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Overglaze decoration,overglaze enamelling, oron-glaze decoration, is a method of decoratingpottery, most oftenporcelain, where the coloured decoration is applied on top of the already fired andglazed surface, and then fixed in a second firing at a relatively low temperature, often in amuffle kiln. It is often described as producing "enamelled" decoration. The colours fuse onto the glaze, so the decoration becomes durable. This decorative firing is usually done at a lower temperature which allows for a more varied and vividpalette of colours, using pigments which will not colour correctly at the high temperature necessary to fire the porcelain body. Historically, a relatively narrow range of colours could be achieved withunderglaze decoration, where the coloured pattern is applied before glazing, notably thecobalt blue ofblue and white porcelain.
Many historical styles, for examplemina'i ware,Imari ware, Chinesedoucai, andwucai, combine the two types of decoration.[1] In such cases the first firing for the body, underglaze decoration and glaze is followed by the second firing after the overglaze enamels have been applied.
The technique essentially uses powdered glass mixed with coloured pigments, and is the application ofvitreous enamel to pottery;enamelled glass is very similar but on glass. Both these latter two are essentially painting techniques, and have been since they began. In contrast, on metal painting in enamel arrived very late, long after techniques such ascloisonné, where thin wires are applied to form raised barriers, which contain areas of (subsequently applied) enamel, andchamplevé, where the metal surface is sunk to form areas where the enamel is poured.
InChinese porcelain, enamels were and are sometimes applied to unglazed pieces; this is called "enamel onthe biscuit" and similar terms.
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Enamel was used in jewellery, applied to metal, from very early on - there are examples in theTomb of Tutankhamun of c. 1325 BC. Enamel was also used to decorate glass by the time of the Roman Empire. Applied to pottery, it is first seen in Persianmina'i ware from the late 12th century, using a group of seven main colours. Presumably the potters learnt the technique from glassmakers.[2]
Slightly later it appeared inChinese ceramics inCizhou stoneware from as early as the 13th century, withuse on porcelain following within a century, though it did not become predominant until later, and the full possibilities were not realized until the 17th and 18th centuries in thefamille jaune, noire, rose, verte group of palettes.[3] Some techniques use thin metal leaf, including mina'i ware as well as the more usual pigments, which are typically applied in a liquid or paste form, painted by brush, or usingstencils ortransfer printing. The Japanesekakiemon style, and other Japanese styles, used the technique from at least the second half of the 17th century. The technique was also developing in Europe, firstly in what the French calledpetit feufaience, and in the 18th century in porcelain, and there appears to have been some influence in both directions between Asia and Europe. From about 1770 to the mid 20th century it was the dominant decorative technique in expensive pottery, mostly porcelain, made in Europe, East Asia,[4] and (to a lesser extent) North America.
In 18th-century England, where the technique was developed, the earliest forms oftransfer printing on pottery, for example by Sadler & Green inLiverpool, were overglaze, although by the end of the century it was normal to print as underglaze.
Today overglaze decoration is much less commonly used, other than in traditionalist wares, as the range of colours available in underglaze has greatly expanded. Overglazes called "lusters" are still used for achieving special effects, such asiridescence ofmother-of-pearl overglazes or metallic look of overglazes made with metal (eg. gold) particles.[5][6]
The kiln used for the second firing is usually called a muffle kiln in Europe; like other types ofmuffle furnaces the design isolates the objects from the flames producing the heat (with electricity this is not so important). For historical overglaze enamels the kiln was generally far smaller than that for the main firing, and produced firing temperatures in the approximate range of 750 to 950 °C, depending on the colours used. Typically, wares were fired for between five and twelve hours and then cooled over twelve hours.[7]
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