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Cob kiln  RSS feed

 
sasha Wharton
Posts: 8
Location: south east kansas
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O so i have never build A. a kiln and B. anything out of cob but now with a lack of resources i am infact building my own design for a cob kiln wich i hope will work so i can therefore spread me knowledge so i cant help poor ignorant people like my self, but i need help i just dont know when the cob is mixed right right now were going off of some youtube videos and a few internet resources if anyone has any advice i would surely appreciate it. did i mention its a dome witch is something else i have never built before.
 
Ryan Basye
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In making a bread oven, they pile up a mound of wet sand in the shape you want and then form the cob around the sand. When its dry you take away the wooden door that you made previously and put where you wanted it, and then drag all the sand out. That is it in a nutshell. I suppose you could look at rocket stove to also give you ideas. Maybe you could put the pottery where the vermiculite or other insulation is supposed to go in a large rocket stove.
 
A Strathman
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While I've not made a kiln either, I do know you can make a buried kiln quite simply by digging a pit, adding fuel and greenware then covering with more fuel and setting the result on fire (this is used for small scale production of terra cotta).

If you want high-fired pieces, you'll have to put some more effort in. I don't know how cob responds to really high temperatures; I suspect the reinforcing straw might carbonize and lose strength and the clay will probably slump under it's own weight if you get it too hot. You'll most likely need at least a refractory brick lining.

Here's a buried kiln design I found in a quick search on Google. It uses electrical heating.

On the same search I found this which has a good survey of different kiln types and some fairly detailed advice on building and operating a kiln, The Self-Reliant Potter: Refractories and Kilns (GTZ, 1987, 134 p.). The link is to chapter 2, the rest of it is available there as well.

Just remember to be careful if you try this out. Keep it well away from flammable things like trees, grass, or your house in case something goes wrong.

Best of luck!

 
Christian McMahon
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This chap built a rocket oven out of cob and then turned it into a 1200 degree kiln. It would be a good place to start. He also discusses his rocket kitchen. I really want to make a rocket kitchen now. You can get an idea about construction if you study the pics.

 
Judith Browning
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Location: Arkansas Ozarks zone 7 alluvial,black,deep loam/clay with few rocks, wonderful creek bottom!
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bump
 
Peter Ellis
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Location: Central New Jersey
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A few thoughts here - There were kilns built in the middle ages that consisted basically of a pit with a trench leading into it for air flow in and another coming out for a chimney. They worked very much like rocket stoves in the sense of a draft that pulled air through the fuel and produced a clean hot burn. These were built with little more in the way of tools than a shovel.

When making a cob oven, you don't include straw in the first interior layer or two, because it will carbonize. There is virtually no risk of the clay slumping in the cob oven application. Outer layers of the cob mix get straw in the usual manner, and typically don't get hot enough to carbonize the straw.

When you are talking about making a kiln out of cob, then you are getting into higher sustained temperatures and the possibility that the clay of the kiln might fail is real. Different clays fire at different temperatures - this is the basis for the "cone system" for judging temperatures in kilns. Cones of different clays that melt at different temperatures are used to indicate the temperature in the kiln - the cones will slump when their melting temperatures are reached.

So if you make a cob kiln and heat it too far for the clay used in the cob - then you could have a problem. You need to know what temperature the clay you want to fire has to get to for good results, and then you need to know that the clay your cob is made of can hold up at that temperature. If you happen to know what kind of clay you are making your cob with, then you should be able to find references that will tell you what temperatures it can handle.

If you don't know what kind of clay it is, then you might want to do some testing. I would make several cones of the clay I was using to make my pottery and of the clay I was using to make my cob. Be sure that you can tell them apart! Then I would mess around building hot fires and putting the two kinds of clay cones into the fire, watching to see which one melted first. If the pottery clay melts before the cob clay, then I would go ahead and build my cob kiln. If the cob clay melts first, then I would start looking for things like good fire brick, because I would not want to trust the cob to hold up.

 
Kris Johnson
Posts: 80
Location: Pahrump NV
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This thread seems pretty dead but I'll attempt a resurrection with my 2 cents.

You need to insulate the cob if you want to reach stoneware temps. Otherwise your kiln will never hold enough heat.

The height to width ratio of your kiln should be equal, the depth can be whatever you want. So if your kiln chamber is 3' tall it should be 3' wide.
Your chimney should be 3 to 3.3 times the height of your chamber. So if your chamber height is 3' your chimney should be 9-9.9' tall.
The chimney area should be equal to the volume of your kiln but in inches. so if your kiln is 3' tall by 3' wide and 6' feet deep its 54 cu.ft,
the chimney area should be 54 inches square or about 8"x8". Your air intake should be equal to the chimney area, what goes in must come out.

These figures will get your kiln to the temps you need to fire clay. They may be a bit "off" but they're better than aimlessly building something.
That being said, people have fired clay for thousands of years using basic techniques, with very spectacular results.
 
Glenn Herbert
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Just found this thread and will put in a few comments from a potter who has built numerous small-scale replicas of medieval English pottery kilns, as well as firing full-scale gas and electric kilns.

The notion that medieval kilns had one stokepit and one exit flue across from it is old theory and quite false. The most common medieval kilns had two arch-topped stokepits on opposite sides of an oval or cylindrical firing chamber four to six feet across, and the gases exited the top, which was most likely open and covered with a layer of broken pottery or turf, giving even heat flow through the whole chamber. (No medieval kiln tops have ever been found, so this cannot be proven, but the covering method was in use up to the early 20th century in England.) It works fine on small-scale kilns (3-6 cubic foot) for earthenware temperatures.

To test your cob material for durability in a kiln, you cannot build a fire that will get hot enough to show anything useful. You really need to make some small samples of your proposed mix and have a pottery or ceramic studio test-fire them under various conditions. Studios are often reluctant to fire unknown pottery, but may agree to make tests for you with appropriate safeguards. My experience with native clays has been that they work fine at earthenware temperatures (cone 06), and may work up to typical electric-fired low stoneware (cone 6) but have melted at typical gas-fired stoneware temperatures (cone 10). Your mileage will vary... I saw a slideshow of a rocket kiln in Latin Amrica which was fired to medium earthenware temperature once for a test, and melted some of the ware they were firing! Their usual temperature was apparently very low earthenware. On the other hand, some traditional pottery-making regions in the US use native clay straight out of the ground and fire it to cone 12.

If you are building a more modern style of kiln, the info above sounds good. There are more technical details that have to be gotten right in this case.

For the cob mixture to use, I have found that clay mixed with dried grass clippings works very well. The most important feature aside from standing up to the fire is insulation, and the grass will burn out and leave lots of tiny voids. It doesn't need to be mixed perfectly, just pulling out obvious stones, mixing the clay evenly to an almost sloppy consistency and then wedging in the grass clippings to leave no grass clumps or pure clay spots does it. I make a frame of bent sticks or withies for a form and pack the clay around it. Applied wet, the lumps join well, and the grass provides wet strength, and draws water out of the clay and stiffens it up much faster than if the grass were not there. For this application, grass works as well as straw, since it will all burn out anyway in the interior structure. I will make the structural walls and arches 1 to 2" thick, and add loose earth around and over that for insulation. This all needs to be sheltered from rain if you want it to last.

For firing, I strongly advise getting a variety of "large Orton cones" for indicating temperatures achieved inside the kiln. Any ceramic supply place will have them. Get at least something like cone 010, 06, 04, 01, and maybe cone 1 or 2... higher temps if you are confident of your process and expect more than earthenware temperatures. Anything fired to less than about cone 010 will be very soft earthenware that will break if you look at it crosseyed. In firing, it is good to leave a peephole that gives a view deep inside the kiln. You will need to see the pots glowing bright orange-yellow before they are any good, and typical earthenware bisque firing of cone 06 has to be a bright light yellow... this is extremely intense heat and can be dangerous to look directly at for long.
 
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