Archive for February, 2006

One of the first rules in the application of underglaze colors is to stir the color thoroughly before using it. This is important because the pigment tends to settle to the bottom of the container and the decorator who neglects to stir the color is left with a brushful of what amounts to little more than tinted water. The color should also be stirred at frequent intervals during the decorating to make sure that the maximum color strength is constantly maintained.

When applying underglaze color, be sure you don’t apply paint too heavily or it may chip or peel off in the firing. On the other hand, color that is applied too lightly will become almost nonexistent after firing. Only by practice (and mistakes) can you determine the correct amount of color for a desired result. Since most beginners are timid about applying color, it might be advisable to use more color than you think is necessary at first.

It is also important to keep underglazes sufficiently fluid. Color that is too thick will make painting very difficult and will pile up, causing defects in the fired piece. Since underglazes have a water base, they tend to dry out when exposed to the air, but you can control the consistency by adding water whenever the paint seems too thick.

Underglazes can be applied to either green or bisque ware. However, decorating on green ware has several advantages, one of which concerns the correction of mistakes. Any unwanted blob of color or incorrect brush stroke can be scraped off the green piece with the edge of a sharp knife or an entire decoration can be wiped off with a damp sponge. A mistake on bisque ware is quite another story. It is almost impossible to remove the color completely and, although the surface may appear clean, a small amount of color invariably sinks into the pores of bisque ware and will show when the piece is fired. Another advantage to decorating on green ware is that sgraffito can be used with the under-glaze decoration, whereas it is quite difficult, to say the least, to sgraffito on a bisque surface.

Before applying underglazes to green ware, make certain the surface is clean by wiping it with a damp sponge. Any dust, finger marks or other surface dirt must be removed before color is applied. In addition to cleaning, sponging the surface will make for easier brush strokes.

A good part, if not most, of your ceramic decoration will probably be done with under-glazes. An underglaze color is a mixture of ceramic materials specially compounded for use on green ware or bisque ware. Although ware decorated with underglaze colors may be fired and left unglazed, usually a transparent glaze is applied over the decoration hence the word “underglaze.”

The popularity of underglaze decoration stems from the fact that almost any type of design is possible, from the most delicate brush stroke to a solid background of color. Underglazes can be painted, sprayed, sponged or spattered with endless variations and combinations possible in each technique. Unlike most ceramic techniques which often dictate the type of decoration, underglaze decoration is comparatively free of such restrictions.

Kinds of underglaze colors

There are two kinds of underglazes available to the decorator and you will probably

Illustration 65

A line design over an irregular area of sponged underglaze gives a light, modern feeling.

have occasion to use both of them, since each has special qualities. The most widely used is the opaque underglaze which has a high clay content, producing an opacity of color similar to engobe. The other type of underglaze may be referred to as “one stroke,” although it has various trade names. This underglaze contains less clay material and a greater concentration of color than the opaque underglaze, resulting in a more or less transparent effect when fired. With “one stroke” underglazes you can obtain a certain amount of shading since the colors are not opaque. Because of the high content of color, this type of underglaze is particularly suited to designs involving fine lines or other areas where a strong color is desired with one stroke.

Dry and prepared underglazes

Underglazes can be obtained in prepared liquid form ready to use or in powdered form. For the ceramist who plans to use underglazes in quantity it is, of course, much cheaper to buy them in dry form. Water is added to the dry underglaze and, if it is to be brushed on, glycerin, mineral oil or gum tragacanth.

Paper masks

Very precise outlines can be obtained with paper masks. Cut the desired shape from newspaper, saturate it with water, glycerin, mineral oil or gum tragacanth and then place it on the dry clay surface. Brush or spray a colored engobe on the piece, covering the paper mask. As soon as the engobe is dry enough remove the paper mask carefully with a pointed tool.

Do not attempt to use large or complicated shapes for your design because the paper may wrinkle and will not lie as flat on the piece as will a smaller, simpler mask. Also, make sure that the mask is thoroughly wet before applying the engobe or it will not adhere to the surface. Glycerin or gum tragacanth is more satisfactory than water since it does not dry as quickly. If you are applying a number of masks, you may find that some have dried by the time all the masks are down, but you can simply re-moisten them quickly with a wet brush before applying the engobe.

After trying these methods of engobe decoration, give free rein to your imagination and combine the various techniques. You will discover endless possibilities that can help make your ceramics pieces of outstanding individuality.

Wax and rubber resist

A bold, free brush technique is possible by means of a wax resist method. The design is painted on the piece with liquid wax and then the entire surface is covered with an engobe. The wax resists the engobe so that the areas painted with wax will remain the color of the clay piece. During the bisque firing, the wax will burn out and disappear completely.

You can use melted paraffin for this method, but it must be boiling hot and is a bit difficult to handle since it cools rapidly when it touches the surface of the piece. You will find the painting much easier if you use a liquid emulsion of paraffin that is available ready made. This can be applied without heating and responds to a brush in much the same manner as tempera paint. It is also much easier on your brushes than hot paraffin which eventually causes the hairs to loosen and fall out. The wax resist material can be removed from the brush by rinsing in water, but this must be done immediately after the brush has been used. If you allow the emulsion to harden in the brush it will have to be cleaned with benzine.

Wax resist permits a freer type of decoration than most engobe techniques. Practice different brush strokes and experiment with different size brushes in order to exploit to the fullest the possibilities offered by wax resist. To vary the technique you can sgraffito designs in the paraffin after it has dried so that when the engobe covers the surface of the piece, it will also sink into the cut areas in the wax, producing lines of colored engobe within the design.

Liquid rubber (rubber latex) can be used in place of wax as a resist material. One of the advantages of rubber is that no bisque firing is required to burn the rubber out as with wax. The liquid rubber can be removed by prying up the edge of the rubber with a sharp tool and peeling it off. This means that the piece can be glazed and fired in a single operation. If you find the rubber latex a little thick for brush work, thin it with diluted ammonia. Also use ammonia to clean the brush immediately after use.

Mishima

Originated by the Japanese, mishima is an inlaying technique in which the design is incised and then filled with a contrasting slip. The clay should be in a leather hard condition for the incising step. Any pointed tool may be used to cut the design-your sgraffito tool will come in handy here-and the lines should be as free as possible. Mishima is best adapted to a freely executed design, although by this we do not mean a sloppily executed design. As with sgraffito, the success of your decoration will depend upon the quality of your line.

Illustration 61
Mishima decoration begins with an incised design.

Illustration 62
A colored engobe is then painted over the entire piece.

Illustration 63
When the engobe is leather hard, it is scraped off with a knife.

Illustration 64
The enrobe remains in the incised lines of the design.

After the design has been incised, brush a contrasting engobe into the cut lines. It is not necessary to be too careful here because no matter how carefully the slip is brushed into the design, some of it will be unavoidably brushed on the rest of the surface as well. Do not attempt to remove the excess slip while it is wet. Wait until the engobe is leather hard and then scrape off the excess, leaving engobe only in the incised lines of the design.

For variation, the contrasting engobe can be painted over the entire surface of the piece and then scraped off in a pattern (bands, stripes or other planned areas). Mishima pieces may be glazed with transparent or semi-transparent glazes or they may be left unglazed and simply waxed, which is a very attractive finish for this technique.