Decorative Glaze Removal Processes Robin Hopper

Appears in the Nov/Dec 2017 issue of Pottery Making Illustrated.

Along with all of the variables of glaze application, there are many glaze removal processes that can add greatly to the richness and diversity of the ceramic surface. Some of these are used in conjunction with the glaze application and some are done after the glaze firing. The processes are sgraffito, wax resist, paper resist, crayon resist, latex resist, sponge removal, finger wipe, glaze intaglio, and sandblasting.

Sgraffito

Sgraffito is probably the simplest form of removal, where a pigment, slip, or glaze is scratched away to reveal the surface beneath (1, 5). It can be done at various stages in the making and decorating processes. When done through pigment or slip on the soft or leather-hard clay, it will have a different quality than when done through slip on dry clay. In the first instance, it will give a more fluid, sharp-edged line; in the second it will usually show a broken line through the grainy quality of the clay. It can be done through once-fired glaze, cutting down into the clay, or through glaze down to bisque-fired clay. A variety of tools can be used, from sticks to knives and old dental tools. Sgraffito techniques can be compared in their visual qualities to pen and ink drawings, as both are essentially scratch processes. The fired result can have all of the expression and richness of this form of drawing.

Resist Processes

The various forms of resisting glazes, slips, or colorants all do basically the same thing, although with quite different qualities. The process makes possible the removal, or resistance, of one material from another by making some form of barrier, which subsequent applications of glaze or color aren't able to penetrate. This can give a wide range of color, texture, and glaze variation, as the resisted area will show one color and the area where the glaze or color hasn't been resisted will show another. Resists can be done on the clay, on the bisque, or on the glaze, depending on the result required.

Wax Resist

This may be done with hot paraffin wax or candle wax, or with cold latex wax or wax emulsion. Wax may be used on the greenware and painted over with pigments to get an underglaze decoration, after which it is then bisque fired. The wax will burn out, leaving the pattern, and glaze may be applied over it. Wax may also be used on the bisque to keep glaze off entirely, to show the character of the clay body, or it may be used between layers of glaze or between glaze and colorants (2). Resist processes may be used in conjunction with any of the previously mentioned application processes.

Paper Resist

Paper resists are usually better done on leather-hard clay, using either slips or colorant brushed, poured, or sprayed on. Absorbent paper such as newspaper can be applied to the piece by wetting with water, after which the slip or colorant is applied. When dry, the paper is removed, leaving a positive/negative image, which can be glazed over or left as is. Masking tape also makes a good material for paper resists.

Crayon Resist

Underglaze crayons may also be used to produce drawings that partially resist the glaze, depending on the thickness of both. They're best used on either bone-dry or bisque-fired clay. Children’s wax crayons may be used in the same way. Some colors may stain, but lighter colors will be unlikely to stain. You can always draw with wax candles as well.

Latex Resist

There are various forms of latex material on the market that may be used for decorative purposes. Almost any rubberized cement or glue will do very well. Some latex materials can be thinned with water, making them more versatile. Some will dry in a sheet, which can be peeled off after the desired effect has been reached, allowing further work to be done on the previously resisted areas (3).

Floor Wax

Liquid and solid floor waxes can also be used for resists, although they are not generally as efficient as other resist materials.

Sponge Removal

Sponges can be used to produce a negative image just as well as a positive image, as described earlier. To achieve this, a damp sponge, patterned or otherwise, may be placed on the slipped or glazed area until the area softens. The sponge can then be lifted, removing the glaze or slip at the same time. Sponges can also be used freely to remove slip or glaze coatings, just by wiping the material away. The area of glaze removal can then be left as is, where the slight residual glaze will usually fuse slightly, or it can be covered with another glaze (see 2).

Finger Wipe or Combing

Finger wiping or combing of glaze is the reverse of finger painting. The removal of glaze produces a negative image. Combs can be made of stiff rubber, leather, or cardboard. Wiping or combing can be done with slips or glazes, particularly glazes that contain a high clay content, in a very fluid, free movement (4). It should be done while the coating is still in a semi-liquid state, for best movement. The process essentially moves some of the material, rather than removing it altogether, giving a variety of thick and thin areas where concentration of glaze has one effect and thinning of glaze has another.

Glaze Intaglio

Glaze intaglio is the name given when one slip or glaze is removed, by any method, and another slip or glaze is put back. It can be an extremely laborious process, but it's a way of obtaining very controlled designs in slip or glaze. It's usually done with a variety of tools, knives, needles, and so on to scratch away the material. The removal can also be effected by using a removable latex resist or by covering an area with hot wax on top of a glaze and removing this after subsequent application of another glaze. The resisted area can then be filled with glaze. The filling in of the glazes is best done with either brush or trailer.

Sandblasting

Sandblasting is a technique more often used in the decorative processes of glass than clay. A high-pressure jet of sand is aimed at an object, removing the surface of the object through abrasion. In clay or glass studios, sandblasting is usually done in specially designed cabinets, with glove inserts for holding the object, a window for viewing, and a nozzle for directing the jet of sand (6). Various types of sand may be used, for coarse or fine jets. The sand jet literally eats off the surface of the clay or glaze, or whatever comes in its path (7). Patterns are made with a resist of some sort, either a latex sheet, as used in stonemason’s workshops, or with masking tape (8). Ordinary masking tape will withstand quite a lot of bombardment before it breaks down.

Using sandblasting, you can remove the skin of glaze or cut right down into the clay (see 7). Different qualities can be achieved by sandblasting at different times in the making process. If it's done at the bone-dry stage, the edges will be soft and rounded. When done on either bisque or glazed ware, the edges are likely to be sharp and crisp. Colored clays can produce interesting effects. For example, the unglazed surface of a porcelain clay, stained with a black stain containing cobalt and manganese, will, after a reduced firing, have a different color from that of the core of the clay and that can be exposed using sandblasting. Sandblasting can be used to give a delicate textured surface when done very lightly over a glaze. It needs to be done with a great deal of care, as the process is very fast and there is no reversing it.

Excerpted from Robin Hopper's book, Making Marks, available on the Ceramic Arts Network Shop.