Overglaze, Onglaze Stain
It has a low firing temperature so it does not react much with the glaze used. That is it has a limited firing temperature range from 700 - 900 °C. Color that changes easily in the firing is red and magenta. If fired at a temperature that is 10-20°C too low or too high, the color will not be the same or disappear so user must observe the results of the test after every firing.
Preparing the colors on the glaze by preparing low-temperature fired stain colors in quantities of 10 – 20 % fused with fired frit at 600-750 °C in the amount of 80-90% , then grind them to incorporate well and then fire them to melt until they become stained glass with glitter. Then, grind and wash the colors again. After that, grind until fine to a grain size 90% smaller than 8 microns. The colors prepared by this method can be used to coat metals and print decorative colors on glass and bottles however the colors on the glaze is not popular to be prepared for use by yourself as it is difficult to prepare frit and requires precision.
The decorative color on the glaze is generally popular with water-based decal printing. Also known as decal on the product that has been glazed. These products have a variety of clay textures and firing temperatures such as Bone China clay, Porcelain clay, Vitreous China clay, Kaolin clay, and Earthenware clay. Therefore, the glaze used has different firing temperatures. This will cause each product to have a slightly different firing temperature depending on the type of coating.
In addition to using the color on the glaze to decorate on the white and clear coated products, it is also sometimes used as a decorative coating on colored products. But be careful not to apply color on the glaze to decorate the glaze that uses copper oxide because the color will absorb the printed color and become gray after firing especially red and yellow. Also, it is still not popular to print the color on the glaze on the dark glaze because the color will be lost except for the white color which has to be printed multiple times to make it extra thick.
Decorative pattern with color on the glaze is a widely used technique especially for creating bright patterns and colors on white tableware products. Therefore, there is a product liability law to protect users of the product. It has direct regulations on the decoration of the color on the glaze on the tableware. The solubility of lead and cadmium was checked by dissolving 4% acetic acid in water and immerse the product in a bath of acid dissolved at room temperature in the dark room for 24 hours. Then measure the content of lead and cadmium of the acid that used to soak the products decorated with color on the glaze. The result has to be less than the pre-set value to be considered safe.
The colors on the glaze available today include offset printing that can overlap and can also be mixed and the color will not react to change after firing. Most of which use Zircon colors which have many types to choose from both with low lead content and no lead content including color on the glaze for faster firing
Remark : Information from Paichit Ingsiriwat's Ceramic Color book