Glass has been produced for thousands of years, it wasn't discovered in the fifteenth century, either AD or BC.

Just as few random examples of glass predating the fifteenth century AD, look at the south clerestory windows in Augsburg Cathedral, which date to the eleventh or twelfth century. Or the fourth century Roman Lycurgus cup, which looks different when lit from in front than when lit from behind due to gold particles suspended in the glass. (A very neat trick, that.) Or the glass ingots found in the Uluburun shipwreck off Turkey, which has been dated to the fourteenth century CD BC. (EDIT: No idea where CD came from. As typos go, that one's pretty odd.)

Could you perhaps elaborate on what prompted the question? EDIT: Are you maybe thinking of the introduction of tin-mercury amalgam mirrors to Europe?