This cone 6 black-burning stoneware is safe. Why?

Saturday 12th August 2017

Black burning bodies are popular with many potters. This one is stained by adding 10% insight-live.com/material/2911">raw umber to a buff-burning stoneware. Umbers are powerful natural clay colorants, they have high iron and also contain manganese oxide. This mug fired perfectly. For both aesthetic and safety reasons. A white engobe, L3954B, was applied during leather hard stage, on the inside and partway down the outside. After bisque, transparent G2926B glaze was applied inside and GA6-B outside (over the engobe it fires amber but over the black clay it produces a deep brown). What about safety? Notice the clay has not bloated, the glaze has no bubble clouds or blisters, that means no manganese fumes were being generated. This happened because the top temperature on the firing was closely guarded to not exceed cone 6. And the body is a cone 10 stoneware, not cone 6, there is insufficient feldspar to trigger release of the MnO from the umber (and its subsequent decomposition to metallic state).

Pages that reference this post in the Digitalfire Reference Library:

Burnt Umber, Raw Umber, L3954B - Cone 6 Engobe (for M340), Can you make a black-burning stoneware using black iron oxide?, Manganese Inorganic Compounds Toxicology, Manganese in Clay Bodies

This post is one of thousands found in the Digitalfire Reference Database. Most are part of a timeline maintained by Tony Hansen. You can search that timeline on the home page of digitalfire.com.