Cob Oven -What type of clay should I buy, where can I buy it?

[CherylACT]

I have begun the process of building a Cob oven in my backyard, so far that means I am almost done constructing a waist hight base out of concrete block and rocks from my backyard. I have located where I can buy a small quantity of fire brick, I know where I can get sand, i know where I can get straw, and I can use the subsoil from my backyard. Clay however is eluding me.

My geotechnical engineer of a sister tells me I have silt in my yard and not clay, my clay seems to be mixed with a type of gravel in my yard. I don't have friends with a stream in their yard and it would just be too weird to show up at a nearby park with buckets and a shovel to take clay from there. Plus I'm not very confident in my ability to tell clay apart from other types of dirt!

So I would like to buy clay, but I don't know what type to buy or where to buy it from. Looking online at pottery clay there are so many types. They all have different firing levels. One was a mexican red clay which would air dry, I was thinking that might be the thing for me. Should I buy something already moistened or should I buy a powder that I can add to my cob mix. Help! Thanks so much! I am really enjoying reading all about everyones ovens they are making, I can't wait to be baking bread and making pizzas in mine!

[dmun]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

Potter's clay is not cheap, even the powdered stuff you mix yourself. (Have you ever done it? It's really hard work.)

There are lots of clay grades. It's been so long since I've been in ceramics that I've forgotten what I used to know. You absolutely don't want the self-setting clays: They have a polymer in them that will burn and stink on the first firing.

If you do a cost comparison between commercial clay and firebrick you may find out why it's been two centuries since builders made their own brick. I think the bottom line is that if you aren't quarrying your own clay, it makes no sense to build your dome with it here on the East Coast where firebrick is so cheap.

[david s]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

Dig your own clay,let it dry, then break it into pieces no bigger that your fist then place in a pile onto a large sheet of plastic. Hose water into it and start squishing it around with your bare feet. It is fun, especially if you include alcohol and girls. Every so often grab the corners of the plastic and heave the mix into the middle again. You can add chopped straw at this stage. Great fun and costs nothing.

[Tscarborough]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

What part of Connecticut? I found geological charts at quadrant scale to tell you where to dig.

[Archena]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

Try calling gravel companies (that do retail/small quantities)- if they don't have it they will likely know who does. Landscapers may also be able to help you find what you need.

Do not buy potters clay - it's overkill and not worth the money for this kind of use.

I'd recommend getting "The Cob Builder's Handbook" by Becky Bee (yes, that's really her name) before you go any further. She covers clay and clay mixes very thoroughly.

Try Biblio.com - they are a clearinghouse for booksellers and often you can find things Amazon can't and usually at better prices (especially if slightly worn copies don't bother you).

[CherylACT]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

Dig your own clay,let it dry, then break it into pieces no bigger that your fist then place in a pile onto a large sheet of plastic. Hose water into it and start squishing it around with your bare feet. It is fun, especially if you include alcohol and girls. Every so often grab the corners of the plastic and heave the mix into the middle again. You can add chopped straw at this stage. Great fun and costs nothing.

Is this the process all the time when you dig your own clay? You have to let it dry first? I have Kiko Denzer's Earth Ovens book on order, I don't know a lot of this stuff yet. It does sound fun although the only girls I will be including is myself and my two school age children I'm thinking alcohol might not fit into that mix, well maybe a little for me!

[CherylACT]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

What part of Connecticut? I found geological charts at quadrant scale to tell you where to dig.

I'm just outside of New Haven, I think I should be able to find it alongside the Housatonic River. One of the companies that sells clay, Shefield Pottery I think it's called quary's all its clay from a deposit right along the Housatonic River further North outside of Hartford, so I'm confident it's around. I'm kind of unsure though where exactally to dig and what the rules are about showing up next to the river somewhere with a few buckets and my shovel!

[CherylACT]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

Try calling gravel companies (that do retail/small quantities)- if they don't have it they will likely know who does. Landscapers may also be able to help you find what you need.

Do not buy potters clay - it's overkill and not worth the money for this kind of use.

I'd recommend getting "The Cob Builder's Handbook" by Becky Bee (yes, that's really her name) before you go any further. She covers clay and clay mixes very thoroughly.

Try Biblio.com - they are a clearinghouse for booksellers and often you can find things Amazon can't and usually at better prices (especially if slightly worn copies don't bother you).

So when I call the gravel companies they would most likely be selling me dry clay right? I have no idea how involved it is to mix it, I just thought I would be thropwing it in the tarp with some subsoil and some water and then adding straw, now that sounds like its not the case? I think I really need a book! I ordered Kiko Denzer's Earth Ovens book but I'll have to check out Becky Bee's book too.

[CherylACT]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

Potter's clay is not cheap, even the powdered stuff you mix yourself. (Have you ever done it? It's really hard work.)

There are lots of clay grades. It's been so long since I've been in ceramics that I've forgotten what I used to know. You absolutely don't want the self-setting clays: They have a polymer in them that will burn and stink on the first firing.

If you do a cost comparison between commercial clay and firebrick you may find out why it's been two centuries since builders made their own brick. I think the bottom line is that if you aren't quarrying your own clay, it makes no sense to build your dome with it here on the East Coast where firebrick is so cheap.

But if I use firebrick then I have to mortar it right, how would I shape it into a dome? Then I'm dealing with insulating it too and it's a all different materials than you use for a cob oven right? I like that I think I can do the whole cob over a sand dome thing. I like also that you insulte with another version of the same cob you used in the first place. Firebrick would require a whole different approach right?

[dmun]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

Clay ovens aren't insulated?

[david s]

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.

[CherylACT]

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.

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.

[CherylACT]

Re: Cob Oven -What type of clay should I buy, where can I buy it?

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.

[Grimaldi]

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?

[turkey]

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.