Underglazes are paints that simply go UNDER a glaze. There are primarily two kinds of under glazes . One, designed primarily to go on greenware and another designed to go on bisque. Underglazes can be translucent or opaque. Even though opaque underglazes need multiple coats in order to obtain complete opaqueness, they are not as controllable for translucent effects. Some have more color pigment and less clay. Others have more clay and less color pigment.
An underglaze designed to go on greenware, is a clay base suspended in a liquid with color pigment and chemicals that produce a color when subjected to heat. Because it has a clay base, it bonds well with the greenware. Most usually, you do not see the final color until after it is fired. There are some companies who add a dye or coloring agent to the ‘paint’ so you can visually discern it from other paints and some companies try to color the paint similar to what it will look like when fired. These dyes however, fire out and are replaced by the true chemical/color pigment. These added dyes are helpful to discern where you have applied which paint to your item. Being clay base, these paints readily soak into your greenware and become one with the piece.
The second type of underglaze, designed to apply to bisque, is a synthetic base with chemicals and color pigment and it lays on top of the bisque, requiring the glaze to permanently affix it into place. As with the clay base paints, the true coloring is not visible until after it is fired. Often, these underglazes when applied to greenware, will not bond as well and have been known to chip or flake off after firing especially if applied too thick.
It is important to know your paints and how they interact with your greenware, bisque and firing. Just because something is called underglaze, does not mean it will suit your purpose.
To read more about underglazes, click here.