Christian here. New to the forum. Owner of Atlanta Specialty Woods in Kennesaw, GA. I have a KD 250 inside a shipping container that has been customized for drying wood. This is more science than fun. I've had 1,500 bf of 8/4" red oak, pin oak, and white oak slabs in the box drying for two months. Air dried for only a week for two. 1.5" thick stickers. 700 bf stacked towards the back. 800 bf stacked towards the front. Baffled as much as possible. Two months later, they are showing 8–30 mc. I removed the dryer pieces and left in the wetter ones. I cut a strip out of the middle of one of the slabs and took the 2" x 2" x 15" long piece to my band saw to test for case hardening. I had a sneaking suspicion... i didn't get the band saw through to create the test piece before I felt the stress in the wood pinching the band saw blade. Crikey!! These things are eaten up with dry/ wet spots and stress. What? Two months?? I need to know what can be done to relieve the stress in these slabs, what caused this to happen, and how to avoid this in the future. I am finding that drying wood requires a scientist with patience... I am neither patient nor super scientific. We have clients waiting on these to finish drying and for us to build their projects. What do I do? Thank you, Christian
what schedule are you following and what is the dry and wet bulbs set at? Were the slabs meant to be sliced into smaller 2 x 2s? Are they to be used/left as slabs.
By chance do you have chickens? ffcheesy
The max recommended drying rate I like to use on 8/4 white oak is about 1%, so if the wood started at say 50% moisture off the mill, then yes, two or more months would be normal and expected. Need a lot more information to diagnose and fix, such as which are what moisture content of each species and cut? Yes, kiln drying is a science, and kiln drying thick green white oak is well up on the "advanced and difficult" level. What is your current temp, what WB and DB? Right off the bat, mixing thick red and white oak, which are very different woods, especially green, is always a recipe for trouble. The pores in red oak are open throughout their length, the pores in white oak are blocked which seal the pores. After all, white oak is used for boats because it's waterproof, and a boat made of red oak will sink....so white oak is never going to behave. i.e. lose water, the same as red oak in a kiln. Their maximum allowable moisture removal rates are different, and drying them thick only makes it worse.
So if the white oak is still at 30% and the red oak is significantly lower, pull the white oak out of the kiln, and finish the red oak with a proper schedule and then go back and finish the white oak, or better yet, let it air dry some and give the wet spots time to disperse. Your load size will be low for a KD250 so you need to monitor your daily moisture removal rates to protect the wood. By the way, if the moisture content on your 8/4 white oak is generally 30%, and the max allowable moisture removal rate I like to use for 8/4 white oak is 1% per day, it ain't getting done for awhile anyway. Also if you have such extensive case to core delta moisture content, I have found the best way to try to even that out is to take it out and let is do so at it's own pace, in the air. I know that's not the answer you want to hear, but time is one way to resolve wet pockets. Or is most of the white oak still significancy higher than the red oak? I would be highly suspicious of any "dry" wood you've taken out to not have a significant case to core differential. How deep are your moisture probes going?
The other way is heat, if you are under about 15%. Sometimes you can turn off the kiln, ramp the heat up to about 130F, and let the wet pockets disperse to the dryer areas of the boards. Then, when the case and core come closer, open the door, or "power vent" and walk the moisture levels back down again. This is kind of an advanced technique, and I hesitate to recommend it, because you can ruin the load, or in extreme cases cause honeycomb. Either way, you need to very closely look at the surfaces of the boards and look for surface checks, because they generally form when the wood is wetter than 30% but don't really become apparent until later.
Anyway, prong test failures are really indicative of a significant case to core moisture differential, but you already diagnosed that in the moisture readings.
You might get some useful information from the recent HobbyHardwood YouTube video, the old guy discusses max allowable moisture removal rates for air drying but does coincidentally touch on why you don't want to dry thick white oak from green simply because of the extended time it would take in a kiln, as well as the chances of drying issues such as what you are seeing. So what you are seeing would actually be expected. Again, I'm sure this is not what you want to hear, but it sounds like you have customers waiting. Basically, you are going to have to tell them it will be awhile....
Chickens are always a good backup plan. Southside's got a mess of them....
Great response from Robert above.
Thick oak is one of the most difficult domestic species to dry. It must be dried very slowly, and you have to be very careful not to have the temperature too high during the first 75% of the drying process.
The bulk of kiln operators won't dry 8/4 oak from green; rather they air dry for 1 - 2 years first in order to minimize problems in the drying process.
What I have successfully done is to add a high pressure (1000 psi) fogging system to my container kiln. When drying 8/4 oak from green, I use the fogger to keep the RH% around 95% in the kiln for the first two months. Temp is kept around 90 degrees F. The high RH% helps to keep the surface from drying too quickly and promotes a smaller delta between core and shell MC%. It also helps to prevent case hardening.
Usually I'm using a 3 month drying cycle for 8/4 oak, as I have fewer problems the slower that it is dried.
Great responses so far. I will strongly agree that time is your friend. Never let a customer talk you into trying to hurry. It rarely ends well. I get some crazy request for kiln dried oak pretty regular, and all I can do is try to educate the customer.