-Reduction firing reduces normal CuO copper oxide to Cu2O to produce bright red coloration in the reaction:
2CuO + CO -> Cu2O + CO2
-Bright red reduction fired colors are usually achieved with very small amounts of copper (i.e. .5%).
-If larger amounts of copper are present, the reaction could precipitate very tiny copper metal particles (colloidal copper) in the glaze melt to yield a red color (i.e. flambé or sang-de-boeuf). It can volatilize at higher temperatures and the fumes can effect the color or other pieces in the kiln.
-Copper luster can be produced by oxidation firing at low temperature glaze (950C) with heavy reduction cooling to leave a metallic layer of copper on the surface. 2-8% copper is required and cooling should be done in 15 minute cycles of reduction, interspersed with intervals where the atmosphere is allowed to clear. This can be carried out in cooling electric kilns by creating reduction through the introduction of flammable materials.
Ceramic Oxide Periodic Table
All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).
Fluoride, when used with copper, can produce blue green colors.
Copper is well-known for its ability to produce blood-red and fire-red colors in steady reduction atmosphere firings where CuO is altered to Cu2O.
Bright red colors are usually achieved with very small amounts of copper (i.e. 0.2-0.5%) in a low alumina base with at least .4 molar equivalents of CaO and plenty of the alkalis. Tin oxide will enhance color. Use of silicon carbide in oxidation (2%) can produce red.
The use of boron in a copper red reduction glaze will give a purple hue.
The following formula produces good purple at cone 10: BaO 0.1, CaO 0.5, MgO 0.1, KNaO 0.2, ZnO 0.1, B2O3 0.15, Al2O3 0.2, SiO2 3.0.
In copper red glazes, barium additions in a high feldspar base will produce turquoise to deep blue depending on how much copper is added. Lithium contributes to the color also.
Large amounts of copper in a glaze give metallic and even graphite effects.