Fire clay for cob oven

Hello all!!  I am going to be building a cob oven in my yard. I’ll be using fire clay and mason sand for the oven. To date I completed the base including an insulated layer beneath the fire brick. I noticed as it’s drying it has some cracks. I guessing this might be due to not having enough sand in my mix. I used 1:1 clay and sand.
Before proceeding, I’d like some clarification on the mixing ratios for the 2 materials.
Thank you all,
Mike

Your mix may be OK, but it needs a binding (or re-enforcing) fibre. In traditional cob, straw is usually used as the 're-bar' that knits the monolith of sand and clay together. Try some test bricks with chopped straw in the mix, also try a few differing ratios of sand and clay, see if you get any better results.

Thank you Chris. I used the straw for my insulting layer in the base. I haven’t seen straw used for the thermal layer. I decided that today I’ll slow myself down and do some testing with clay to sand ratios. I had read using a 1:1 ratio for the thermal layer. Last night after I posted this I saw you can use anywhere from 1-3 times the sand to fire clay.