Cob oven starter questions

[Earthmonger]

Hi there everybody, I'm just in the planning stages right now, but am hoping to build a cob oven this summer up at my wilderness retreat in east/central Wisconsin (about 50 miles north of Green Bay). I am using Kiko Denzer's book as my inspiration, but I do have a few basic questions before truly getting started.

First and most importantly is clay. The earth at my land is basically a thin layer of topsoil (6-12 in) on top of sand, sand and more sand, with no clay in sight. Digging clay elsewhere and transporting it to the site is impractical in my case. I have seen a couple of threads that discuss buying dry powdered clay in large bags much as one would buy cement mix. I have looked online at Home Depot and other major retailers and not found anything that looked like a likely prospect. Actually, one thread here gave a link to a 50 lb bag of "fire clay" on the Home Depot website, but it is not available in my area. Can anyone recommend a source for good clay? Or at least help me with some key terms to use when calling local building suppliers to describe just what it is I'm looking for.

Second is the foundation. Denzer's book stresses the need for a foundation that goes down to the soil's frost line. Unfortunately that is pretty impractical here in the frozen north, as I would have to dig down 2-3 feet or more. Am I misunderstanding something, or is this overkill? Is the foundation needed for anything other than stability? How thick does it really need to be?

Lastly, the base. I've seen photos of a lot of ovens on this site with a hollow base made of brick or stone, with the inside being available for wood storage. I've also seen a couple with a timber base. This would be better for me as wood is more available to me than brick, plus I'm experienced in working with wood but not with masonry. However, I have misgivings about making a wooden base for my wood-burning stove, you know? Is this really safe? Are there any special considerations? What do people recommend?

I'm sure more questions will pop up as the project moves along, but hopefully this will help me get started. Thanks for any help you can provide.

[Wiley]

Re: Cob oven starter questions

Earthmonger,
If there is a redimix batch plant (place where you order concrete to be delivered by a truck) they will have a settling pond where they allow the water they used to wash the sand and gravel (used in the concrete) to set and the fines to settle out. The sediment is usually rich in clay. My neighbor (who is really into cob construction) gets it from such a place. I remember once he even had them deliver the stuff. Literally dirt cheap (pun intended) the stuff is a waste product to the redimix plant. They will probably charge for transport but not for the stuff. Be sure to go to the plant and feel the product. The heavier stuff settles first and is more sandy, the finer clay stuff is usually an inch or six on the top. Talk to the guy who will scoop it out (with loader) if you can convince him to highgrade the stuff for the layer you want you will be better off.

As for a base, sure use wood but be sure to insulate it well from the heat of the WFO; the oven probably isn't going to last that long anyway. While with care (and a climate like Arizona) they can last a long time, usually they need to be rebuilt within 5 -7 years. My neighbor's WFO (and like I said he is into cob and knows it's limitations) had his tarp blow off during a stormy period a couple of months ago, while he was away for a couple of days. I'll take a picture as somewhere I have previous "before" pictures so one could get "before" and "after" ideas. Total rebuild. Here's the "after" photo (which I asked permission to take). Note after he came home and discovered the amount of damage and called it a rebuild he did not bother to recover and so this photo shows continued damage. I cannot find the "before" picture on my computer but did post it here on the fourm so a search of cob ovens should (perhaps) bring it up.

As for freezing and jacking and all the usually fun stuff frost does to structures...I would suggest building the whole as "floating" with a larger base than usual. The whole structure may lift and settle as a unit. You may have to re-level if you get unequal frost heave and the structure re-settles at an angle. Be sure to have good drainage, which from all the sand you say you have, shouldn't be a problem.

Building with cob one has to keep in mind what they build is by its nature ephemeral. Rebuilding his WFO is not what my neighbor wanted/had planned to do this summer. If your summer retreat is something you plan on having for a long time you might want to reconsider the investment in building a brick WFO rather than cob, just MHO.

Hope this helps,
Wiley