Stain colors use in glazes must be sintered at a temperature higher than the sintering temperature of the glaze so that the color does not disappear after firing. In addition, the stain must match well with the chosen glaze then the colors will appear as intended. Because some stains are not suitable for glazing with zinc, some stains are not suitable for glazing with lead, and some stains are not suitable for glazing with magnesium, etc.
However, when grinding colors in the glaze, the glaze should be thoroughly grinded first then add the color near the end of grinding, continue grinding for a while or if it is a fine finished color, grind the clear glaze separately and add the pigment to the glaze then stir the ingredients together.
Color pigments prepared for various glazing must be suitable for producing glazes which is different from the mixed color in the clay both the raw materials used in the production and the reactions that occur during the firing. The structure of the color mixture in the glaze is not as strong as the color mixed in the clay. Therefore, it will react by melting with the glaze. After firing, the fine pigment reaction is stable and will be melted into the glaze.
When the factory produces the colors used in glazing, the test is carried out with a drag glazing method using a metal-coated squeegee by taking the old standard color to compare with the new color and then fire it. After firing, the test color will be clearly seen in the same test base before being measured with a colorimeter.