Stain colors used on glazes (Overglaze, Onglaze Stain) have low firing temperatures and therefore do not react much with the glaze used. They have a limited firing temperature range of 700 - 900 °C. Colors that are easily changed during firing are red and magenta. If fired at temperatures too low or too high, 10-20 °C, the color will not be the same or may disappear. Therefore, those who use the color must always observe the test results after firing.
Glaze pigments are prepared from 10-20 % synthetic low-temperature fired saturates combined with 80-90% frit fired at 600-750 °C . These are then ground together and fired to melt into shiny colored glass. They are then ground and washed again, then ground further to achieve a pigment size of 90% smaller than 8 microns. The pigments prepared by this method can be used to coat metal and print decorative colors on glasses and bottles. However, glaze pigments are not commonly prepared for home use due to the complexity of frit preparation and the need for precision.
The most common way to decorate a glaze is to print water-based decals onto glazed bowls and cups. These products come in a variety of clays and firing temperatures, such as bone china, porcelain, vitreous china, hard kaolin, and earthenware. Therefore, the glazes used vary in firing temperature, resulting in slightly different firing temperatures for each product depending on the type of glaze.
In addition to using glaze colors to decorate white, clear-glazed products, they are sometimes also used to decorate colored glazes. However, care should be taken not to use glaze colors to decorate copper oxide glazes, as the color will absorb the printed color and turn it gray after firing, especially red and yellow. Printing glaze colors on dark-colored glazes is also not recommended, as the color will be lost, with the exception of white, which requires multiple printings, resulting in extra thickness.
The application of colored glazes to decorative patterns is a widely used technique, particularly for creating vibrant patterns and colors on white tableware. This has led to user protection measures and product liability laws that directly regulate the use of colored glazes on tableware. The dissolution of lead and cadmium is tested by dissolving 4% acetic acid in water and immersing the products in a bath of acid at room temperature and in the dark for 24 hours. The acid used to soak the colored glazes is then tested for lead and cadmium. The results must be lower than the specified values to be considered safe.
The glazes currently available are offset-printed, which can be overlaid with different colors and can be mixed without changing the color after firing. Most use zircon colors. There are many types to choose from, including low-lead and lead-free types, as well as glazes for fast firing.
Note : Information from the book Ceramic Colors by Phaichit Ingsiriwat.