In a small way, glass had been used for mirrors by the Romans; but the background was a dark one, and the image was no more plain than it had been on the polished metal surface. By the sixteenth century, even before the invention of plate glass that followed a hundred years later, the mechanical surface of the glass had been  improved to such an extent that, by coating it with a silver amalgam, an excellent mirror could be created. Techsilvenically this was, according to Schulz, perhaps the highest point in Venetian glass-making. Large mirrors, accordingly, became relatively cheap and the hand-mirror became a common possession. (Mumford 1934:128-9)
MUMFORD, Lewis. 1934. Technique and civilization. New York: Harcourt, Brace and Company.