Polished metal and obsidian mirrors have existed from ancient times, and because of this, historians have usually passed over the introduction of the glass mirror as if it was just another variation on an old theme. But the development of glass mirrors marks a crucial shift, for they allowed people to see themselves properly for the first time, with all their unique expressions and characteristics. Polished metal mirrors of copper or bronze were very inefficient by comparison, reflecting only about 20 percent of the light; and even silver mirrors had to be exceptionally smooth to give any meaningful reflection. These were also prohibitively expensive: most medieval people would only have glimpsed their faces darkly, reflected in a pool of water… More at Lapham’s Quarterly.
One to read later for sure.