Vitreous (Porcelain) Enamel: Fused Glass Coatings on Metal

The protective function of enamel on everyday utensils is an innovation of the 19th century. Older enamel works have a decorative character throughout. The first known enamel work is 3500 years old and was found as grave goods in Mycenaean tombs on Cyprus. The ancient Egyptians also knew enamel work, both on gold and on iron. The Celts reached a further high point in enamel technology in 500 BC with blood enamel.

In the Middle Ages, enamel was used in the art of goldsmithing. The first flowering of enamel art occurred around the year 1000 (cellular enamel, cloisonné), possibly promoted by the Empress Theophanu, who came from Byzantium (compare the cover of the Codex aureus Epternacensis).

Delicate flexible gold strips were soldered onto a metal plate in such a way that they formed the outline of the desired figure. The resulting cells (cloisons) were filled with different coloured enamel powders and heated until the mass melted. This was repeated until the required height of enamel was reached. A central workshop for this technique was probably located in Trier. An important example of this art is the Pala d'Oro in Venice. Even in the early period, translucent glass flows were often used, which allowed the golden metal background to shine through.

The Meuse region emerged in the 11th century with works using the pit enamel process (émail champlevé), whose heyday was in the 12th century in the Cologne area. Here, opaque enamel colours were preferred, the surfaces of which were no longer separated by bars, but filled depressions that were dug out of the metal with a burin. Later, blue work from Limoges (Limosin enamel) gained in importance until the 13th century and was widespread throughout much of Europe. In the 14th century, translucent enamel, now applied to silver grounds gridded in relief, returned. In small plates the design, usually figural, was engraved or cut so as to form a very flat but sharply defined relief, and then the whole surface was covered with transparent enamel of different colours. Where the layer became thinner, the silver shone through and gave the bright spots, while shadows prevailed on the thicker layers. This resulted in an extremely delicate effect of the enamel work.

A typical type of enamel work for the early modern period is painter's enamel, which originated in Limoges in the 16th century. Here, the coloured areas are no longer separated by bars or metal parts, but colours applied with a brush and merging into one another enable miniature-like, fine, pictorial representations. In order to avoid distortion of the support material (and thus cracks in the enamel), the reverse side of the main representation was also covered with enamel (contreémail [contre-émail]), which could also be decorated. Important artists are Pierre Reymond, Jean Courtais and Léonard Limousin. In the 17th century this technique was replaced by enamel painting. In this technique, only the metal oxides were painted onto the white enamel ground and fired on. The technique was used especially in France and Switzerland, but also in Germany. Watch covers and tobacco boxes were typical applications of this decorative art. After the decline of enamel art in the middle of the 18th century, it was not until a century later that a revival in the field of ecclesiastical goldsmithing brought a revival of the old enamel art. This revived the medieval techniques, and in the later 19th century one also remembered the decorative possibilities of the Renaissance and imitated (not infrequently with the intention of forgery) the models of the 16th century. Important sites were Aachen, Cologne, Vienna, Mechelen, Brussels, Lyon and Paris. In the 20th century, in the spirit of contemporary trends (Expressionism, New Objectivity), enamel art was also revived in the Werkkunstschulen.

Stimulated by demand, the Japanese, Chinese and Indians also began to take up and perfect the art of enameling again.

From a chemical point of view, enamel is a melting mixture. The glass-forming oxides are silicon dioxide (SiO2), boron trioxide (B2O3), sodium oxide (Na2O), potassium oxide (K2O) and aluminium oxide (Al2O3). The oxides of titanium, zirconium and molybdenum serve as opacifiers. To ensure that enamel adheres firmly to metallic substrates, it also contains cobalt or nickel oxide. The ceramic pigments most commonly used are iron oxide pigments, chromium oxides and spinels.