I think I answered my own question. Calcium carbonate releases carbon dioxide on heating (to above 825 °C in the case of CaCO3), to form calcium oxide.
CaCO3 → CaO + CO2
So I believe my bubble was due to the release of carbon dioxide. Answer, buy some new kiln shelves to use for heating above 825 deg cent as in combing and pot melts.
Whiting or calcium carbonate
I think I answered my own question. Calcium carbonate releases carbon dioxide on heating (to above 825 °C in the case of CaCO3), to form calcium oxide.
CaCO3 → CaO + CO2
So I believe my bubble was due to the release of carbon dioxide. Answer, buy some new kiln shelves to use for heating above 825 deg cent as in combing and pot melts.