it could case harden while air drying... Do people usually cut up a board every time and do the prong test? From what you mentioned I could avoid case hardening green lumber I put in the kiln by drying slower above 46%, or will it always happen to some degree?

From Professor Gene Wengert, forum technical advisor:

It is impossible to avoid in conventional drying. In air drying, the casehardening is partly relieved at night when the RH reaches 100% RH. Given enough time, the small relief at night adds up to little stress in well AD lumber.

From contributor R:
One thought for conditioning... I have a Nyle L200 as well, and when I'm ready to condition, I turn the heat up to 160, crack the door, and stick in a power washer wand with an adjustable misting tip on the end. It really atomizes the water, and gets circulated immediately through the stack, and didn't cost anything since I already had the power washer. The water isn't steamed, but it seems to work well.