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Raw Umber

Oxide Analysis Formula
CaO 5.00% 0.64
MgO 2.00% 0.36
Fe2O3 51.00% 2.30
MnO 13.00% 1.32
Al2O3 3.00% 0.21
SiO2 13.00% 1.56
H2O 13.00%n/a
Oxide Weight 627.01
Formula Weight 720.70

Notes

Umber is a sedimentary high-iron oxide mineral containing significant manganese (e.g. 5-20%) and clay (also calcia, quartz). It is a brown earth pigment that is darker than the ochre. It is highly valued as a permanent pigment both 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.

The raw form of umber is preferable over burnt umber in many ceramic applications because it is less expensive yet supplies the same metal oxide pigments. Burnt umber can be more suitable where its lower LOI is advantageous (especially in glazes to avoid surface defects).

Manufacturers of the raw material quote its hazards as that of common clay, they refer to it as "natural iron oxide pigment". From a safety point-of-view that makes raw umber a much-preferred body colourant over manganese dioxide. The combination of iron and manganese natural to raw umber delivers dark color effectively while using a fraction the amount of MnO that would be typical if pure manganese dioxide were employed. That being said, during firing the material can still generate manganese fumes if fired past cone 6, these have their own hazards. However, put into perspective, a body having 10% raw umber contains only 1% MnO (if the umber contains 10% MnO) embedded into the matrix. Glazes, by contrast, are on the surface and routinely contain many times that percentage of pure MnO.

Umber, when used as a colorant in clay bodies, also acts as a flux. 10% umber will convert a buff burning cone 10 stoneware into a black burning cone 6 stoneware. Umber can also be used in casting bodies, even porcelains, as a source of color, it should not affect the deflocculation properties (in fact, it should improve it). In our testing, 10% umber in M370 produces a highly vitreous almost black porcelain (without evidence of bloating). Raw umber also increases the percentage of water needed for plastic bodies.

Umber quality needs to be monitored for use in production, batches can have high concentrations of plus 100 mesh particles, these can produce fired specks that bleed up through glazes.

Related Information

Raw Umber original container with occupational health advisories

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Sold by Arlimin but made by New Riverside Ochre Company.

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 umber is bubbling the underglaze in these clay rocks

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Clay rocks with butterfly pattern

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 at cone 6, on the right to cone 4. Thus, at some point between cone 4 and 6, the body is starting to generate gases of decomposition (likely from the raw umber). Glazes can bubble the gases through but this underglaze cannot. To continue working at cone 6 I made my own body by mixing Mason 6600 black stain into a porcelain.

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

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A black stoneware mug

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).

6% black stain vs 10% raw umber in a body

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These bars were fired to cone 5. Until we looked at the stained version of the body the umber version did look pretty dark in color. The unstained stoneware already has about 2% iron but that does not seem to assist the umber in developing color nearly as much as it does the stain. Even at only 4%, the stained version is still darker. The umber generates some gases of decomposition at cone 6 (affecting glazes), limiting its use there, with the stain that is not an issue.

Two shipments of raw umber - particles over 170 mesh

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The newest shipment, right, has almost no larger particles at all. The previous one, on the left, had all of these oversize particles in only 100 grams of powder. These produced fired speckle that bled up through glazes on the black clay body made using this material.

A zircon-opacified glaze pinholes on a raw umber stained body

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Umber generates significant gases here as certain particles within it decompose on approach to cone 6. While the gas escape enhances the appearance of variegating glazes that can heal the pinholes, for stiff melt ones like this zircon white, the effect is somewhat of a disaster!

Links

Typecodes Colorant
Metallic based materials that impart fired color to glazes and bodies.
Materials Burnt Umber
URLs https://www.nrochre.com/products/natural-iron-oxide-pigments
About natural iron oxide pigments: Ochre and Umber
Hazards Manganese Inorganic Compounds Toxicology
By Tony Hansen
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