Ruth McCallum-Howell

A so-called Elixir of Life, a potion made from gold, was discussed, if not actually manufactured, in ancient times. Colloidal gold has been used since Ancient Roman times to colour glass an intense shades of yellow, red, or mauve, depending on the concentration of gold.

In the 16th century, the alchemist Paracelsus claimed to have created a potion called Aurum Potabile (Latin: potable gold). In the 17th century the glass-colouring process was refined by Andreus Cassius and Johann Kunchel, and was to become known as Precipitus of Cassius, or more commonly Purple of Cassius

First, prepare something called aqua regia, a combination of aqua fortis and spirit of salt. The proportions can vary: Cassius himself says 2:1; 1 Use the aqua regia to dissolve some gold: a gold coin will be easiest to obtain. The dissolution will happen more quickly if the gold is shaved or cut into small pieces, and heat will speed the reaction. The solution will be yellow-coloured. In a separate vessel, dissolve some tin filings in just enough aqua regia to make a solution. You need a large container for this, as the solution will effervesce and you might otherwise lose some of the liquid. Fill a third container with clean water. Stir in first some drops of the gold solution and then about half as many drops of the tin solution. A red substance will precipitate out. Repeat these last two steps until all of both solutions are used up. Periodically collect the precipitate and wash it.2 You may use separate containers for each collection, but not everyone suggests this. The colouring material made by precipitating gold in a tin chloride solution is often called purple of Cassius, named after Andreas Cassius, who described it in his treatise De auro (1685).3

It was known before then, however: and the process to make it is described in a fourteenth-century Bolognese manuscript of painters' recipes.4 This was not the only technique to obtain red colours from gold; eighteenth- century versions of Antonio Neri's Arte vetraria describes heating a gold powder until it turned purple and using this to create red-coloured glass.5 Purple of Cassius, however, had a noted advantage over other techniques; its colour was stable in the presence of high temperat-ures. This made purple of Cassius an excellent colouring material for the vitreous colours: ceramics, glass, and enamels. And, as the basis for the Chinese famille rose porcelain colours that were fashionable in the seventeenth and eighteenth centuries, purple of Cassius is the unusual example of a colouring material adopted from the West into Asian manufactures.6 Ruby-coloured glass based on this gold-tin solution is closely associated with Johannes Kunckel, glassmaker and alchemist to Elector Friedrich Wilhelm of Brandenburg. Kunckel had described experi-ments to calcine gold with aqua regia to make a transparent red colour.7 The affiliation of the best examples of this product with German and Bohemian glassmakers was exploited by Mayer Oppenheim when he applied for patents in Britain to make ruby and garnet coloured glass.