My situation is different than others, I have a dedicated space for a foundry furnace so mine does not have to be portable. That affects the design. I'm also very concerned about the health hazards of airborne ceramic fiber particles. Maybe overly concerned, however I decided to make a brick furnace to avoid dryout and spalled refractory. Firebrick is also lighter than most refractory and individual bricks can be replaced if necessary. This is what I came up with. First a page of history. I started melting aluminum in a loose stacked brick furnace with a home made propane burner. This was hard firebrick I had laying around. Next step was to buy some soft firebrick and make a burner tile. Tapered wooden plug with Greencast 94 refractory. Greencast 94 is a high alumina refractory which is rated for 3,500F. Several outdated bags were given to me. Then I melted aluminum in this furnace for a year or so. It was surprising to me how good this worked. The outside of the firebrick stayed cool, less than 100F. The brick lasted well with direct flame impingement, but there were lots of air leaks and I wanted to be able to go to higher temperatures eventually for brass and bronze. After a little research and study I decided to build an all brick furnace in a 55 gallon drum. It's working good for me so if there is interest I'll detail my build in a few posts.