Decorating and glazing

Additives can be worked into the clay body prior to forming, to produce desired effects in the fired wares. Coarse additives, such as sand and grog (fired clay which has been finely ground) are sometimes used to give the final product a required texture. Contrasting colored clays and grogs are sometimes used to produce patterns in the finished wares. Colorants, usually metal oxides and carbonates, are added singly or in combination to achieve a desired color. Combustible particles can be mixed with the body or pressed into the surface to produce texture.

Agateware: So-named after its resemblance to the quartz mineral agate which has bands or layers of color that are blended together. Agatewares are made by blending clays of differing colors together, but not mixing them to the extent that they lose their individual identities. The wares have a distinctive veined or mottled appearance. The term 'agateware' is used to describe such wares in the United Kingdom; in Japan the term neriage is used and in China, where such things have been made since at least the Tang Dynasty, they are called marbled wares. Great care is required in the selection of clays to be used for making agatewares as the clays used must have matching thermal movement characteristics.

Banding: This is the application, by hand or by machine, of a band of color to the edge of a plate or cup. Also known as lining, this operation is often carried out on a potter's wheel.

Burnishing: The surface of pottery wares may be burnished prior to firing by rubbing with a suitable instrument of wood, steel or stone, to produce a polished finish that survives firing. It is possible to produce very highly polished wares when fine clays are used, or when the polishing is carried out on wares that have been partially dried and contain little water, though wares in this condition are extremely fragile and the risk of breakage is high.

An ancient Armenian urn.

Engobe: This is a clay slip, often white or cream in color that is used to coat the surface of pottery, usually before firing. Its purpose is often decorative, though it can also be used to mask undesirable features in the clay to which it is applied. Engobe slip may be applied by painting or by dipping, to provide a uniform, smooth, coating. Engobe has been used by potters from pre-historic times until the present day, and is sometimes combined with sgraffito decoration, where a layer of engobe is scratched through to reveal the color of the underlying clay. With care it is possible to apply a second coat of engobe of a different color to the first and to incise decoration through the second coat to expose the color of the underlying coat. Engobes used in this way often contain substantial amounts of silica, sometimes approaching the composition of a glaze.

Litho: This is a commonly used abbreviation for lithography, although the alternative names of transfer print or decal are also common. These are used to apply designs to articles. The litho comprises three layers: the color, or image, layer which comprises the decorative design; the cover coat, a clear protective layer, which may incorporate a low-melting glass; and the backing paper on which the design is printed by screen printing or lithography. There are various methods of transferring the design while removing the backing-paper, some of which are suited to machine application

Gold: Decoration with gold is used on some high quality ware. Different methods exist for its application, including:

• Best gold - a suspension of gold powder in essential oils mixed with a flux and a mercury salt extended. This can be applied by a painting technique. From the kiln the decoration is dull and requires burnishing to reveal the full color
• Acid Gold – a form of gold decoration developed in the early 1860s at the English factory of Mintons Ltd, Stoke-on-Trent. The glazed surface is etched with diluted hydrofluoric acid prior to application of the gold. The process demands great skill and is used for the decoration only of ware of the highest class.
• Bright Gold – consists of a solution of gold sulphoresinate together with other metal resonates and a flux. The name derives from the appearance of the decoration immediately after removal from the kiln as it requires no burnishing
• Mussel Gold – an old method of gold decoration. It was made by rubbing together gold leaf, sugar and salt, followed by washing to remove solubles

[edit] Glazing

Main article: Ceramic glaze

Glaze is a glassy coating applied to pottery, the primary purposes of which include decoration and protection. Glazes are highly variable in composition but usually comprise a mixture of ingredients that generally, but not always, mature at kiln temperatures lower than that of the pottery that it coats. One important use of glaze is in rendering pottery vessels impermeable to water and other liquids. Glaze may be applied by dusting it over the clay, spraying, dipping, trailing or brushing on a thin slurry composed of glaze minerals and water. Brushing tends not to give an even covering but can be effective as a decorative technique. The color of a glaze before it has been fired may be significantly different than afterwards. To prevent glazed wares sticking to kiln furniture during firing, either a small part of the object being fired (for example, the foot) is left unglazed or, alternatively, special refractory spurs are used as supports. These are removed and discarded after the firing. Special methods of glazing are sometimes carried out in the kiln. One example is salt-glazing, where common salt is introduced to the kiln to produce a glaze of mottled, orange peel texture. Materials other than salt are also used to glaze wares in the kiln, including sulfur. In wood-fired kilns fly-ash from the fuel can produce ash-glazing on the surface of wares, and the use of an ash and clay mix can result in alkaline glazes, as used in Catawba Valley Pottery in the eastern United States.

[edit] Firing

Firing produces irreversible changes in the body. It is only after firing that the article can be called pottery. In lower-fired pottery the changes include sintering, the fusing together of coarser particles in the body at their points of contact with each other. In the case of porcelain, where different materials and higher firing-temperatures are used the physical, chemical and mineralogical properties of the constituents in the body are greatly altered. In all cases the object of firing is to permanently harden the wares and the firing regime must be appropriate to the materials used to make them. As a rough guide, earthenwares are normally fired at temperatures in the range of about 1000 to 1200 degrees Celsius; stonewares at between about 1100 to 1300 degrees Celsius; and porcelains at between about 1200 to 1400 degrees Celsius. However, the way that ceramics mature in the kiln is influenced not only by the peak temperature achieved, but also by the duration of the period of firing. Thus, the maximum temperature within a kiln is often held constant for a period of time to soak the wares, to produce the maturity required in the body of the wares.

The atmosphere within a kiln during firing can affect the appearance of the finished wares. An oxidising atmosphere, produced by allowing air to enter the kiln, can cause the oxidation of clays and glazes. A reducing atmosphere, produced by limiting the flow of air into the kiln, can strip oxygen from the surface of clays and glazes. This can affect the appearance of the wares being fired and, for example, some glazes containing iron fire brown in an oxidising atmosphere, but green in a reducing atmosphere. The atmosphere within a kiln can be adjusted to produce complex effects in glaze.

Kilns may be heated by burning wood, coal and gas, or by electricity. When used as fuels, coal and wood can introduce smoke, soot and ash into the kiln which can affect the appearance of unprotected wares. For this reason wares fired in wood- or coal-fired kilns are often placed in the kiln in saggars; lidded ceramic boxes, to protect them. Modern kilns powered by gas or electricity are cleaner and more easily controlled than older wood- or coal-fired kilns and often allow shorter firing times to be used. In a Western adaptation of traditional Japanese Raku ware firing, wares are removed from the kiln while hot and smothered in ashes, paper or woodchips, which produces a distinctive, carbonised, appearance. This technique is also used in Malaysia in creating traditional labu sayung.