cob oven, cracks in test mix

[zsklar]

Hey yall, unsure if there is a better forum but anyone in here have experience with diagnosing cracks for a cob mix?


I made a mix to start building up the height of the foundation and provide a containment ring for the oven

clay source: a brick company's clay soil. I was told it was close to 80% clay

I did a 2 parts sand to 1 part clay mix

didn't measure the amount of straw

the result was really hard but also sort of easy to pull off of the stones.
not enough sand?
not enough straw?

thank you!!!

[SableSprings]

Welcome zsklar! We'd prefer you attach or upload jpg files rather that refer to a site (like Imagur) that want to install its app. I took a screenshot of your post pic from them and removed the link. Be aware that this system requires jpg files to be <1 MB (prefer 800x600 pixels) and you can only add 6 pics per post. The straw is intended to burn out & the cobb hardens when SLOWLY heated in stages over several days to drive out the moisture. Expect minor cracks, but after a proper curing with controlled, incremental firings they should be minimized. Unfortunately most people are not patient enough and overheat the cobb (and masonry) ovens by building too big of a fire too quickly. Curing your oven depends on a number of factors, thickness of clay, insulation (above & below), type of clay, etc. Also be aware that cobb ovens are very susceptible to weather damage and will absorb water quite easily causing spalling & more cracking as water is turned to steam in the cobb.

Good luck!

[Petter]

If it crumble and cracks, it shrinks too fast and/or too much. Add more sand and/or dry more slowly.

[SableSprings]

Zsklar, is there any "real" reason that you are using cobb for the foundation? I know it's what Kiko Denzer did/does, but using concrete blocks is going to be much more stable, durable, quicker, & a lot easier to work with to create a solid foundation for your oven. Just my two cents worth since I also considered going the cobb route well over a decade ago......and I'm very glad I chose to go with a masonry build instead.