I finneshed cleaning the water heater tank and marked out 2 lines round the middle at 4 and 10 inches from the cap weld. I then used an angle grinder with a cut off wheel to cut the tank apart.

After that I used the cut off wheel again to cut the top off of the chimney just under the weld. I used the wire wheels and the drill to clean up the edges.

The tank is 16inches in diameter and I am thinking that 3inches of refactory cement around the walls will be ok. That will give me a 10inch bore to work with. My intended crucible is a bit over 6 inches which will give me 2 inches around it for the fire to do its thing