Pottery and porcelain could be decorated before or after glazing, or the glaze itself might form the decoration — as, for example, with the splashed greens, yellows and browns of English Astbury-Whieldon pottery of the 18th century. Three-dimensional, relief or incised designs were always done before applying the glaze, while coloured designs could be added to the piece before glazing ( in the case of underglaze colours) or after (overglaze colours, or enamels, and gilding).

Underglaze Colours

After an initial firing — but before the glaze was applied — a design could be painted or printed onto the ceramic body with a solution containing a metal oxide, such as cobalt, chromium, manganese or iron. When these were fired at high temperature, the oxide matured to produce a colour that appears to come from within the body. Cobalt, for example, would mature from black to blue. The shade of colour produced depended on the firing temperature and other conditions as well as on the metal oxide used. Copper, for example, produced green or red depending on the amount of oxygen in the kiln, and brown and black came from iron and manganese.

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Underglaze colours were restricted to those pigments that could withstand the high temperature of the kiln. Cobalt blue has long been the most widely used in both Oriental and Western ceramics. The others are iron red, manganese purple, chromium yellow, and copper green or red.

Overglaze Colours and Gildings

When a design was applied on top of the glaze the colour borders can often be seen or felt with the fingertips. On porcelain, colours were applied with enamel paints, of which a fa1 wider range was available than ofunderglaze colours, since they did not have to withstand such high temperatures. Different coloured enamels harden at different temperatures, so those fired at the highest temperatures were applied first, followed by those needing the next lowest temperature and soon. Gilding almost always came last.

The colours used on a piece can help point to its place and date of origin. For example, pink enamel—from a mixture of gold chloride and tin chloride — first appeared on Chinese export porcelain after 1720. It varied with firing temperature from pale pink to purple.

Painting or Printing?

Ceramic designs could be either hand-painted o1 transfer-printed onto the body, or sometimes a combination of the two. An outline or cartouche, for example, might be transfer-printed but the details then painted in by hand. Transfer-printing, developed in the 1750s, speeded production and enabled British manufacturers to compete with cheap Oriental imports; at first, only one colour could be printed on at a time, but multi-colour transfer-printing was developed in the 1840s. Transfer-prints could be applied either under or over the glaze.

It may sometimes be difficult to distinguish between transfer-printing and painting. But there are clues, usually best picked up by close examination with an eyeglass. Shaded areas, for example, will be crosshatched on a transfer-print taken from a copperplate engraving. In a painting they will be colour- washed (although Chinese porcelain with a design copied from a European engraving may show crosshatching done by hand).

On a printed image the vertical lines, particularly, are consistently straight and even because its perspective is related to the flat surface on which it was drawn. It gives a different impression from a painting, where the artist turned the vessel as he worked. In some cases it is possible to recognise the ‘hand’ of an individual artist. Very few pieces are signed, but some factories added a code to the factory mark to indicate who painted the decoration. The signature of a renownedartist such as Francois Boucher or Angelica Kauffman invariably indicates a transfer print as these artists never directly paintedcommercial porcelain.

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Pottery and Porcelain Decoration: Glaze, Gilding, Painting or Printing?