|Monthly Tech-Tip |
Umber is a processed from manganese-enriched goethite, a naturally occurring inorganic iron oxide. It is a brown earth pigment that is darker than the Ochre because of its manganese and iron oxide content. It is highly valued as a permanent pigment either in the raw or burnt state. Umber is lightfast, insoluble in water, resistant to alkalis and weak acids and non-reactive with cement, solvents, oils, and most resins.
This is preferred in ceramics over ochre because of its much higher iron content.
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.
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.
Sold by Arlimin but made by New Riverside Ochre Company.
The body is Plainsman Coffee clay, it is stained black with 10% raw umber. I painted Amaco Velvet white underglaze over the black clay (in the leather hard state), then over-painted the colors. When they were dry enough to handle, I cut the black lines using a Kemper WS sgraffito tool. The rock on the left is fired to cone 6, on the right to cone 4. Thus, at some point between cone 4 and 6, the umber in the body is generating gases of decomposition. Glazes can bubble it through but this underglaze cannot pass it. To continue working at cone 6 I made my own body by mixing Mason 6600 black stain into a porcelain.
Metallic based materials that impart fired color to glazes and bodies.
About natural iron oxide pigments: Ochre and Umber
Ochre (pronounced o’-ker) is a natural, mineral, earth pigment. Chemically, it is a hydrated ferric oxide, chemical formulation: FeO(OH). Ochre is inorganic, chemically inert, non-reactive with cement, mortar or brick, and non-toxic.