Did some hunting and research.
For the castings we need a high density and abrasion resistant mix.
130-145 lb cu/ft
The normal stuff may work 'able cast 3000'
Found a specialty product for high/frequent temp swings $80 for 50lb
Has high fused silica content, glass.http://www.ablerefractory.com/products%20castable.html
The materials for kilns are really not suitable for a coal firebox as a kiln has no abrasion just heat.
Cone 10 here we come!
Each cone is made to melt at a different temperature.
Cone 10 melts at 2350 degrees which is considered high fired stoneware.
At this high temperature, the clay becomes vitreous and is the strongest of the clay bodies.