When clay is fired, heated to a high temperature in a kiln, its physical and chemical structure changes. Before firing clay can be reclaimed and recycled by adding water and remixing, but once it is fired it cannot.

The first stage of firing green clay, wet clay, is the bisque stage. At this stage all of the chemical water has left the clay. this leaves a very porous structure behind. IF you paint water onto the surface of a bisque piece of clay, it rapidly absorbs like a bone dry sponge.

Underglaze

Underglaze is made from pigments mixed into clay. When fired, underglaze does not seal the surface like glaze. Underglaze is often applied with a paintbrush directly on bisque ware. It can also be applied on greenware. Underglazes can be mixed together like paint.

As underglaze is fired to higher temperatures, less of the color will remain. To obtain the richest and brightest color from underglaze, use a clear glaze on top. This is similar to how the top layer of varnish on wood brings our the color from the wood or the stain or how the top layer of varnish on a painting can accentuate the colors underneath. The final color of underglaze is effected by the glaze put on top, even if the top glaze is clear. Many ceramic shops have test tiles and glaze samples to give you an idea of what to expect with different combinations of underglazes and glazes.

Glaze Temperature

Ceramics are fired to different temperatures measured by pyrometric cones. Cone 10 is hotter than cone 06. The exact temperature may vary between manufacturer but the cone melting point is generally the same.

It is important to match the glaze temperature to the kiln temperature. If you use a cone 06 glaze in a kiln fired to cone 10, then you will have a puddle of glass, the melted cone 06 glaze, at the bottom of your ceramic piece glueing it to the kiln shelf. This will ruin your work, damage the kiln shelf, and likely ruin others’ work below yours in the kiln. Pay attention to glaze and kiln temperatures.

Underglaze can touch a kiln shelf without fusing to the kiln. This can be useful to add color to the feet of pots or the bottom of ceramic sculptures.

Glazing Process

Underglaze Application

1. Make sure that your ceramic piece is free of dust. Generally it will be at the bisque stage. To remove dust, wet a sponge, wring all the water out of the sponge,, then lightly wipe the surface of the piece to lift way any dust without adding moisture to the ceramic piece. If you add moisture, then the underglaze will not adhere to the surface.
2. Get a clean pallette or container for the underglazes that you want to use. Pour a small amount of underglaze out onto your pallette. Do not work directly from the underglaze container, to avoid contaminating the underglaze.
3. Use a brush to paint on underglaze. Be quick and decisive with your brush strokes. If applied too thin, the color will not be visible, applied too thick and the underglaze and delaminate from the clay.
4. Be aware of the moisture content of the clay. If it is too wet then the underglaze will not rapidly absorb and dry when applied.
5. Wax can be painted on before the underglaze to serve as a mask or stencil. The underglaze will not absorb where the wax is.
6. Once the underglaze is complete, clean all of your tools and work area.

Glaze Application

1. Make sure to use the correct cone temperature glaze for the temperature of the kiln.
2. Select the bucket of glaze. Remove the lid.
3. Use the large metal paddle to stir and thoroughly mix the glaze. Make sure the paddle is clean before using and clean immediately after use.
4. Use tongs to hold your ceramic piece.
5. Dip it directly into the glaze and then pull it out.
6. Allow the excess to drip back into the container.
7. Wipe away any glaze that will come in contact with kiln furniture. Pay careful attention to the sides near the bottom of pieces that may have large amounts of glaze that can run down the side and onto the kiln shelf. It is good practice to have a buffer zone of no glaze on the sides at the bottom.
8. Place your glazed piece on the cart labeled for the correct temperature kiln.
9. Clean up your tools and work area.