Underglazes are pigments derived from various oxides. These are used to paint pottery in various ways, depending on the particular method used. They are called underglazes because they are usually applied under a coat of clear glaze. This clear glaze melts in the kiln when fired and acts like a protective glass coating. Underglazes behave differently under different conditions and temperatures. The pigments sometimes fade or disappear altogether at high temperature. It is always advisable to test your colors as some will survive the kiln firing and keep their brilliance, while others may not. Red colors can be especially difficult to achieve when firing to higher temperatures.
Mix the colors each time you dip the brush, as the pigments are not dissolved, but merely suspended in the watery solution. Do not overload your brush, as it will cause the underglaze to drip down the side of your pottery before you even have a chance to finish your design. Wait for any unwanted drips to dry and use fine sandpaper to remove the stained area.
Continue with painting in even flowing strokes and do not apply too many coats as it will either clump up or fall off as it dries. Paint abstract designs, lettering, flowers or patterns. Dip the painted pottery in clear glaze, clean the foot and fire. This glaze coating will protect the underglaze. Fire to test new underglazes before painting a lot of pottery.
Prepare your pottery by dipping bisque in a clear or white glaze. Let it dry and begin to use the underglazes as you would painting on unglazed pottery. Watch for excess paint on your brush as you may not be able to clean off any drips. Don't press too much paint into the underlying clear or white glaze as it will rub off.
Gently paint on the glaze until you become accustomed to the texture. Keep in mind that it will be a balancing act between having enough paint on the brush while not having it drip or wash away the unfired glaze which is coating the surface. Test the results of applying the underglaze on top of the clear or white glaze. Examine the results and adjust your underglazes for better results the next time around.
Premixed underglazes are available in pottery supply stores and come in all colors.
Bisque the clay items you wish to glaze. Clean the bisque with a moist sponge or cloth before painting. Test the color on a piece of paper before you apply it to the pottery. Use a separate brush and container, like a jar, for each color. Use less water to get darker shades.
Some underglazes are toxic and should be used with caution.