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This type of clay is made by a number of North American manufacturers. Generally, raw or burnt umber are used as the stain (10% or more). The umber is a clay that matures at low temperatures, so it acts as a flux in the body. Some manufacturers make the mistake of adding it to a cone 6 body and this is the result: bubbling of the glaze (in this case it is a transparent) and bloating and warping of the body. The umber is decomposing and potentially gassing metal fumes. However, if the umber is added as a stain to a cone 10 clay, it fluxes it to mature the resultant body at around cone 6 and these gassing problems do not occur.
Hazards |
Manganese in Clay Bodies
Manganese is used to stain clays (using black) and to impart fired speckling (as a decorative effect). It is dangerous? |
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