The final mix for the arch mud (some people insist on calling it "cob", or "clay") was a challenge. The soil on site is basically beach sand with some silt and humus. Which for the sub-hearth I added a little left-over potters clay. But this was going to be insufficient for the structural requirements of the arch unless I wanted to re-hydrate lots and lots of potters clay. So I looked about town and found some high clay content sub-soil from a city project that they let me have. The clay content was very good, but the gravel/stone content was pretty "good" too! I mixed this soil with the sand from my yard, and some other silty-clay soil that I came by in my travels. In retrospect, I think I could have increased the sand content, maybe up to double, but it worked well with minimal cracking.
It's hard not to get some cracking in a four inch thick mass of wet mud as it dries! I just filled them, and put another layer over the whole thing.
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