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 violent 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.
Since these stains tend to be refractory, crawling of overlying glaze can sometimes happen where a 'dry' underglaze lacking sufficient flux prevents the glaze from gaining a foothold on the body underneath.
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.
Silica based blues are fusible, tending to soften the glaze. This tendency can be overcome by the addition of china clay to the glaze.
Underglaze Color: 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 also 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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