These are two 10-gram balls (formed by dewatering the glaze on plaster) of low temperature glazes (G1916J, G1916Q) containing only frit and kaolin fired to 1250F. The carbon is part of the LOI of the kaolin (that hardens and suspends the glaze). Yet these glazes have much lower carbon content than ones made from raw materials.
Ceramic materials, especially clays, often contain carbon and organic compounds. When they are fired in a kiln, these must burn out, often producing complications.
G1916Q - Low Fire Frit 3195 Glossy Transparent
An expansion-adjustable cone 04-02 transparent glaze made using three common Ferro frits (low and high expansion) and a suspension strategy that produces an easy-to-use slurry.