(Sources: Cobalt Oxide)

-Cobalt is a powerful and stable colorant used in glass, glaze, enamel, and even paint. As little as 2 PPM can produce a recognizable tint, thus cobalt is often cut in a medium to make it easier to weigh and distribute in a mix.

-It is not volatile even at 1400C.

-Various raw forms are available and all break down to cobaltous oxide (CoO), which is the stable form that combines with the glass melt to produce color. These include black stable cobalto-cobaltic oxide (cobaltosic oxide) Co3O4, which has a 93% conversion ratio and decomposes to liberate oxygen at 800C. Grey cobaltic oxide (Co2O3) is 90% CoO and mauve cobalt carbonate (CoCO3) has 63% effective stain content. Cobalt dioxide (CoO2) is not marketed for ceramics.

-Because cobalt is quite soluble in glaze melts, it has little or no opacifying effect.

-Although cobalt has a high melting point by itself, it is a powerful glaze flux, dissolving readily in most glazes, especially alkaline and boron types. This active nature causes it to diffuse, making it difficult to maintain a clean edge on painted decoration, especially overglaze. Decorated areas employing color in or on glaze can thus be locally more melted depending on the concentration of CoO or its particle size, this can result in increased tendency to blister or crawl in some glazes.

-It is very color dependable under both oxidizing and reducing furnace conditions, fast and slow firing. Cobalt is used in a wide array of decal inks, underglaze colors, body stains, and colored glazes.

-Cobalt is a trace element in vegetables and an important vitamin (B12) in stock raising. Cobalt metal is used in steel and chrome alloys.

Properties

• Body Color - Violet, Lilac

  In magnesia glazes a color range is from violet to lilac is possible.

• Fusion - 1810C

  From The Oxide Handbook

• Glaze Color - Blue

  Cobalt is a classic and reliable blue colorant at all temperatures and in most types of glazes. The shade of blue can, however, be affected in many ways by the presence of different oxides. Cobalt is powerful and often less than 1% will give strong color. If the color needs to be toned down, additions of iron, titanium, rutile and nickel may work.

• Glaze Color - Blue Soft

  Cobalt is often calcined with alumina and lime for soft underglaze colors. Stains often employ mixes of alumina, cobalt, and zinc for softer blue colors.

• Glaze Color - Yellow

  Cobalt is used in combination with manganese and selenium to mask excess yellow coloration (yellow plus blue gives green which is masked by the pink of selenium).

• Glaze Color - Blue Slate

  Combinations with iron and manganese can give a slate blue.

• Glaze Color - Blue-green

  With barium shades of blue-green are possible.

• Glaze Color - Blue-black

  With chrome and manganese blue-black and black are common.

• Glaze Color - Blue-green

  With chrome and copper, cobalt can yield tints from pure cobalt blue, to greenish-blue, to the green of chromium. These effects work best when silica is not too high and there is adequate alumina.

• Glaze Color - Purple

  When cobalt occurs with manganese (i.e. 1-3% cobalt carb, 3-5% manganese carb), purples and violets can be made. Less cobalt will lighten the color. This effect works well in magnesia glazes. In high magnesia glazes, 1-2% cobalt alone will give purple. Add tin to move the color toward lavender.

• Glaze Color - Lavender, Purple, Violet, Pink

  With adequate SiO2 and high MgO (0.4 molar), purple, violet, lavender, and pinks can be made using 1% or more CoO. Mimimizing boron, alumina, and KNaO will help prevent it from turning blue. Note that the high MgO will generally make the glaze matte and it could suffer some ill effects associated with excessive MgO.

• Glaze Color - Red

  With MgO, SiO2, and B2O3, red, voilet, lavender, and pinks can be made.

Pictures

• Cobalt in a transparent glaze

  

Authors

• Tony Hansen (Owner)

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