deep-red glass deriving its colour from gold chloride. Originally known in the ancient world, its rediscovery was long sought by European alchemists and glassmakers, who believed it had curative properties. A Hamburg physician, Andreas Cassius, in 1676 reported his discovery of the red colouring properties of a solution of gold chloride, subsequently called purple of Cassius. Ruby glass was produced c. 1679 by a Potsdam chemist and glass technologist named Johann Kunckel von Löwenstern (Kunckel von Löwenstjern, Johann), who kept the recipe a secret. The difficulty in producing this colour lay in the fact that the glass at first appears gray and turns red only on reheating. This secret was rediscovered in the glassworks at Ehrenfeld at the end of the 19th century. Meanwhile, Bohemian glassmakers produced a ruby shade using copper, and glassware flashed with a thin ruby-glass casing became a characteristic 19th-century Bohemian product.