How to Make a Gas Fired Smelting Furnace shows a small furnace that you can use to melt down aluminum, brass and copper; preheat small, thick pieces of iron and steel for brazing or forging; case harden soft steel; make up alloys and bake vitreous enamels on metals. You can use either LP or city gas.
Making your own is not hard, and it saves a ton of money over a purchased model, but they may be less easy to use.
If you can cast metal then you can build (or repair) just about anything. The US Navy puts small foundries on many of their ships because they don’t have the space to keep all the replacement parts they may need.
There are better books on this subject available, but this one is free, and it does work.
The next burner I am going to make (soon hopefully) is to make a waste oil burner. They use old motor oil and air pressure and are capable of melting steel – they also can make pretty good rocket engines….How to Make a Gas Fired Smelting Furnace