[Dutchoven]

Stan,
It is quite true that cracks are. I think what is going on is firstly that you used concrete as a render...probably without any layer of material to allow slippage(aluminum foil works well). Firing it fully so soon did not help either because the concrete holds lots of water and heating it so high creates steam which has to find a place to go.

The best method would have been to insulate first using ceramic fiber blanket and then vermiculite cement(vermiculite and Type N mortar) before coating with a layer of render. How thick is the current concrete render? It will become part of the thermal mass of this oven so you could now insulate with both the insulators mentioned above or just the vemiculite cement with a slip layer in between and then coat the whole thing in another layer of render. You would both have a more efficient oven and have less of a likelyhood of the render cracking(although it still happens sometimes).
Hope this helps!
Dutch