Commercial stains are prepared from minerals found in nature, significantly modified for the use of ceramists. They are refined, blended with one another, mixed with silica and alumina, fired to bond the various materials, and ground to a fine powder.
Stains have created a whole new color palette for the ceramist. The colors are brilliant, safe to use, and very reliable. This will enable you to make a reasonable prediction of the way the piece will look after the firing. Stains are added to the glaze in varying amounts, usually 2–12% of the total recipe.
Using stains for colourful brushwork decoration is very popular, and many people just mix the pure stain powder with water and paint it on using watercolour techniques. In general, a stain should be used under a transparent glaze. The ware can be decorated at the green or biscuit stage. However, under-glaze stains must have maximum chemical compatibility with the overlying glaze for correct colour development. This is achieved by dispersing the stains in a medium of clay, frits and fillers. When used as underglazes, surfaces coming into contact with food must be covered by a food-safe transparent glaze
Do not underestimate the sophistication and refinement of commercially prepared underglazes.