In its strictest sense, the term "enamel" applies to the use of a glossy protective paste or covering applied to the surface of glass or metal.  In firing, the paste is ultimately fused to that prepared surface.  Firing temperatures are fairly low, between 700-800 degrees centigrade.  Though it is seemingly fragile, the enamel is durable and has a brilliance of color that is rarely achieved through other media.  The earliest known enamels are Mycenaean jewels (ca. 13th century b.c.) decorated with dazzling cobalt blues and brilliant greens.  The earliest continental European enamels appear sometime around the 2nd century a.d.  The technology is thought to have been carried there by the Sarmatians.