Under the Glaze
"Underglaze" as a term can mean any decoration that is applied, almost always in a fluid form, on the pottery surface before any glaze is applied. In this blanket sense, underglazes can encompass slips, engobes, and stains, as well as products that are marketed as underglazes.
Commercial underglazes used to be formulated to be basically highly pigmented colored slips: raw pigment, plus clay, plus water. Like all slips, they were made to be applied to the wet or leather-hard clay before it was bisqued.
Today, most underglazes on the market are formulated more like engobes. They are created using fritted material, which reduces shrinkage and allows them to be applied to bisqueware. (In some cases, they can be applied to both greenware and bisqueware.)
Besides liquid underglazes, there are increasing options available for underglaze application. These include underglaze pencils, crayons, chalk, and semi-moist pads which can be used like watercolors.
Underglaze Product Reviews
At their simplest, stains are raw ceramic pigments suspended in water. Commercial stains are carefully prepared and blended so that pigments are consistent and reliable in color and how they handle.
If you are use raw oxides or carbonates to make your own stains, be sure to use a respirator and rubber gloves as you work with them. Find out which raw materials are hazardous, and how to handle them safely.