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Thread: Bowl Drying Kiln

  1. #1
    Join Date
    Aug 2009
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    Bowl Drying Kiln

    Well, due to my absolute impatience with waiting for rough turns to dry, I decided to build a kiln. I researched the heck out of it and finally landed on a somewhat simple design (until I started trying to automate it...). One big trick is that my space is very limited. I found some great unused space under my clamp cart that would allow me to build one that will hold 3 or 4 bowls.

    It's nothing more than a box framed with 2x4's rippe in half, and then skinned with 1/4" ply I had left over from various projects. I made sure to seal all of the joints with caulk and lined the floor, ceiling, walls, and door with insulation. Heat is generated by two light bulbs on the bottom, (1) 125W heat lamp and (1) 100W standard bulb. These get the temperature up to about 95 in 30 minutes. You'll notice a thermostat in the photo that automatically shuts off the lights when the desired temp is reached. It is hinged so that I can fold it ack out of the way to load bowls. I have a large equipment fan mounted in the top to keep the air circulating. I also have a thermometer/hygrometer (relative humidity) mounted to the door that has a remote sensor mounted inside. There are some vent holes in the top of the box with covers that I can open if the relative humidity gets too high.

    I haven't thried it yet, but will be sure and repost with some of my results. My thinking is cooking it at 95 -100 with a RH of around 40%-50% should give me pretty good results. I welcome any feedback.
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  2. #2
    Join Date
    Jan 2006
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    North central Pa Tioga Co.
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    I am interested to see how this works for you, I have seen old refrigerators turned into kilns keep us informed! Looks good.

  3. #3
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    Jan 2005
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    Keep us informed Clark as I am interested in building one.
    Bernie

    Never put off until tomorrow what you can do the day after tomorrow.

    To succeed in life, you need three things: a wishbone, a backbone and a funnybone.



  4. #4
    Clark, good idea for space saving, different woods will react differently, be very careful with cherry

    different shapes make a difference in wood warpage

    bowls warp vs. hollow forms warp very little using wood from the same tree, try it

    my hollow form i turn to finish thickness

  5. #5
    Clark:

    This is an interesting idea. I'm far from being an electrician, so how do you wire the thermostat to turn the lights off and on?

  6. #6
    Join Date
    Aug 2009
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    Quote Originally Posted by Greg Just View Post
    Clark:

    This is an interesting idea. I'm far from being an electrician, so how do you wire the thermostat to turn the lights off and on?
    My background is in the electronics industry, so for me figuring out the wiring was not too big of a deal, though time consuming. The biggest problem I ran into was that here in Texas, it is impossible to find a thermostat that is rated to switch 120VAC since we do not have base board heaters, etc. This probably is not a big deal in other parts of the country, so the wiring could be simplified. Never the less, it forced me to have to work with two different voltages, which made it more complex.

    For me, I had to install 12VDC transformer and a 12VDC relay. I have the thermostat switching the 12VDC to the relay. The hot leg of power for the lights are running through the normally open side of the relay. So, when the thermostat turns on, it actually is only turning on the relay. Having installed a transformer did simplify one thing for me , though, and that was the fan. I had access to a number of old fans that were pulled out of equipment racks, but they were all 12VDC. Without the transformer, I would have had to order a 120VAC fan.

    THis was a long way of answering your question. To be more direct, you would interrupt the hot leg of the lights (or relay) and tie the wires into the W and RH terminals on the thermostat.

    Charlie - thanks for the info on hollow forms. I have not done any yet (havent really turned many bowls for that matter...) but was wondering if they warped like bowls do.

  7. #7
    Join Date
    Feb 2006
    Location
    Mason Michigan
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    Clark,

    I built this one and have done one load and I overcooked it.http://www.sawmillcreek.org/showthre...highlight=kiln Only one bowl cracked but the others are really really dry. I had trouble figuring out the thermostat but I think I have it now. I have not been able to turn for a while but should be getting back to it soon.
    I am going to be building a dehumidification kiln this spring that will be 4x 8
    A few hours south of Steve Schlumpf

  8. #8
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    Jan 2010
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    Lewiston, Maine
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    Very interesting. Anxious to see how it works for you. Good luck & keep us informed.

  9. #9
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    Quote Originally Posted by Pete Jordan View Post
    Clark,

    I built this one and have done one load and I overcooked it.http://www.sawmillcreek.org/showthre...highlight=kiln Only one bowl cracked but the others are really really dry. I had trouble figuring out the thermostat but I think I have it now. I have not been able to turn for a while but should be getting back to it soon.
    I am going to be building a dehumidification kiln this spring that will be 4x 8
    Thanks for the link to your thread. I really appreciate the source you used for the thermostat. I imagine that I will probably change mine out to a more industrial one after I determine if it is worth while or not.

    You said you overcooked the rough tuns. How were you determining when to remove them from the kiln? My thought was to use the weighing method: take the weight of a sample bowl before putting it in the kiln. weigh it again after the 1st stage of drying, then weigh it every 1-2 days after that until the weight stabilizes. Based on my understanging of the physics, the weight should at first drop significantly. During the second/third phase of drying it should fluctuate up and down until it reaches an equilibrium.

    My only thought on yours drying out too much is that the RH (relative humidity) got too low in your kiln. Your original thread stated that you left in in there until the RH was 35%. In the reading I have done, I seem to recall that you want to keep it up around 50% during the entire process. I found an incredible amount of information on this process from bow making forums and web sites. They have to keep their wood from becoming too dry so that the bow doesn't break when under pressure from shooting or even stringing. I found a great link that provides what the moisture content equilibrium is at different RH levels:
    http://www.fpl.fs.fed.us/documnts/fp.../fplgtr113.pdf. In chapter 3, it gives a lot of info related to this. Based on the charts (page 57, I think), at 90 degrees and 50% RH, the moisture content of most wood will balance out at about 9%. I also found another source that provided these general values (RH on the left and moisture content on the right):

    30% = 5%
    40% = 8%
    55% = 10%
    65% = 12%
    75% = 14%

    I guess my only question is what is the ideal moinsture content for bowls? Keep in mind that the bowl will continue to try and equalize with the environment it is in after you remove it from the kiln. However, if you remove it, then finish turn it, and then immediately apply a finish, any further movement should be minimized. At least, that is my understanding....

  10. #10
    I build a simular kiln last month (http://www.sawmillcreek.org/showthread.php?t=131269). I put my first load of bowls in it ten days ago. I ran it for two days with just the fans blowing, then set the thermostate for 70 degrees. Yesterday, the Humidity was 59, so I moved the thermostate up to 80 and opened a vent. Today the Relative Humidity in the kiln is 49. I am just playing this thing by ear, don't know when I need to do what. Thought I would just play with it until I figure it out. Thanks for posting a source for information on the subject, I'll look that over.

    I used the T110 Single Stage Waterproof Thermostate from: http://www.littlegreenhouse.com/acce...controls.shtml

  11. They have a great article in the winter issue of Woodturning Design #24.They go through the steps to make one out of an old fridge.By adding a lock on it you can put it outside.

  12. #12
    Join Date
    Jan 2005
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    Goodland, Kansas
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    Clark thanks for the link. That is some very interesting reading.
    Bernie

    Never put off until tomorrow what you can do the day after tomorrow.

    To succeed in life, you need three things: a wishbone, a backbone and a funnybone.



  13. #13
    Join Date
    Nov 2008
    Location
    Southwest Missouri
    Posts
    30

    wood kiln from fridge

    Would anyone happen to have a copy of the Woodturning Design #24 magazine that Robert mentioned that they would be willing to part with? Or even possibly posting the plans? I have an old fridge and an old upright freezer I have been saving for such a purpose.

    Thanks
    Jeff

  14. #14
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    Nov 2006
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    Remsen Ia.
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    Here is the wedsite with the article on it. Ron

    http://www.rudieswoodwork.com/kiln.htm

  15. #15
    Join Date
    Nov 2008
    Location
    Southwest Missouri
    Posts
    30

    kiln plans

    Thanks Ron,

    Thats what I was looking for

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