Gas fired furnace,

Gas fired furnace,
if you have a need for one this will show you how to make a simple by very good furnace.

From Joe.
One thing I noticed that could have been overlooked is a cap for the furnace. Two concentric metal bands welded to make two circles about 5 cm high with the larger band the same diameter as the furnace and the smaller, concentric band about 5 cm in diameter. Set the two rings on a 'release surface' and fill in the space between them with fire cement.
In use the removable cover (with a hole in the middle) helps to retain heat, allowing less fuel and time to get to the melting temperature.
I happen to have a commercially made "muffle furnace" and that is the way it is set up. The only difficult part is finding some way to pick the cover up while it is hot.
One other thing I noticed is that the jets are set up to shoot directly into the burning chamber. On all the other muffle furnaces I have seen one or two nozzles shoot into the center at a tangent - creating a swirling pattern for the burning gasses.
http://www.small-farm-permaculture-and-sustainable-living.com/support-files/gas_fired_furnace.pdf