FornuisThere was already bread baked befor there were furnaces. On, under or beside the fire or the smoldering ash.
A still used technique is packing fish or meat in leaves and cover them with clay to bake in or nearby the fire. If you cook one egg like that the leaves cause the shrinking clay or loam not to break the egg.
Bread was baked in a clay pot. The first ovens were probably a big earthwork pot that was put over this one to keep heat.