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Homemade kiln?

Joined
Oct 23, 2018
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Midland, TX
Ok now, remember I'm just a newbie at all this so don't rake me over the coals too bad. I found a youtube video where a guy made a box kiln and it dried his large bowls in about 24 hrs. So my 2 bowls were already predicted to warp and crack on me, so I thought, what the heck. I thought maybe I can dry them a little faster and slow down some warping and/or cracking. I built the box kiln and put both bowls in it. They set all night, probably a total of 15 hrs when I pulled them out. Well both bowls did warp, no noticeable cracks that weren't already in them. But at entry last night, my smaller bowl moisture content was over 14% and the more parabolic bowl was over 16%. When I opened the box today, the small one measure 4.6% and the other was 6.2%. So do you think they dried too fast and this is not a good idea or were they destined for death either way? Btw, here are a couple pics of the box kiln. As you can see on the side is a shop light taped to the side. Under that is a 4" fan that I cut a hole in the box for it sit in the side. The opposite end has a small vent hole for air escape. It was in the 50's this morning in my shop but I could feel nice warm air from the box vent.
 

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hockenbery

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All green wood bowls will warp as they dry.
The process of twice turned bowls is that the thick bowl warps as it dries.
When the warped dry bowl is turned round it stays round.

About the only way I know to prevent warping is to used
PEG which through a chemical process replaces the water in the wood.
Finishing is problematic with PEG.

If your bowls were 3/16” wall thickness they would have dried in several days so the kiln speeded thing up a bit.
Generally when people use kilns to dry thick bowls for returning the kiln takes a week to a month.
 

Bill Boehme

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A woodturner in Midland, Texas needs a kiln about as much as an Eskimo needs a refrigerator. Your two bowls were nearly dry before putting them in your kiln and they were fairly thin so they would have finished drying in a few days anyway. There are places where a kiln is essential such as Hawaii, New Orleans, parts of Florida, and the Amazon jungle. Some places that are very dry use a kiln to slow down the rate of drying.

If you were a production turner making dozens of bowls a month from sopping green wood then a kiln might be useful. A properly designed kiln dries the wood at a precisely controlled rate that might be faster or slower than air drying. Drying wood is a complex process. If dried too fast the wood will be case hardened. It may not have cracks, but case hardened wood has high internal stresses. When you finish turning case hardened wood it will change the balance of internal stresses and the result will be further warping and possibly even cracking.

All of the pieces should be at approximately the same high moisture content when loaded into the kiln and the kiln shouldn't be opened until they all come out at the same time.

Don't even think about using PEG 1000 (polyethylene glycol with a nominal molecular weight of 1000). When I was a beginner about fifteen years ago I had the same urge to speed up the process and PEG was the latest fad. Although promoted as being inert and stable I found that it breaks down to a lower molecular weight liquid mess after about a dozen years.
 
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Not an expert or even vaguely knowledgeable about kilns but I recall noted woodturner, Glenn Lucas, putting rough turned bowls in a shed or building that was a big as my living room and kitchen combined. I can't remember if it was a kiln or just storage, allowing the wood to dry naturally. What I noticed was that he had dozens of bowls ready for final turning.
 
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Glenn does use a kiln, but I don't think he gave the particulars about it. I do think there was steam involved, which is one method of relaxing tension in the wood, but there are many varieties of kilns. I had a friend who used a dehumidifier with a light bulb for a heat source. It worked fine, but he had to have a drain line from the dehumidifier to let the water out. Most use a light bulb in an old fridge or freezer with a computer type fan. If your bowls survived with no cracking, you did it right, and or had the proper method for the particular wood you were using. Every piece is different, as is every different species... I turn green to final thickness, and the more they warp, the better they seem to sell.... "It's so organic!" Here in wet western Oregon, a 1/4 to 5/16 thick bowl (no I don't measure) is totally dry in a week to 10 days max. No heat.

robo hippy
 
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Here in Mid PA, I stack my wood with some air spaces between it, in a metal pole barn for at least six months. 4 inch thick Yellow poplar goes from wet to dry in about 6 months. 12 inch thick walnut takes about 2 summers Not as fast as a kiln but faster than if just covered over. The temps on a 95 degree summer day get up to about 120 in there, if I don't leave the doors open. .
 

Emiliano Achaval

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Ok now, remember I'm just a newbie at all this so don't rake me over the coals too bad. I found a youtube video where a guy made a box kiln and it dried his large bowls in about 24 hrs. So my 2 bowls were already predicted to warp and crack on me, so I thought, what the heck. I thought maybe I can dry them a little faster and slow down some warping and/or cracking. I built the box kiln and put both bowls in it. They set all night, probably a total of 15 hrs when I pulled them out. Well both bowls did warp, no noticeable cracks that weren't already in them. But at entry last night, my smaller bowl moisture content was over 14% and the more parabolic bowl was over 16%. When I opened the box today, the small one measure 4.6% and the other was 6.2%. So do you think they dried too fast and this is not a good idea or were they destined for death either way? Btw, here are a couple pics of the box kiln. As you can see on the side is a shop light taped to the side. Under that is a 4" fan that I cut a hole in the box for it sit in the side. The opposite end has a small vent hole for air escape. It was in the 50's this morning in my shop but I could feel nice warm air from the box vent.
I made me a kiln with a broken down "Lei" cold case. Looks like a soda & water case that you see by the check out stands at supermarkets, but taller. I like it because I can see what I have inside, and I don't have to open it all the time to see how are the things drying... I use the heavy duty filament light bulbs for the ovens, anything higher and my % of pieces cracking goes way up. For a hobbyist turner, I would not recommend a kiln...
 
Joined
Feb 22, 2020
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Coruña, Spain
Hi, a bit off topic but somewhat related. I read the thread and it's quite interesting.

I just built two fridge kilns and are ready to be tested. I'm a bit worried about safety as the shop I'm in is in another residential building that isn't mine and is'nt monitored.

For testing them I was thinking of filling them starting with a 40w bulb and only turning them on while I'm there, about 12h per day. Can anyone tell me what this will do to the bowl blanks? They are Cherry, alder and a bit of banana wood with a mixture of sizes from 6" to 18"

My hypothesis is that it will slow down the process but not actually harm the pieces minding that the temperatures are not increased or reduced too rapidly.

How do the bulbs fair in a humid environment? Anyone have experience with safety issues or care to look at photos of this build?
 
Joined
Jan 22, 2018
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Cameron, Illinois
Almost all bowls I turn are twice turned. Turn wet, seal, and sit drying for 6-8 months or so. I spend winters harvesting blanks (bowl and spindle) that will be turned the following fall. It is just a cycle I've started, and right now, ive got about two thirds wet bowls and one third dry, that i can grab anytime to final turn. It takes a little while to build up a stock, but worth it.

If I was going to build a "kiln" it would be with an upright freezer, and I probably would use a 40W or less bulb with a cheap thermostat to keep it from overheating. Would have holes in top and bottom to provide airflow through convection. Slow and steady moisture removal...

Even a commercial kiln wouldn't dry wood as fast as you did, as it would cause too many problems with quality.

Moisture removal needs to be a gradual process to prevent problems, particularly on thicker woods. Heat and airflow can remove moisture from the surface rapidly while the interior maintains a much higher moisture content. This creates stresses, cracking and warping.

Although not as fun because there is no electricity involved, you could probably dry those bowls in a week or less using a paper bag or box.

Thomas: as long as your light fixture as rated for outdoor use, I wouldn't worry about the bulb. IMO, your plan of only drying for 12h at a time is perfect. Very similar to a solar kiln. Heat and moisture removal when the sun is out. At night, it cools and the wood is conditioned by equalizing moisture content throughout the thickness.
 
Joined
Feb 22, 2020
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Coruña, Spain
Thanks Tim!

I also have a good bunch of bowls air drying and have got pretty good success so far. The problem would be the time and space. I've got my shelves full and more bowls to rough turn.

I'm hoping this kiln speeds the process along. It's the same system as the one you described
 
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