I have been setting aside material to build a furnace and I have done a lot of research on same. There's more out there on the internet than one can absorb. In all the things I have seen, I notice one common theme that has me confused. Why do all the furnaces have a big hole in the top to let the heat out? It seems like trying to bake a cake with the oven door open. I'm guessing that if one measured the temperature of the flame coming out the top it's pretty much the same as the temp. at the source of the flame. Well, maybe a bit less since some of the energy goes into the stuff you're trying to melt, but at the rate of "flame flow", not much. If I were to use an electric furnace (kiln) to melt the material, I'd surely close the lid to keep the heat in, but all the "bucket" furnaces I see seem to go to great lengths to let it out. Why is that?