Bronze Metal Clay...

must be fired in a reduction atmosphere.  Activated coal carbon is used for this purpose.  Stainless steel containers are used to hold the carbon along with the bronze pieces, but the stainless steel flakes off the container as it expands with heat and releases a layer of soot inside the kiln as it cools down.  This problem is finally resolved with "The Ceramic Firing Pot" which is available through

Ceramic Manufacturing Services.  It fits perfectly inside my kiln leaving room for the heat to circulate.   After firing a load of bronze metal clay, I was delighted to see that my kiln is totally free from the debris that would have appeared if I were still firing in stainless steel!