Enamel is a type of glass product reduced to powder, and then melted by fervent heat into a liquid condition, which, when it has hardened, returns to its vitreous state. Enamel has been used from very early times. The first allusion to it is by Philostratus, in the year 200 A. D., where he described the process as applied to the armour of his day. "The barbarians of the regions of the ocean," he writes, "are skilled in fusing colours on heated brass, which become as hard as stone, and render the ornament thus produced durable." Enamels have special characteristics in different periods: in the late tenth century, of Byzantium and Germany; in the eleventh century, of Italy; while most of the later work owes its leading characteristics to the French, although it continued to be produced in the other countries. It helps one to understand the differences and similarities in enamelled work, to observe the three general forms in which it is employed; these are, the cloisonne, the champleve, and the painted enamel. There are many subdivisions of these classifications, but for our purpose these three will suffice. In cloisonne, the only manner known to the Greek, Anglo-Saxon, and Celtic craftsmen, the pattern is made upon a gold ground, by little upright wire lines, like filigree, the enamel is fused into all the little compartments thus formed, each bit being one clear colour, on the principle of a mosaic. The colours were always rather clear and crude, but are the more sincere and decorative on this account, the worker recognizing frankly the limitation of the material; and the gold outline harmonizes the whole, as it does in any form of art work. A cloisonne enamel is practically a mosaic, in which the separations consist of narrow bands of metal instead of plaster. The enamel was applied in its powdered state on the gold, and then fused all together in the furnace. [Illustration: GERMAN ENAMEL, 13TH CENTURY] Champleve enamel has somewhat the same effect as the cloisonne, but the end is attained by different means. The outline is left in metal, and the whole background is cut away and sunk, thus making the hollow chambers for the vitreous paste, in one piece, instead of by means of wires. Often it is not easy to determine which method has been employed to produce a given work.