How to Make a small Gas Furnace

Step 3 Add the Refractory

This is the easiest step, though it needs you to get your hands dirty. Using a spoon or a knife or your fingers, coat the inside of the can with refractory. Make it fairly smooth, too, this will make sure there are no nasty eddies or swirls inside the can that can do unpredictable things. The refractory should be between 1/4 and 1/2 inches thick. Mine was somewhere between those. Don't obstruct the pipe!