Victorian Cloisonné Enameled Pocket Watch.
Enamel is a type of allochromatic glass that consists usually of quartz sand, iron oxide, potassium oxide (potash), and borax (flux). These components form a transparent and colorless fondant after firing at temperatures between 700 and 900 degrees Celsius. Their plethora of colors are established by the addition of different metal oxides and/or chlorides. After thoroughly crushing and washing these materials a hydrated mass is formed (a fondant) which is then applied on a suitable, and completely clean metal (typically gold, silver or copper alloys).
Enamel can be applied to metal surfaces with engraved lines, engraved cells, and raised borders that are fused at high-temperature ovens creating a vitreous (glassy) surface. The fine art of enameling has many schools including (but not limited to) champlevé taille d’épargne, guilloché, cloisonné, plique-à-jour and miniature painting (french enamel).