Over Fired Bisque For Raku? in Clay and Glaze Chemistry Posted July 10, 2010 · Report reply I just had a full kiln load of bisque Way over fire. It was suppose to fire to 06 but went to at least cone 8. I connected the wires wrong to the thurmo couple & everything came out burnt. ** Question. I know for raku, the clay needs to be open & UN vitrafied. Will it still be Ok to Raku without cracking? Thanks, If you really did hit ^8 then the clay will be too vitrified to withstand the thermal shock of a raku firing and it will crack. Sorry.