Copper is an active flux and may increase melt fluidity and may increase crazing because of its high thermal expansion. Bright red 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 colour (i.e. flambâeš or sang-de-boeuf).
Copper lustre 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.