Black core

This happens when enough carbon is not removed from the clay body during the firing process. All clays contain certain carbonaceous content and oxidize during burning at around 1400 ° F, releasing carbon dioxide into the clay body. If there is inadequate oxygen in the kiln atmosphere, then carbon cannot be removed from the clay, which causes it to weaken. It is more likely to happen during a “reduction” firing, in which the amount of oxygen in the kiln atmosphere becomes lowered.