drying wood in pottery kiln

[Stephen White]

ok I have done some searching and while this question seems to be asked from time to time, it also seems to be answered by folks with no real kiln knowledge. We have several electronically controlled kilns and two of them are pretty decent size. While kilns glaze fire over 2k degrees they also do what is called candeling. This is a process where when pottery being bisque fired to partial vitrification (first firing) and the potter suspects all of the dried pots may not be thoroughly bone dry. The temp will be slowly raised and will be held at just below boiling for a number of hours to get all moisture out. Now that temp is usually about 180-190 (water boils at 212) but I think it can be set as low as you want and then held at that temp for as long as I want. I was reading that wood is often dried at about 130-135 but it is often determined by wood species.

Our two larger kilns are also fitted with a vent that pulls the internal air out as it fires. There are three drilled holes in the lid and a fitted spring loaded cup on the bottom and a $400 fan that sucks out the airflow and is vented outside. In pottery this is to vent off gasses as the temps reach 1000-1200 degrees and to create a good firing atmosphere.

It strikes me that that really covers drying wood. If starting at room temp, I raise the temp say 10 degrees an hour and hold at 130, with the vent going to pull out moisture. This is electronically controlled so the ramp can vary and different temps can be programmed at different points.

If the answer is that this sounds like it would work, how long would I need to do this on say a chunk of green just cut wood to be able to turn it?

[Richard Coers]

You can't raise the temperature of wood that fast. But that depends at what moisture content you start with. Most wood kilns start around 90 degrees and go up, but that would be based on the wood species and wood thickness. If you pour the heat to wood by dramatically raising it 10 degrees every hour, it will case harden and then the super dried "skin" has be softened to the core moisture so the core moisture can come out. In a commercial kiln that can be accomplished by injecting live steam into the kiln. If you continue to rush it, you get the case hardening and then honeycombing in the interior where the wood cells shatter. You can also case harden and trap the moisture in if your moisture meter doesn't read deeply enough. Then when you turn it or resaw it, the wood moves like crazy. I suggest you look at kiln schedules that the government prints out to get a better idea of how to dry wood. It's not as simple as pouring the heat to it. I'm suggesting you do the research because when I usually explain how wood must be dried, no one believes me and I find myself arguing with "kiln experts" that have never dried wood before. Another consideration is for your kilns. Woods like red and white oak put out a highly acidic moisture during drying. All commercial kilns are made of stainless steel. Your kilns are probably stainless steel too, but I don't know what happens to the nichrome heating coils with all that high humidity acidic air inside at low temps like 100 degrees.

[Stephen White]

You can't raise the temperature of wood that fast. But that depends at what moisture content you start with. Most wood kilns start around 90 degrees and go up, but that would be based on the wood species and wood thickness. If you pour the heat to wood by dramatically raising it 10 degrees every hour, it will case harden and then the super dried "skin" has be softened to the core moisture so the core moisture can come out. In a commercial kiln that can be accomplished by injecting live steam into the kiln. If you continue to rush it, you get the case hardening and then honeycombing in the interior where the wood cells shatter. You can also case harden and trap the moisture in if your moisture meter doesn't read deeply enough. Then when you turn it or resaw it, the wood moves like crazy. I suggest you look at kiln schedules that the government prints out to get a better idea of how to dry wood. It's not as simple as pouring the heat to it. I'm suggesting you do the research because when I usually explain how wood must be dried, no one believes me and I find myself arguing with "kiln experts" that have never dried wood before. Another consideration is for your kilns. Woods like red and white oak put out a highly acidic moisture during drying. All commercial kilns are made of stainless steel. Your kilns are probably stainless steel too, but I don't know what happens to the nichrome heating coils with all that high humidity acidic air inside at low temps like 100 degrees.

Hey thanks for the response I just grabbed 10 degrees an hour out of thin air. It could start at 90 and be one degree every 2 hours or any other combination I think. I was looking for my controller manual but its a digital setting so I think it can be any thing but I think your right it will not work because it could trash my kiln which at several thousand bucks would not be worth drying a little wood for.

I think I will just go buy some blanks

[Kyle Iwamoto]

Disclaimer. I am one who has ZERO knowledge with a clay kiln. I agree that you cannot rush wood drying. 10 degrees a day would be too fast. 10 degrees in 1 hour or 2 hours makes little difference. If you can set 90 degrees and hold it for a day or 2 that could work. I use a fridge kiln (Cindy Drozda) and a 40 watt bulb takes a week to raise it 10 degrees........

[Stephen White]

Disclaimer. I am one who has ZERO knowledge with a clay kiln. I agree that you cannot rush wood drying. 10 degrees a day would be too fast. 10 degrees in 1 hour or 2 hours makes little difference. If you can set 90 degrees and hold it for a day or 2 that could work. I use a fridge kiln (Cindy Drozda) and a 40 watt bulb takes a week to raise it 10 degrees........

you could do that. You can raise it to whatever temp you want and hold it for however long you want. My kilns fire to 2300 degrees over 10-12 hours for glazing so a day or 2 at 90 is no problem. You can use any ramp schedule to 90 or whatever that you want with the electronic controller but now I'm worried about the 'acidic' moisture that Richard mentions. The whole ideal of candeling is to draw off the moisture out of clay because if there is any moisture at all in the clay when the temp moves above boiling the clay pot will literally explode at some point in the climb and mess up both the kiln elements and bricks. The vent is constantly pulling in new air through the lid and moving the old air out the bottom but not sure if the wood moisture Richard talks about would still trash my elements and that would cost hundreds of dollars and easily offset the value of the wood I dried. While the kiln is not made of stainless steel they mostly do all have stainless steel outside skins holding the fire bricks together.

There are now some old kilns with electronic controllers out there, someone should experiment with one and see if it works. I use mine for our pottery business so can't take the risk. Still it sounds promising. Even the older kilns with just a sitter might work on the lowest candling setting. They just have switches and the lowest switch might be close to 180 though than 90. Maybe removing a row of elements though would get you back to the 90 level. Really old electric kilns can had for free or really cheap as often folks just ant to get rid of them. The vents are not as cheap but they are built to run for hours on end.

[Alex Zeller]

How sealed is the kiln? I think that could be the key to how well it works.

[Stephen White]

How sealed is the kiln? I think that could be the key to how well it works.

They are not air tight and have the vent holes. Could probably seal one a little better but they are not going to be air tight. The acidic moisture mentioned make me not want to do it in one of our production kilns but if we move to a larger shop at some point I might grab an old kiln that has worn elements for free or cheap and give it a try. If/when I do it I will find this thread and update it.

[Dave Mount]

I don't know anything about pottery kilns, but I do about lumber kilns, and I don't think this approach will work, at least not as easily as presented. Lumber kilns operate at high temperatures, but they also operate at high humidities until the wood is pretty dry. If you hit green wood with dry heat (as I suspect the pottery kiln exhaust would be) it will dry too fast on the outside and check.

To dry wood you need to do two things. The first is obvious, evaporate water. The second is that you have to move water from the interior of the wood to the surface of the wood fast enough that the outside doesn't shrink too much faster than the inside, which is why checks form.

In lumber kilns they control both temperature and humidity. High temperature helps move water from the inside to the surface of the piece faster. High humidity (initially) keeps the water near the surface from evaporating too fast. I've pasted below a kiln schedule for drying 1" cherry (not that cherry is of particular note; other temperate hardwoods are generally similar). Note that the first drying step is 130 F at 81% humidity, which is the same as 100% humidity at 123 F. Although the pottery may be putting off some moisture, I'm betting you'd have to inject a ton of moisture into the exhaust from a kiln to achieve that. I suppose you could add misters into the flow, but you have to have some feedback control, because if the moisture is too high, the wood won't dry.

In regards to how fast, kiln schedules of 15 days are not uncommon for drying domestic hardwood lumber. Depends in part on how much it has air dried first.

If you want to read about drying wood in kilns, the USFS Forest Products Laboratory has a ton of free publications.

Best,

Dave

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[Stephen White]

I don't know anything about pottery kilns, but I do about lumber kilns, and I don't think this approach will work, at least not as easily as presented. Lumber kilns operate at high temperatures, but they also operate at high humidities until the wood is pretty dry. If you hit green wood with dry heat (as I suspect the pottery kiln exhaust would be) it will dry too fast on the outside and check.

To dry wood you need to do two things. The first is obvious, evaporate water. The second is that you have to move water from the interior of the wood to the surface of the wood fast enough that the outside doesn't shrink too much faster than the inside, which is why checks form.

In lumber kilns they control both temperature and humidity. High temperature helps move water from the inside to the surface of the piece faster. High humidity (initially) keeps the water near the surface from evaporating too fast. I've pasted below a kiln schedule for drying 1" cherry (not that cherry is of particular note; other temperate hardwoods are generally similar). Note that the first drying step is 130 F at 81% humidity, which is the same as 100% humidity at 123 F. Although the pottery may be putting off some moisture, I'm betting you'd have to inject a ton of moisture into the exhaust from a kiln to achieve that. I suppose you could add misters into the flow, but you have to have some feedback control, because if the moisture is too high, the wood won't dry.

In regards to how fast, kiln schedules of 15 days are not uncommon for drying domestic hardwood lumber. Depends in part on how much it has air dried first.

If you want to read about drying wood in kilns, the USFS Forest Products Laboratory has a ton of free publications.

Best,

Dave

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sounds like you know what you are talking about. Yeah the vent is pulling in airflow from the room (with my top loading kilns that is through the top) and exhausting out the bottom through a hose and outside. Sounds like even if you could get the temp, temp ramp and humidity right the time would get ya. Running a pottery kiln for a day or three on such low temps is probably fine for maybe hundreds of cycles but doing that for 2 weeks not so much.

Still old electric kilns with worn out elements (meaning they won't fire to 2000 degrees anymore) are pretty available for free or very cheap so might be worth someone experimenting with it if you have 220 power available. My plates full and hearing about the wood acidity moisture from the drying wood scared me off using one of my production kilns.