Black Coring, Black Core

Black coring usually occurs during a reduction firing and is a result of fast firing and/or lack of oxygen in the kiln between 700 and 900C (usually in the bisque firing). If body carbon fails to oxidize to CO2 it steals oxygen from Fe2O3 (reducing it to FeO, a powerful flux.) This FeO will then flux the body, sealing it and preventing the escape of remaining carbon in the body. This produces the characteristic 'black core' you see on ware cross section. The more iron in a body, the greater the risk of this problem if firing is not right. Once iron is reduced to it is very difficult to reoxidize it back to Fe2O3.

Note that electric kilns can also produce this problem, depending on the carbon and iron content of the clay, density of the pack, available airflow, and speed of the firing.

Out Bound Links

• (Articles)

  Organic Matter in Clays: Detailed Overview

  A detailed look at what materials contain organics, what its effects are in firing (e.g. black core)...

In Bound Links

• (Glossary) Bloating

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