Enamelling is a process of applying a thin coat of finely ground glass on to metal. The enamel is made from a combination of silica and soda ash, coloured with a small amount of metal oxides. Raw materials are smelted together at a temperature of between 1,150 and 1,450 °C into a liquid glass that is then poured out of the crucible into a water bath. The resulting frit, a combination of silica and oxides, is then ground mechanically or with a mortar and pestle into a fine powder. Various colours can be produced depending on the metal oxides introduced during the smelting process. The ground enamel powder is then applied to the metal and heated, using a kiln or torch, to a temperature of between 700 and 820 °C.
The glass melts and fuses to the metal base creating a thin layer of coloured glass. Enamelling has evolved over the centuries and metalsmiths have applied enamel to their metal objects using a variety of techniques including champlevé, cloisonné, Limoges, plique-a-jour, basse-taille and grisaille. Irish metalsmiths of the eighth and ninth century frequently used the technique of champlevé to adorn their metalwork. In this technique, the enamel colours are applied to depressed areas of the copper alloy. These depressions can be created during the casting process, or through etching and engraving. The cast metal parts are then filled with the ground glass powder and placed in a kiln to fuse the glass on to the metal. The use of enamels was also a feature of 11th and 12th century monastic metalwork including The Cross of Cong and St Manchan’s Shrine.
St Manchan’s shrine features fifty-three unique enamelled panels. The geometric patterns are highlighted in red and fused onto a yellow enamel background. Griffin Murray states that an alternative method was used to create the champlevé technique on the shrines panels. The panels were filled and fused with yellow enamel and allowed to cool. The panels were then gently chased or scribed to remove yellow enamel to create the geometric pattern. The red enamel was applied into the void and fired. Each enamel panel was then fixed onto a copper alloy backing sheet and riveted into the metalwork frame. During the British Museum restoration in 1970 it was found that the yellow colour was created using cubic lead tin oxide (British Museum 1970) and the red enamel was created with cuprous oxide (Bourke 1970).