Certain colors may employ controlled quantities of calcium or barium compounds as modifiers.
Dark mazarine blue through royal blue to matte blue. Generally darker and more violet in shade than zircon-vandium blues.
Specific Gravity: 3.5-4.5
Excellent in most types of glazes and bodies for firing in oxidizing or a slightly reducing atmosphere at temperatures over 1450C.
Suitable for blending with most other types of ceramic colors including zircon-vanadium blues.
Cost is low to moderate, increasing as the cobalt content increases.
Note: This description is taken from information provided by Blythe Matthey and may or may not apply to stains of this family from other companies.
Cobalt-silica based blues are fusible, tending to soften the glaze and make it runny. Thus the host glaze may need to be hardened by the addition of kaolin, for example. Cobalt-alumina stains are more stable.
Underglaze Color: Cobalt-alumina blues are suitable for use under all types of glazes, firing in either an oxidizing or a slightly reducing atmosphere at temperatures up to 1450C (cobalt-silica blues are not as refractory so under glaze decoration may have fuzzy edges). Cobalt-silica blues do find wide applications as 'oven blues', the color being applied over the glaze and fired at the glaze firing temperature.
Suitable for all types of bodies and slips for firing at temperatures up to 1450C. Not suitable for bone china bodies since cobalt content of the blue combines with the phosphate content of the body producing an unstable violet or reddish shade.
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