Generally speaking, s consist of some combination of salts, metallic oxides, alumina, silica and natural clays. These different materials each convey different colors to the final pigment. A combination of zinc, chromite, cobalt and alumina, for instance, produces a blue- , with the ratio of chrome to cobalt allowing for a bluer or greener result. These mixtures exhibit more stability than oxides, natural coloring agents that can react with one another or with the heat of the kiln to spoil the intended result. An alcohol base serves as a solvent to keep the pigments from oxidizing.