BY enamel is understood a coloured vitreous substance, which by a properly regulated firing may be applied for decoration on an excipient of metal, clay, or any material capable of enduring a high temperature without melting. Enamel is generally opaque; when translucid it becomes a true glass, comprised of a colourless matter or "flux," serving as a vehicle for the metallic oxide that forms the colour Its opacity of enamel is produced by the presence of oxide of tin, which alone yields a white opaque substance.

In the process of enamelling on faience the white enamel is first spread upon the clay, and is combined by fusion with the coloured glasses with which it is charged, for the purpose of forming varied enamels.

In the enamelling upon metals several processes have been employed, the first and most ancient of which is the so-called "cloisonnage." It consists in tracing on copper, prepared in plaques, some ornamental design in cells composed of very thin strips of gold, fixed in an upright position, and bent to form the outlines of the pattern. They are attached to the metal plate by means of gum, and after filling in the cavities with the coloured powders, intended to form the design, the piece is placed in a furnace, kept open so that it may be watched while firing, and withdrawn the moment it is perfectly fused. But whatever care may have been taken to fill up the metal cells with the enamel, the fusion always causes it to sink below their level in the middle of each cavity. Hence the necessity of polishing and pumicing the piece in order to obtain a perfectly even surface, and restore to the enamel its proper lustre that has become somewhat deadened while cooling down.

A process much akin to this is that known as "champleve" enamel. Here the partitions are reserved in the thick metal, all the parts being hollowed out that are intended to receive and serve as cells for the colouring matter. This method is also known as enamel "a tailles d'epargne." When finished the effect closely resembles that of the cloisonne enamel.

Enamels "de basse taille," or translucent upon relief, are of a more complicated nature. The metal chased and hollowed out like a bas-relief, and the colours laid on, being of various degrees of thickness according to the depth of the cavities filled in by them, impart a very decided relief to those places and become shaded. Here coloured glasses and not enamels are employed.

Painted enamel need not require description. Every one knows that it is executed on a black, blue, or red coating applied to the copper and overlaid with white layers, superposed to express the lights and hatchings on the grey to restore the dark shades.

Having said so much in explanation of the various descriptions of enamel, we will not stop to discuss the origin of the word, on which the learned have hitherto failed to arrive at a unanimous conclusion. Nor will we re-open the question as to whether the Egyptians were acquainted with the cloisonne method of enamelling. The monuments that have been brought to light are sufficiently numerous to show it applied well with a rare talent, as may be considered to have settled the point. From Egypt the Art passed doubtless to Greece and Italy. It is at least certain that the employment of enamel work in jewellery is very general, as shown by the exquisite specimens preserved in our galleries. The Art, however, may have found its way to Italy from two quarters : Egypt, as above stated, and the extreme East, which may have transmitted it through Lydia to the artists of Etruria.