I needed to make a crucible large enough for some future projects. It has been a long process, and at no point was a sure I would succeed, but I believe I have. Coming this far, I think its safe to show how I went about it.
I planned to cast the crucible using a dense castable refractory. I chose it because of its strength, conductivity and the fact that I have a bag and a half left over from making the furnace.
The first thing to make were the wooden patterns.
I didn't have stock large enough to turn the outside pattern, which is 10" tall and 8" wide. Instead, cedar was cut and glued up to form a "barrel." This barrel was then sanded and carved to shape as it was too big to fit on the lathe!
Below is the internal pattern. This was composed of glued up cedar 4x4 posts. I use a lot of cedar. It is all reclaimed from rebuilding our deck.
Now this was the mold that I cast the crucible in. It was a wooden box in which I had poured concrete backfill and a plaster face around the patterns. The plaster would give me a smooth layer that could be sanded and repaired in case the pouring didn't work well. It came out well, but I'm still happy I used the plaster. The crucible, after it had set up in the mold got stuck. Plaster was easier to break away from the crucible than concrete would have been.
With my green crucible de-molded, a kiln was in order. This is old firebricks stacked up. Barbecue charcoal is the fuel.
It became rather hot in there...
And the crucible! The patch is from my initial attempt at firing, which I will not speak of.
I tried to fire it in the furnace with the evaporator oil burner. I'm supposed to be able to control it, but I failed. It malfunctioned, spat out a huge flame and spalled the crucible a bit. It hadn't really fired yet, so I patched it. The patch appears to have completely adhered.
Now for some melting!