Decorating and Glazing
Pottery may be decorated in a number of ways including:
- In the clay body, for example, by incising patterns on its surface.
- Underglaze decoration (in the manner of many blue and white wares).
Underglaze may be applied by brush strokes, air brush, or by pouring the underglaze into the mold, covering the inside, creating a swirling effect, then the mold is filled with slip.
- In-glaze decoration
- On-glaze decoration
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 coloured clays and grogs are sometimes used to produce patterns in the finished wares. Colourants, usually metal oxides and carbonates, are added singly or in combination to achieve a desired colour. Combustible particles can be mixed with the body or pressed into the surface to produce texture.
Agateware: Named after its resemblance to the quartz mineral agate which has bands or layers of colour that are blended together, agatewares are made by blending clays of differing colours 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 colour 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.
Engobe: This is a clay slip, 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 colour of the underlying clay. With care it is possible to apply a second coat of engobe of a different colour to the first and to incise decoration through the second coat to expose the colour 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 colour, 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 colour
- 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
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