Silvering is a separate and older process than silver mirrors, as in the metal silver applied to a glass surface. The Wikipedia page on silvering points to the

Silvering is a separate and older process than silver mirrors, as in the metal silver applied to a glass surface. The Wikipedia page on silvering points to the 1400s in Europe, and the 10th century for the eastern Mediterranean : https://en.wikipedia.org/wiki/Silvering https://en.wikipedia.org/wiki/Silvering