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  #11  
Old 07-24-2010, 08:01 PM
david s's Avatar
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

Cheryl,
Yes you need to dry the clay out before adding water to it. It is very difficult to get water into clay that already has some water in it. It becomes impervious rather than porous. If you let it dry right out, break it up, it will suck up the water into itself really easily. I have recycled tons of potters clay over the years using this kind of method, although I haven't been too successful getting girls to join in. usually end up doing the job by myself, all alone, so sad.
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  #12  
Old 07-25-2010, 06:41 AM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

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Originally Posted by david s View Post
Cheryl,
Yes you need to dry the clay out before adding water to it. It is very difficult to get water into clay that already has some water in it. It becomes impervious rather than porous. If you let it dry right out, break it up, it will suck up the water into itself really easily. I have recycled tons of potters clay over the years using this kind of method, although I haven't been too successful getting girls to join in. usually end up doing the job by myself, all alone, so sad.
Okay so then if I buy a dry clay in a bag I've saved this step then I'd think. That does make sense thta it's hard to get water into already moist clay. I think maybe dmun's comment about it being hard to mix up your own potters clay from powder applies more to making pottery and not cob.
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  #13  
Old 07-25-2010, 06:50 AM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

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Originally Posted by dmun View Post
Clay ovens aren't insulated?
Well yes clay ovens are insulated I think but I'm building a cob oven not a clay over. Most of the pizza oven kits actually seem to be moulded of clay which you put together and seal with refractory mortar and then cover with insulation like an insulation blanket but thats not what I'm building. (I would have liked to but I don't have an extra $4,000.00 to spare!) I'm building my oven out of cob. Cob is a mixture of clay, subsoil, water and straw. You don't insulate a cob oven with anything more than more cob, however that insulating layer of cob has more staw in it. As far as I have gathered you use the same mixture of clay water subsoil and straw to insulate as you did to build the original dome, you just add a higher proportion of straw which offers the insulating quality. Of course I'm pretty much a hack at this, I'm trying to pull together as much info as I can.
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  #14  
Old 07-25-2010, 08:43 AM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

Just throwing this out there as an idea on sourcing clay that would work in your cob mix...but have you considered using bentonite in your mix (available at farm supply stores)? How about clay cat litter?
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  #15  
Old 07-25-2010, 01:42 PM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

bentonite isnt that also used as heavy drilling mud in the petrolium indutry?

as for insulation, for a cob oven i think clay and sawdust is the norm.

For buying clay 50/50 mix of fireclay and grog as been done, and I have heard craft crank is also recomended which has grob built in, but have no experience of either.
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  #16  
Old 07-25-2010, 06:25 PM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

Cat litter has additives that would likely be bad ideas in really hot ovens. Also, have you priced that stuff lately?

I doubt you could work it well enough - cat litter clumps but doesn't hold together well.
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  #17  
Old 07-25-2010, 11:36 PM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

I know that people say not to buy potters' clay, but I bought a bag for $30 which was enough to do the whole dome - saved a huge amount of time and gave a great result. It was a very fine dry powder in the bag - easy to dry mix with sand and aggregate in a wheelbarrow, much like mixing concrete.

Cheers,
Mick
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  #18  
Old 07-26-2010, 06:47 AM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

Quote:
Originally Posted by Grimaldi View Post
Just throwing this out there as an idea on sourcing clay that would work in your cob mix...but have you considered using bentonite in your mix (available at farm supply stores)? How about clay cat litter?
I like the ideas but cat litter wouldn't be good as mentioned becasue they add chemicals to it. Who knows what horrible stuff would be realeased if I heated it to 800 degrees F! Bentonite might be a possibility...
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  #19  
Old 07-26-2010, 06:48 AM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

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Originally Posted by BackyardPermaculture View Post
I know that people say not to buy potters' clay, but I bought a bag for $30 which was enough to do the whole dome - saved a huge amount of time and gave a great result. It was a very fine dry powder in the bag - easy to dry mix with sand and aggregate in a wheelbarrow, much like mixing concrete.



Cheers,
Mick


Okay, this is what I needed to know. What did you buy a 50 lb bag?
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  #20  
Old 07-26-2010, 03:57 PM
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Default Re: Cob Oven -What type of clay should I buy, where can I buy it?

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What did you buy a 50 lb bag?
Yes, I think so (25kg here, maybe 30kg). The mix was 1:1:1 clay:sand:fine aggregate, and then 10% portland cement. You also need fibres - cutting up natural rope works well, about 3 feet of rope cut into inch-long pieces and then separated into the fibres. I think that's better than using straw - either way, you need the fibres to prevent cracks.

Cheers,
Mick
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