Although colorants are added to bodies, most people think of them as materials that transform a colorless transparent or opaque glaze into a colored glaze. Colorants can be raw metal oxides (e.g. iron oxide, chrome oxide), metal oxide containing materials (e.g. rutile) or man-made powders which are smelted from metal oxide and stabilizer mixes (stains). Potters and smaller companies often use raw colorants and color-containing raw materials whereas industrial manufacturers employ stains. Unlike stains which are prefired, the color of raw colorant powders often bear no resemblance to the color they will produce in a glaze. Raw metal oxide powders are often available as carbonates or oxides (other forms are typically available also, but they are not usually used in ceramics for solubility or others issues). Carbonate colors gas during firing as the carbonates decompose and volatilize. This can be a source of glaze defects when the gassing happens after the glaze has begun to melt (copper carbonate is known for this issue). Color-containing raw materials, like rutile, often vary in their chemistry and mineralogy and are the least reliable option for coloring glazes. Not all metal oxides produce color (e.g. tin, titanium, zircon), but they can affect existing color and stabilize it and opacify the glass.
Iron oxide is available in many colors. Here are three.
How can there be so many colors? Because iron and oxygen can combine in many ways. In ceramics we know Fe2O3 as red iron and Fe3O4 as black iron (the latter being the more concentrated form). But would you believe there are 6 others (one is Fe13O19!). And four phases of Fe2O3. Plus more iron hydroxides (yellow iron is Fe(OH)3).
MgO can destroy the rutile blue variegation effect
The rutile blue variegation effect is fragile. It needs the right melt fluidity, the right chemistry and the right cooling (during firing). This is Alberta Slip GA6C recipe on the right (normal), the glaze melt flows well due to a 20% addition of Ferro Frit 3134 (a very low melting glass). On the left Boraq has been used as the flux (it is a calcium borate and also melts low, but not as low as the frit). It also contains significant MgO. These two factors have destroyed the rutile blue effect!
How do metal oxides compare in their degrees of melting?
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
Cone 10 Reduction firing is the home of the most magic oxide in ceramics: iron
It is a powerful glaze flux, variegator and crystalizer, a colorant of many characters in bodies and glazes and a specking agent like no other. And it is safe and cheap!
Mason stains in a cone 6 clear base
These are Mason stains added to cone 6 G2926B clear liner base glaze. Notice that the chrome tin maroon 6006 does not develop as well as the G2916F glossy base recipe. The 6020 manganese alumina pink is also not developing. Caution is required with inclusion stains (like #6021), if they are rated to cone 8 they may already begin bubbling at cone 6 is some host glazes.
Chrome oxide powder
Maroon and white mug before and after firing: What a difference!
The outer glaze is Ravenscrag GR6-E Raspberry, the bright maroon color is a product of the surprising interaction between the 0.5% chrome oxide and 7.5% tin oxide present. That small amount of chrome is only enough to give the raw powder a slight greenish hue, hardly different than the clear liner. While this color mechanism appears to be effective, it is delicate. A maroon stain is actually a better choice. It would fire more consistent would be less hazardous to use. And the raw glaze will be the same color as the fired one!
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