Colorants

Oxides and carbonates added to the base glaze recipe or clay body to give color. Oxides and/or carbonates of iron, copper, cobalt, and chrome are used as colorants, among others.

Colorants can vary in purity and composition from batch to batch, and these variations can cause significant differences in how a glaze recipe turns out. A lot of experienced potters have a tale of "the day the colorant changed" (or a material became unavailable) and threw off their glaze recipes. If you find a colorant that gives you pleasing results, it's not a bad idea to stock up on that batch of it.