Crystal engraving is done with fine sandblasting or diamond points, as on this Serpent vase, created by René Lalique in 1924. Coloured crystal is obtained by adding metallic oxides to the glass mixture. Until the 19th century, Bohemian crystal makers were the experts in coloured crystal, although as early as 1668 Frenchman Bernard Perrot claimed to have found the formula for red glass, based on gold and arsenic. Sixteen years later, the alchemist Johann Kunckel rediscovered Cassius’ purple, made with gold, hydrochloric and nitric acids, and tin. The ruby red colour only appears during the annealing process.