How long ago was stained glass invented?

Answer 1

Probably at the same time that glass itself was invented. The sand from which glass is made naturally contains minerals that will cause the glass to be colored. The earliest glass made was was not clear but strongly colored (often dark, sometimes even nearly opaque).

The first glass makers therefor began working simultaneously in two opposite directions to make various types of glass that they wanted:

1. Trying to develop ways to remove these minerals from the raw sand or to bleach out the resulting color in the finished glass, in an attempt to produce clearer more transparent glass. Due to the difficulties of completely removing all of these minerals (especially iron based minerals, which produce a brown color in the finished glass) the development of effective bleaching agents to remove these colors from the finished glass. (note: the purple bottles and purple insulators that are now popular on the antiques market were originally a clear transparent glass made using a permanganate bleaching agent to remove the "iron brown" color from the finished glass, after long exposure to ultraviolet light this bleaching agent broke down to manganese compounds that themselves colored the glass purple. Glass makers long ago replaced this bleaching agent with others that do not break down on exposure to ultraviolet light.)
2. Trying to identify which specific minerals produced which specific colors in the finished glass, in an attempt to produce stained glass of desirable colors.

Answer 2

They were invented in Medieval times however, they were also used in ancient Egypt and Roman Italy.