Ruby glass
Ruby glass is deep red glass. It is one of the most prized colours in stained glass and is also one of the most difficult to make. Medieval red was often made with copper, which gave a strong red that tended to be very dense, so that it is usually flashed onto clear glass, since a thick red would look almost black.
From the seventeenth century, gold chloride was used to make a clearer and more brilliant red, known as gold ruby, which was a luxury in its day. Selenium and cadmium compounds are used today for red and orange glass, and some of the strongest reds are among the most expensive sheet glasses.
Because a thin flash of red can be removed by etching or abrasion, ruby flashed glass has long been used for heraldry, lettering and patterns in doors and windows.