Underglaze; is when ceramic colors are applied on to bisque or clay and covered with transparent glaze before firing. The glaze on top gives it a strong, permanent finish and is used in the mass production of pottery for hotels and restaurants as it can sustain the heat and the powerful substances used in industrial dishwashing.
Colors can be bought as powder or as liquid ready to use. If powder, the potter can create his own colors. The colors are usually made with a small portion of flux to help them integrate with the clay below and the glaze that is put on top. They must be mixed with varnish or oil if applied to bisque and gum if to unfired clay.
Mixing the colors is hard work, the powder must be very dry as water cannot come into contact with the liquids it is going to be mixed with, if there is any doubt spread the powder over a piece of newspaper and leave it in a warm place to dry. On top of a nonabsorbent surface, such as glass or china put the powder and mix it slowly with the liquid, pressing and stirring with a pallete knife until it becomes a smooth liquid of the correct consistency for your work.
The suppliers of the colors usually specify in their catalogues the best way and temperatures they should be fire at. Ware can be fired in two ways; first the colors then covered with a glaze and fired again or the colors and the glaze can be fired together. The firing should bring out the strength of the colors and melt the glaze to a clear, shiny layer of glass. Read more about: In-glaze / On-Glaze