|Monthly Tech-Tip |
C-Red clay (also called Carbondale Red) has 12% iron oxide. It is also refractory so easily handles cone 10R. However with that amount of iron is it pretty well impossible to prevent black carbon coring in glazed ware. This cup simply fell to pieces when just touched. Each of these small pieces can be broken into many more with just light finger pressure. Yet the unglazed test bars were unbreakable by hand! The glaze-body bond is also strong. What is demonstrated here is the extreme compression under which the glaze is attached to the clay, breakage relieves it. That means the black coring is increasing the thermal expansion of the body, as the piece cools in the kiln the body is contracting thermally more than the glaze, putting the latter under more and more compression.
A common fault in reduction gas fired ceramic ware made from iron bearing clays. The interior cross section of the clay turns black.