[marsh2]

I am wondering if the conical shape was used to bury the bottom in the earth to maintain cool temperatures (natural refrigeration) or fire ashes to heat.

[concentric circles]

I was wondering about the shape as well. Could it be as simple an explaination that it was easier to make than other shapes?

[Nipplemancer]

maybe it was before the invention of the table and just pressed it into the earth to keep it upright.