Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.) 10. Decoration (introduction...) 10.1. Decoration and design 10.2. Glaze decoration 10.3. Engobe decoration 10.4. Terra sigillata

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)

10. Decoration

Decoration is a very big field, which deserves a separate book to cover it in detail. Here we will only discuss some of the main techniques for using glazes and engobes.

10.1. Decoration and design

Good decoration is always related to the design of the pot. It should be used to emphasize and enhance the shape of the pot, rather than being applied randomly.

There are several approaches to decoration;

Banding

Plain or decorative bands of color are painted around the pot, usually by spinning the pot on a banding wheel and applying color with a brush. The bands are placed where they emphasize changes in the curve of the pot, for example, at the rim, the belly, the shoulder.

Area decoration

Decoration is placed inside a defined area, such as a circle. Again this should be done to emphasize the natural curves of the pot.

Overall patterns

These are patterns that are repeated around the pot, often expanding and contracting as the pot does.

Contrasting shapes

These are strongly shaped areas of pattern or color that contrast with the shape of the pot.

10.1.1. MOTIFS, STYLES, LOCAL INPUTS

There are as many motifs and styles of decoration as there are cultures in the world. In traditional cultures, motifs are selected from mythology and familiar designs. Nowadays, with the mixing of cultures around the world, pots are often designed for what can be marketed for export, and design tends to be based on fashion rather than tradition.

The potter selling to tourists will generally choose traditional motifs, since tourists are interested in the culture of the area.

10.2. Glaze decoration

Glaze decoration is done with the glaze itself or with colorants under the glaze or on top of the glaze.

10.2.1. UNDERGLAZE

Underglaze decoration is decoration that is applied under the glaze. It is affected by the transparency and fluidity of the glaze.

Underglazing is usually done under a transparent glaze in order to show it clearly. However, beautiful effects can be obtained under opaque or semiopaque glazes.

A variety of pigments and oxides may be used.

Metallic oxides

The more fusible metallic oxides can be used directly as underglaze pigments, mixed thinly with water. The satisfactory ones are red iron oxide, cobalt carbonate, manganese dioxide and copper carbonate. Designs made with oxides alone will often run with the glaze. Refractory oxides, such as chrome oxide and rutile, can cause crawling.

Oxides mixed with glaze

Metallic oxides can be mixed about 50/50 with glaze, which will prevent the problem of crawling. However, the decoration will usually flow with the glaze and should be designed with this in mind.

Underglaze pigments

These are pigments that are specially prepared by fritting metallic oxides in a base glaze that fires hard but is not fluid. Rather than preparing them yourself, it is usually better to purchase commercial underglazes from a supplier. These are supplied for venous firing temperatures and firing conditions in a wide range of colors. Not all colors can be used under all conditions, and suppliers can usually tell you which are suitable for oxidation and reduction and what type of base glaze will develop the best colors.

10.2.2. ON-GLAZE

On-glaze decoration is applied on top of the unfired glaze. It may be done with a contrasting color of glaze or with metallic oxides or glaze pigments.

Figure 10.2.2.C. Double glazing with the high-surface-tension glaze on top. This draws itself into islands leaving the bottom glaze in irregular patterns.

Application is done by brushing or spraying. Even more than underglaze, on-glaze decoration will tend to flow with the glaze. If distinct patterns are desired, a stiff, viscous glaze will give the best results.

Figure 10.2.2.D. Double glazing with the low-surface-tension glaze on top. This produces a different effect.

Double glazing

Glazes high in surface tension (see page 30) tend to form into small islands on melting. This may cause crawling, but it can also be used as a decorative effect by applying two different glazes on top of each other. The glazes must have different degrees of surface tension. This is achieved by adding clay or talc to one of the glazes. The colors should be contrasting. The best results are obtained with a light-colored glaze at the bottom.

10.2.3. OVERGLAZE

Overglaze decoration is often called "china painting". The pot is glaze-fired as usual and then is decorated with special low-temperature enamels that fire at around 700°C. The enamels are prepared from color pigments and low-temperature frits and are best purchased from commercial suppliers. They are available in every color and have the advantage of firing to true colors, making them suitable for elaborate painting effects. They also stay where they are applied, as there is no chance of the glaze running at this low temperature.

Overglaze is available as powder, which must be mixed with a medium. This is best done by grinding the pigment and medium on a glass plate with a thin palette knife.

Sometimes plain water is used -this works well when filling areas with solid colors. It helps to add some water-soluble glue (white glue) to provide dry strength.

For more elaborate painting, pigment is mixed with special oils. The best is oil of lavender, which is thickened as desired with a thicker oil. The consistency is controlled very much like with oil paint.

Some suppliers have ready-mixed overglaze, which comes in tubes. This can be used directly, without grinding.

Special metallic or iridescent overglazes are known as "luster". These are available commercially as liquid gold, platinum and a variety of mother of pearl colors. They also are fired at 700°C. NOTE: Lusters take on the same surface as the glaze, i.e. a matt glaze will produce a matt luster and a shiny glaze will give a mirror-like effect.

Overglazes are applied by brushing or by spraying.

Overglaze transfers or decals

Most commercially sold decorated dinnerware is decorated with decals (sometimes called transfers), which are made from overglaze that is silkscreen-printed onto decal paper. These are available from suppliers in a range of standard designs and can also be custom-made (in large quantities). They are applied to already glaze-fired ware.

The decal is soaked in water until the design can easily be slid off the paper. The wet paper is placed in the correct location and is carefully slid from under the design, leaving the design adhered to the pot. The design is carefully smoothed, dried and fired like standard overglaze.

10.2.4. REGLAZING, MULTIPLE GLAZING

Reglazing means applying glaze and firing an article that already has been fired once. It is sometimes necessary when glazes do not work correctly the first time -they may be too thin, underfired or not the right color.

Multiple glazing is the process of glazing and firing an article two or more times in order to get special glaze effects that cannot be achieved in one firing. It often involves first glaze firing the pot at a high temperature and then glaze firing with lower temperature glazes, in order to get special colors or textures. For example, a pot may be fired to cone 10, then be refired with cone 06 glazes to get bright colors. It may be fired several times at cone 010 for overglazes and lusters.

Reglazing or multiple glazing makes an article more expensive, but it can also be sold at a much higher price.

Reglazing hints

Because already fired ware is no longer porous, it is difficult to apply enough glaze. It helps to first heat the article (as hot as you can hold in your hand), and spraying is the most effective way to apply more glaze.

For multiple glazing, glaze can be specially prepared by adding cellulose gum (CMC). This thickens the glaze and gives it better handling strength.

10.3. Engobe decoration

Engobe is a specialized type of clay slip that is used for decoration under the glaze. The engobe shows through a transparent or semitransparent glaze and can have the range of color that is possible in glaze.

10.3.1. ADVANTAGE/DISADVANTAGE COMPARED TO GLAZE DECORATION

Engobes stay where they are applied and do not run with the glaze. This makes it possible to do designs with sharp edges or a lot of detail.

Engobes are often used on dark clay bodies in order to provide a bright, white background for glazes.

10.3.2. ENGOBE MAKING, ADJUSTING TO BODY

Engobe is generally prepared as a white base and then colored with appropriate coloring oxides. If you are already using a white clay body, this becomes an engobe simply by thinning it with water. A dark body will require a white engobe formula that fits it correctly.

The main problem with engobe is getting a good fit between engobe and clay body. It must have about the same amount of shrinkage as the body or it will tend to flake off or crack. The engobe should also mature at the same temperature as the clay body in order to provide a strong clay-glaze interface. Engobes can be applied at three different stages:

Leather-hard

This is the best stage for applying engobes, as it permits the widest range of decorating techniques (brushing, incising, inlaying, stencil etc. -see below). The engobe must have enough clay in it to shrink at the same rate as the body.

Bone-dry

Engobes for bone-dry application need to have less shrinkage, so that they adhere to the body.

Biscuit

Engobes for biscuit application are more like underfired glazes.

Some typical engobe recipes are (from. D. Rhodes: Clay and Glazes for the Potter):

Temperature range	Cone 08 - 1			Cone 1 - 6			Cone 6 -11
Body condition	DAMP	DRY	BISC	DAMP	DRY	BISC	DAMP	DRY	BISC
Kaolin	25	15	5	25	15	5	25	15	5
Ball clay	25	15	15	25	15	15	25	15	15
Calcined kaolin		20	20		20	20		20	20
Leadless frit	15	15	15			5			5
Nepheline syenite				15	15	20			5
Feldspar							20	20	20
Talc	5	5	15	5	5	5
Quartz (flint)	20	20	20	20	20	20	20	20	20
Opacifier (zircon)	5	5	5	5	5	5	5	5	5
Borax	5	5	5	5	5	5	5	5	5

Usually engobes designed for plastic clay will not fit on bone dry or biscuit clay and vice versa.

Engobe composition

Engobe is made up of a mixture of plastic and nonplastic ingredients. Recipes for engobes look like those for glazes with a high percentage of refractory ingredients.

For white engobes, the plastic ingredients are china clay and ball clay. The amount of ball clay can be adjusted to get correct shrinkage.

The nonplastic ingredients are feldspar and quartz, and for low-temperature engobes frit is sometimes added to lower the vitrification point.

Small amounts of borax are often added to give better dry strength and to fuse the other ingredients together.

Engobes are often deflocculated like a casting slip, and in fact you can often use a casting slip which fires at the same temperature as your clay body.

10.3.3. COLOR OXIDE ADDITIONS TO ENGOBE

Coloring oxides are added to engobes as a percentage, as with glazes. However, since the color is diluted by the glaze over it, larger amounts are required. You should also remember that the oxide reactions will depend on the type of glaze being applied and on whether oxidation or reduction firing is used. Typical colors and oxide amounts are:

Red iron oxide

1-5%	light green to light brown
5-10%	brown
10-15%	dark brown to black

Copper oxide or carbonate

1-5%	green or blue, red in reduction

Cobalt oxide or carbonate

1-5%	light to dark blue

Chrome oxide

1-5%

green

Manganese dioxide

1-10%

purple-brown

Nickel oxide

1-5%	grey or gray-green

Titanium dioxide or rutile

1-10%

tan, or mottled colors

Commercial glaze stains

1-50%

produces the color of the stain

As with glaze colorants, the most interesting colors are usually obtained by mixing combinations of oxides.

10.3.4. APPLICATION METHODS

A wide variety of decoration techniques can be used with engobe. Leather-hard ware permits the largest variety of techniques and usually has fewer technical problems compared to application on bone-dry or biscuit ware. Work flow for leather-hard engobe decoration is:

- Apply the engobe.
- Biscuit-fire.
- Apply the glaze.
- Glaze-fire.

Dipping, pouring

This is done the same as with glazes. The engobe should be just thick enough to completely cover the clay. Too thick application will often crack, especially on rims. If applied leather-hard, pouring and dipping should be done quickly, so that the pot does not get too soft from absorbing water.

Brushing

Brushing is one of the most satisfactory methods, especially for making bands or areas of engobe. The technique requires some skill in order to get an even coating. The brush should be fully loaded with engobe and should spread it evenly.

Spraying

The engobe must be thin enough to flow through the spray gun. It should be applied in several even coats, taking care to keep a smooth surface and to cover all areas equally.

Scratching or "sgraffito"

To get fine lines, engobe is applied to an area and, after it sets, it is scratched with a sharp tool. This is called "sgraffito", which means "scratching". The clay color shows as a line.

Inlay

Lines are scratched on the leather-hard pot and then filled with engobe. The excess engobe is removed with a metal scraper after it sets, or with sandpaper after the pot is bone-dry. This results in a smooth surface, with the engobe lines contrasting with the clay colon

Stencil

Paper or plastic stencils are placed on the leather-hard pot, and engobe is brushed or sprayed over them. Afterwards the stencil is removed leaving the design of the stencil.

Trailing

Usually called "slip" trailing, the engobe is applied by allowing it to flow from a device with a small opening, which produces raised line decoration. It is easiest to use a rubber bulb (such as an ear syringe available in pharmacies) or plastic containers used for soap or cosmetics. The opening can be made smaller by inserting small metal tubes.

Engobe hints

Engobes will show most clearly under a fully transparent glaze. However, semitransparent or even opaque glazes can give beautiful effects, clouding the engobe colors.

Sophisticated decorators can take advantage of different glazes over the engobe to produce different colors. Complicated effects can result from applying different glazes to different areas of the decorated piece.

10.3.5. ENGOBE PROBLEMS

Often engobe will come off the pot. This almost always is caused by a different shrinkage rate of clay body and engobe and usually happens before firing. In many cases the engobe is applied too thick.

Engobe shrinks more than clay body

In this case, the engobe will develop cracks and will flake off, with the flakes curling away from the ware. The solution is to reduce the amount of plastic clay or substitute raw clay with calcined clay. Deflocculating usually helps.

Engobe shrinks less than clay body

In this case, the engobe will flake off, especially on rims and sharp edges, and the flakes will be flat. The solution is to add more plastic clay or to substitute calcined clay with raw clay.

Flaking after firing

This is caused by differences in firing shrinkage between clay and engobe. Usually adding flux to the engobe will help.

Spit-outs

Application of engobe to biscuit ware sometimes causes the engobe to lift off in small bubbles. This may only show up after glaze firing, but it arises during application. If the biscuit ware is very porous, it absorbs the water in the engobe so fast that air inside the body comes under pressure. When the air is released, it may blow out the engobe layer where the air escapes. The solution is to reduce the absorption by dipping the biscuit in water some time before engobe application.

10.4. Terra sigillata

The technique of coating pottery with terra sigillata was used by Roman and Greek potters and is still used by traditional potters in India and Nepal. It produces a thin, opaque and low gloss finish to pottery.

10.4.1. PREPARING TERRA SIGILLATA

Terra sigillata is made from clay. For temperatures below 1100°C local sedimentary clays are more suitable. The finer the clay particles the better. Such clays normally contain iron and fire to a red colon It is more difficult to produce white-fring terra sigillata from ball clay or kaolin.

by weight:	Clay	0
	Water	0

Sodium metaphosphate + 0.5%

The best result is obtained when ball milling the clay. Some clay can be prepared without ball milling. After ball milling the batch is transferred to a container and left for 24 hours. The coarse particles will settle and the upper 2/3 of the batch is siphoned off. A bucket with a tap placed 1/3 up is useful for regular production of terra sigillata.

Colors can be made by adding color oxides or pigments. First the terra sigillata is dried and the color oxide is added in amounts similar to what is mentioned for engobes (by dry weight). Then water is added and the batch is again ball-milled for 4 hours. It is then ready for use.

10.4.2. APPLICATION

The terra sigillata should be adjusted to a density of 1.15 to 1.20 for application on leather-hard clay. For dry and biscuit ware more water is added to obtain a density of 1.05 to 1.10. The ware should be clean and dust-free before application.

Application can be done by dipping, brushing and spraying. After drying the gloss can be improved by polishing the surface with a cloth.

10.4.3. ADVANTAGES

The use of terra sigillata makes it possible to produce attractive decorations on low-fired pottery without using glazes. The coating gives a dense, glossy and impervious surface. A very beautiful glossy black can be produced by placing the terra sigillata items in a ridded pot filled with sawdust. This is fired in a normal firing either in a kiln or in a traditional pottery firing. The strong reduction will change the normal red color to black.

The use of terra sigillata coatings as an intermediate layer between body and glaze is reported to reduce crazing and bubbles in the glaze.