The composition of the glaze. This not only includes the colorants in the glaze, but also other glaze materials that interact and effect that colorant or combination of colorants. The temperature to which the glaze is fired. Some colorants are volatile and will dissipate into the kiln atmosphere if fired too high. Others give different colors at different temperatures. The kiln’s atmosphere during firing and, in some cases, during cooling.
Chrome-red: needs lead glaze fired at cone 08 or below.Chrome-yellow: needs lead-soda glaze fired at cone 08 or below, or will begin turning green.Chrome and zinc yield brown.Chrome plus tin yields pink, grayed pink, and warm browns. Color depends on proportions of these oxides in glaze and in relation to each other.Small amounts of chrome plus cobalt can yield teals at cone 9 and higher when fired in reduction. Magnesia glazes aid in producing nice colors.
In glazes with a high magnesia content, very small amounts of cobalt can give a range from pink through blue violet.Magnesia and cobalt in glazes fired at cone 9 or higher can yield blue mottled with red, pink, and purple. Very hard to control and duplicate due to the narrow temperature and atmospheric range.Cobalt and rutile can produce in mottled and streaked effects.Cobalt with manganese and iron will yield an intense black.
In alkaline glazes, copper will produce turquoise.Copper yields a lovely range of greens in lead glazes.Copper in barium high-fired glazes produce intense blue and blue-green in both oxidation and reduction.Copper in low-fire raku glazes can yield metallic copper. Over time, however, the glaze will oxidize to green.
Iron-containing clay bodies that have been fired but are not mature, such as bisqueware, often are a salmon or yellowish pink color. If a pot is glazed with a lower-temperature glaze and fired below the clay body’s maturity temperature, a salmon, ocher or reddish brown color will show through.
High-fire glazes containing bone ash and iron can yield persimmon reds and oranges. Iron and tin in high-fire glazes result in a mottled cream color, breaking to red-brown in thin areas. Iron fluxes in reduction atmospheres. It is less active and can sometimes even act as a refractory in oxidation atmospheres. Iron in high-fire reduction can yield lovely, delicate iron-blue and celadon green. High-fire, high-iron content glazes fired in reduction will yield glossy dark brown or brownish black. In thin areas, the iron may reoxidize during cooling. Reoxidization will result in those areas turning red or gaining red highlights.
In high-alkaline glazes, manganese yields rich blue-purple or plum.At cone 6 and above, manganese produces brown.In lead glazes, manganses yields soft purple tinged with brown.
In boron-containing glazes rutile produces pronounced streaks or spots, especially in glazes containing other colorants.In fluid glazes, rutile encourages opalescent blues.Rutile increases opacity.
Antimony: used for yellow in low-fire glazes.Cadmium and Selenium: very similar, producing bright reds. Both burn out extremely easily. Handle with care.Gold: gives a range of pink, red and purple.Ilmenite: as a colorant, very similar to black iron oxide.Iron Chromate: produces shades of gray, brown and black. Iron chromate plus tin may produce a pink or reddish brown; if applied with a brush, can yield black hazed or haloed by pink. Handle with care.Platinum: gives gray.Silver and Bismuth: used in luster overglazes.Uranium Oxide: gives red, coral and yellow colors. Note: even fired into a glaze, uranium remains radioactive. Handle with care.
