Carbon Coring, Black Coring

carbon-cored pottery

A gray or black layer under the surface of a fired clay piece. Carbon or black coring is undesirable, as it makes the piece weaker, can discolor glazes, and is associated with bloating.

The two causes of these phenomenon are incomplete burnout of organic materials that contain carbon and sulfur, and early or heavy reduction during the glaze firing. Incomplete burnout of carbon during bisque firing is due to firing too fast, and/or the kiln being poorly vented and lacking enough oxygen to combine with the carbon and sulfur (which would enable them to turn to gases and burn out of the pottery). The black/gray is caused when the carbon and sulfur, or early/heavy reduction, strips the oxygen from any red iron in the clay body, turning it to black iron, which is a strong flux.

Clay bodies that contain no iron, such as porcelains, do not have this problem, and so are often used in work intended to undergo early/heavy reduction, such as pieces glazed with a carbon trap shino.

time well spent

closeup view Jack Troy cup, links to Jack Troy artist page

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link to newest page of ceramic artist links, including link to Scott Parady, pictured

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