Nice work, I like that you are using commercial refractory. My first furnace is made from my own mix with alumina, sand, clay and perlite. It still works great and has no cracking what so ever. I have 2 sacks of some commercial refractory that is going to make my new bigger furnace. I am working in opposite of you as I started with a smaller furnace and now I am making a larger one. My burner is pipe with a a very small hole drilled into it and then set perpendicular to a larger pipe, propane is the fuel type. This is for melting aluminum, the new furnace will be waste oil run and the commercial refractory is Kruzite Hi-Temp for over 3200 degrees, gonna try cast iron.
Keep the pics coming and thanks for sharing.