"Cob" fireplace mixes.

[Mike Carter]

>What would your opinion be on an inner layer (or perhaps entire thing) made
>with a mix of concrete, say ten or fifteen percent?  Would this up the
>strength sufficiently?
>
>Cheers, Rog.

The fireplace design and mixes we use are:

Firebox: standard fire brick.

Behind that, in a 3" layer, special mix #1 which is:
        Perlite - 2-3 scoops (64 oz. cups)
        Sand - 1 part (3 gal. bucket)
        Fireclay - several handfuls
        Sifted road base (crushed limestone, a bit like clay, a bit like
sand) - 1 part

Then regular Cob with straw in back.

For the flue, in a 2-3" layer, we use special mix #2 which is:
        Sand - 3 parts.
        Clay - 2 parts, finely sifted (local dirt)
    (this gives a very sandy mix)
Then regular Cob with straw around that.

We have another mix that includes some Portland cement for building the
throat and smokeshelf.
This mix and the special mix #1 were worked out with a mason.  Time will
tell how they work out - we haven't had any structural issues with these
mixes to 15 feet high.

With some ideas from this list, we may use cement in the Cob for the actual
chimney above the wall.

Mike Carter