Five pieces of struck ruby gold glass. Ruby glass is deep red in colour due to additions of gold or copper. The metalis dispersed as miniscule particles, called colloids, throughout the glass. In olden times an opaque red glass was made using copper and the use of copper was continued throughout the Middle Ages. It was not until later (17th century) that ruby glass became consistent in quality. This was due to the work of the German alchemist and glassmaker Johann Kunckel, who perfected the use of gold chloride. In the twentieth century it was discovered that cadmium slenide and zinc will also give a deep ruby colour.