• Materials for Ceramics

Burnt Umber

Description: Hydrated Calcined Iron Oxide, Fe₂O₃, H₂O, MnO2, SiO₂

Notes

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.

Related Information

Raw Umber vs. Burnt Umber

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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.

The covering power of an engobe is amazing. If they are not over-fluxed.

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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.

A cone 6 black-burning stoneware with a porcelain surface. How?

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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 some manganese oxide. Could a white engobe produce a porcelain-like surface on such a clay body? Yes. L3954B engobe was applied during leather-hard stage to this Plainsman Coffee Clay mug (on the inside and partway down the outside). After bisque, transparent G2926B glaze was applied inside and GA6-B outside. Notice the GA6-B over the engobe fires amber but over the black it produces a deep glossy brown. The engobe was mixed into a thixotropic slurry, as explained on the page at PlainsmanClays.com (see link below), and applied in a relatively thin layer. This porcelain-like result is a testament to the covering power of a true engobe. It is no wonder they are so popular in the ceramic tile industry - a red-burning body can be turned white as a porcelain, which enables all the marvellous glazing and decorating they can do.

Dark Umber-Stained Engobes on M340 at cone 6

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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.

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