I have been using a charcoal based backyard furnace for melting aluminum and want to try making a propane based furnace. I have watched several videos on making them and I have a question. Is there an optimum space between the outside of the crucible and the furnace chamber inner wall? It seems that some space is needed for the heat to efficiently travel around the chamber before exiting the top, but none of the videos I have seen address this. Most talk about the thickness of the chamber walls and how to optimize the propane flame. So anyone have any recommendations?