Building Cob oven using bagged fire clay

Hi All,

I love this site. So many interesting topics.

I'm finally getting around to building my cob oven.
I am reusing a base structure from an old outdoor fire place.
I bought the Kiko Denzer book but I'm not clear on how many 50 LB bags of fire clay I'll need for a 27" oven.

I think I read somewhere that in total I'll need about a half ton of sand and 3 bales of straw.
I'm assuming the powdered bagged fire clay is more pure than anything I can dig so I'm guessing a 1-3 or 1-4 clay sand ratio.
The measurements in the book seem to be by volume not weight.
I plan on using clay and sawdust to embed two layers of beer bottles on their sides to insulate the base. then fire bricks for the bottom of the oven.

Not sure how much clay and sand I'll need for the 3" inner layer and how much clay, sand and straw I'll need for a 6" insulation layer.

Any advise is much appreciated.

I'm getting excited.
I've wanted an oven like this all my life.
My aunt had one when I was a kid and she made the best bread.

Thank you in advance.

Puppy

How much are you paying for those bags of clay?

This seems like something that could get quite expensive. The fireclay that I used for my masonry stove, cost about $1 Canadian per pound dry weight. I used it only for the bricks adjacent the fire, and was able to build the entire stove for $55.

Dale

The fire clay is $24.50 for 50 pounds.

That's another reason I was wondering how much I would need?

Thank you

Dom

Do you have anywhere you could dig clay out of the ground? Unless your region has no clay (like Florida), there is clay good enough for building an oven in most parts of the world. If you don't know of any resources, asking a friendly contractor would probably do it; they will know where there is clay under the surface, and may let you take some from an excavation pile at a job.

If you do use commercial clay, I have some very rough figures...
A 27" oven, not counting base structure, 9" thick, would run somewhere around 10 cubic feet, plus say 6 cubic feet for the insulated floor, around 2/3 of which will be bottles, so 12 cubic feet of cob mix total. As pure as commercial clay will be, I suspect a 4:1 or 5:1 sand mix for the thermal layer would be plenty strong. For the insulating layer, having that at least half, maybe 2/3 straw sounds good, and maybe half to 2/3 sawdust for the floor layer. So I would guesstimate you will want 2 to 4 cubic feet of clay. Wet boxed clay is around 100 pounds per cubic foot, and dry bagged clay will be in the same ballpark once prepared for use. So it looks like 4 to 8 50-pound bags of clay.

A half ton of sand would be around 10 cubic feet (1000 lb @ 100 lb/cu.ft.), so that is probably more than you need for this project, but unless it is expensive I would get it anyway, as it can always be used for something.

Two regular square bales of straw would probably be enough, or again, three bales will be plenty and leave you with some mulch material.

Glenn,

Thanks for all your advise.
I did some research and it turns out my area is very clay rich. in the late 19 early 20 century my general area was a big clay pit industry. I'm going to try to find a construction site in my area and see if I can get some clayish, clumpy looking soil into my pickup.

I'll take some before during and after pictures to post.

Really appreciate the advice.

Dom