This type of clay is made by a number of North American manufacturers. Generally, raw or burnt umber are used to stain the color (10% or more). Umber contains a high percentage of manganese. The umber also fluxes the body and 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. If the manganese is added to a cone 10 clay, the umber will flux it to mature around cone 6 without these problems. These bubbles are happening because the umber is decomposing and potentially gassing metal fumes. A functioning kiln vent is a must to fire this type of body.
|Hazards||Manganese in Clay Bodies|