Besides ornamenting a piece with form decorations, colored clays or slip and mineral colors, known as under-glaze colors, may be used. Both may be obtained from a ceramics supply company. The slip, sometimes also called engobe, may be made of different colors of native clays having a shrinkage similar to that of the body of the vase upon which it is to be used.
Mix the slip to a creamy consistency by adding water to the wet clay or to the finely pulverized clay flour. Plan the design by painting with a water-color brush on a plaster slab. After the design has been worked out, paint it on the wet "green ware." The slip should not be too thick. If it is, it will crackle.
A trailed design may be applied with thicker slip by filling an eye dropper with slip and trailing on the decoration. The slip should be thin enough to flow evenly, but it should not be watery.
Designs may be scratched directly on the surface of the ware. The slip is scratched away to the body of the piece. This method of decoration is known as "sgraffito." It comes from the Italian "sgraffiare" which means "to scratch." After a piece which has been decorated with slip has been fired once, a transparent glaze is applied and fired.
A spiral may be cut through the slip to the body of the clay by pressing a pointed wooden tool through the slip so that as the wheel is turned the tool takes off the slip. The piece should be covered with only a thin coating of slip. The body of the clay should still be wet. The slip may be applied with a sponge by patting it on smoothly, or it may be sprayed on from a fly sprayer.
Underglaze painting offers a wide range of color and design. Underglaze colors may be purchased from ceramic supply houses in 1-ounce packages. Larger amounts may also be obtained. There are many colors from which to choose. These colors are painted directly on the biscuit-ware.
Plan the design. Use a brush in making a number of trial sketches. The shape of the brush strokes, variations in the thickness of the long lines, and gradations of color made by adding water to the brushful of paint can all be employed to good advantage.
The dry colors are mixed with gum arabic and water. Dry lumps of gum arabic are dissolved by soaking them overnight in cool water. For a small amount of color, dip the brush in the gum arabic, pick up some of the dry color on the brush, dip the brush in water, and mix the paint on a glass or a nonabsorbent surface. A larger amount of color is mixed by dipping out half a teaspoon-ful of gum arabic, adding the dry color, and mixing a few drops of water. If too much gum arabic is added, it will blister and peel. If there is too much dry color used, it will powder off. Too much water will make the color pale and the lines thin. Practice until the right amount of each ingredient is used.
Trace or draw the design on the biscuitware. This may be done with a lead pencil or a piece of charcoal. Do not use carbon paper. Do not erase with a rubber eraser. The glaze will not adhere to an oily surface.
After the design has been painted on the bowl with even strokes, a transparent glaze is applied. Spraying is the best method. If it is to be brushed or dipped, the glaze should be thicker, but care must be taken not to dissolve the underglaze colors.
When the piece is fired, the colors will be about as deep as the wet underglaze colors were. The glaze will look like a thin coating of glass. Underglaze colors may be added to colorless glazes for tinting. Only a small amount of color is necessary. A test piece should be fired before putting an experimental glaze on a piece of pottery.