|Monthly Tech-Tip |
Description: Hydrated Calcined Iron Oxide, Fe2O3, H2O, MnO2, SiO2
When raw umber is calcined (or burnt), the brown color intensifies (because of MnO content) and any plasticity from the clay is negated. This produces a stable material, resistant to temperature and weathering. The material is used to stain many products, especially in construction. In ceramics it is used to darken the fired color of clay bodies, slips, engobes and glazes.
The term "burnt umber" can be quite broad, there are many manufacturers and choices, mainly because the coloring market for cement or paint, for example, is so large and widespread. Burnt umber is an engineered material whose color is controlled by heat treatment and additives.
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, L3954B, 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. This one is stained by adding 10% 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).
This is the standard Plainsman L3954D white engobe recipe with the 10% Zircopax switched for Burnt Umber. The result is a dark, rich, ultra-gloss brown (almost black). The engobe is applied inside and half-way down the outside. The mug on the left is glazed inside and out with the base GA6A Alberta Slip cone 6 recipe (but uses Ferro Frit 3195 instead of 3134). The one on the right has the same glaze on the outside but the G2926B clear transparent on the inside (it is micro-bubbling). This engobe works even better with a black stain.
Metallic based materials that impart fired color to glazes and bodies.
|By Tony Hansen|
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