Bell ovens come in two types - The oven with a bell shaped cavity that is accessed
via a hole in the top, shuch as a tandoor, and a bell shaped pot that sits over a hot slab to
enclose the bake.
The tandoor bell oven is a large clay structure with an open top. A fire or embers in the bottom of the vessel warm up the entire oven. Bread dough can be pushed onto the hot inner walls of the oven to bake in the heat of the fire.
All of the best nan bread is cooked in these types of oven. Ancient ovens would have been solid fuelled, the modern counterparts are usually electric or gas powered.
There are a number of commercial suppliers of modern Tandoor ovens,
The Clay Oven Company
The traditional bell oven is a clay or iron bell shaped hood that covers the bake on a griddle or stone slab over a fire.