Commercial underglazes are manufactured so they can be applied either before or after firing, depending on the type of pottery. For greenware pottery the underglaze is applied before the clay is fired, while bisque-fired clay can be coated with a commercial underglaze after the clay is fired in a kiln. Most commercial underglazes carry a label on the packaging providing instructions on how best to use the paint-like substance; in most cases they will carry a maximum temperature for firing in a kiln of cone six -- a mid-range firing temperature which equates to 2230F, according to LMRA Pottery Studio. When a commercial underglaze is fired beyond its recommended firing temperature the texture and coloring of the underglaze may be altered.
To apply a commercial underglaze to the surface of a clay product.it can be brushed, sprayed, sponged, dipped or poured. If a commercial underglaze is left in storage for a prolonged period of time the colorings can settle and separate within the container, so to re-merge the chemicals within the glaze must be mixed. In some cases the underglaze can become to thin for use; in such cases the excess dried glaze from around the edge of the container must be scraped into the mixture to obtain a good consistency.
Commercial underglazes can be mixed after purchase by hand or by using a powered mixing tool. If the underglaze becomes too thick for use. you can place the required amount of the glaze in a container and add water to thin the glaze. If the underglaze is too thin, you can remove the lid of the container to allow excess liquid to evaporate.
When used to cover a surface using a brush, a commercial underglaze often leaves behind brushstroke marks; in order to avoid leaving brushmarks the underglaze should be applied in three coats, one horizontal, one vertical and finally a second horizontal coat. Once applied, a potter can often experiment with firing a commercial underglaze past its recommended firing temperature to change the effect and often the color of the glaze.