Ceramic stains (noun, pl.) can refer to ceramic colorant oxides suspended in water or to prepared coloring oxides (commercial stains). Colorants generally are sold in powder form and commercial stains may be either sold in powder or liquid form. Stains can be used by themselves as an underglaze color, in slips, in clay bodies, in glazes, painted on glazes, and in overglazes.
Many commercially available stains are fritted for safety, then reground. Some, especially oranges, reds, and yellows, have their volatile coloring oxides encapsulated in a coat of zirconium silicate. This manufacturing technique has significantly increased the firing range for many colorants.
Depending upon the strength of the colorant, full color can be achieved in a transparent glaze anywhere from 1-5%, in an opaque glaze from 5-10%, and in slips and clay bodies from 10-15%. Stains, like all colorants, can result in very different colors dependent on firing range and, especially, firing atmosphere. In addition, glaze components can also have a significant impact. When using commercial stains, note and follow all directions for use, including what types of glazes are appropriate for that particular stain.
Because ceramic stains consist of metallic oxides, always consider safety. Even with fritted stains, do not have food or consumable liquids where you are working with stains, use a respirator with dry materials, and control all dust created.
Note: There are also organic dyes which are sometimes used to color raw stains. These dyes help potters have a better idea of what a fired ceramic stain may look like (but is by no means exact). Because they are used only to help identify raw stains, they are sometimes referred to as distinguishing stains. These organic compounds burn out during firing and do not effect the final product.