A broken test bar of ball clay fired to cone 10 reduction. Notice the black carbon core. Ball clays commonly contain carbon, many have a noticeable grey color in the raw state because of this. Notice it has not burned out despite the fact that the clay itself is still fairly porous, the firing was slow and the temperature reached was high. Ball clay typically does not comprise more than 30% of a body recipe so its opportunity to burn away is sufficient. However some specialized bodies have a much higher percentage.
A common fault in reduction gas fired ceramic ware made from iron bearing clays. The interior cross section of the clay turns black.
Ceramic materials, especially clays, often contain carbon and organic compounds. When they are fired in a kiln, these must burn out, often producing complications.