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Umber is a hydrated iron oxide that also contains significant manganese, calcia, alumina and silica. When umber is calcined (referred to as "burnt" because it is done to lower temperatures), the color intensifies. The material is used to color many products (e.g. for construction), in ceramics it is used to darken the fired color of clay bodies or engobes.
Burnt umber can be any of a variety of natural and synthetic iron oxides (there are many manufacturers and choices, this is because the coloring market for cement, for example, is so large and widespread). A variety of impurities are common. Some companies condition the color of their umbers by adding various colors of iron oxide.
These are the same material, however the one on the right has been burnt to 600F. At this surprisingly low temperature the color transforms into a deep redish brown.
This cone 6 mug is made from a black clay (containing 10% burnt umber). The engobe on the inside is covered by a clear glaze. The color is the same as if the engobe were on a white or buff firing stoneware. Engobes get their covering power from the fact that they do not melt. If you see an engobe with lots of frit it will likely melt too much, be suspicious.
Black burning bodies are popular with many potters. They are normally manufactured by adding around 10% burnt or raw umber to an existing buff-burning cone 6 stoneware. Umbers are powerful colorants, they have high iron and also contain manganese (the latter being the primary source of the color). But these clays can be troublesome. First, good kiln venting is needed to avoid breathing the dangerous manganese metal vapors. Micro-bubble clouding/gloss-loss in the glazes and blistering/bloating of the bodies are common. But this mug fired perfectly. Why? The umber was added to a cone 10 stoneware instead (and it has fluxed the body to mature at cone 6). The mug has been white engobed on the inside and partway down the outside during leather hard stage. After bisque it was clear glazed on the inside giving a flawless surface (using G2926B) and dipped in GA6-A Alberta Slip base amber-clear. The GA6-A over the black clay produces a very deep, rich, almost black ultra-gloss surface.
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