Light straw clay insulation couldn't handle the heat

Hi all,

I recently completed my first cob oven build. Yeeha.

I insulated with about 6 inches of light straw clay. I fired my oven for the first time last night and cooked some delicious pizzas.

This morning I noticed a stream of smoke coming out of a crack in the plaster. I bit of digging revealed that pretty much all of the light straw clay has burned out, or is in the process of smouldering away.

What happened here?

It seems that this is a common material to insulate cob ovens with. Why did mine burn out? Not enough clay slip coating the straw? Is there maybe a path from the interior of the oven, through 4 inches of cob, into the insulation layer where flames may have gone? Anybody experienced this before?

Fire triangle



You need to exclude one of these to prevent combustion.

Your system has fuel (straw) - you are aiming to keep both air and heat away from it. The straw can char if it gets sufficiently hot, even with the absence of air.

Without knowing a lot of details about your design, I suspect that your cob wall between the fire and the insulation is too thin (hence your straw is overheating), and there is an air leak somewhere. Cob cracks when drying so you might need to hunt around for it.

Personally I wouldn't use clay-slip and straw for this application for precisely this reason. Too many opportunities for failure. On the plus side, your oven will likely still work fine even with the insulation partially charred away. It may just be a bit less efficient.

Interesting. Thanks for the info. What do you use for insulation instead?

https://clayoven.wordpress.com/ Clay ovens no insulation I think.