After setting up – as Origlia says in his Istoria dello Studio di Napoli (1753-54) – “a furnace similar to those used by glaziers” and “a chemist’s laboratory with every type of burner”, the Prince of Sansevero dedicated himself night and day to the most varied experiments. Some of these aimed to produce artificial gems and a special way of colouring glass.

He counterfeited various types of “hard stones, such as the blood green jasper, various kinds of agate, and lapis lazuli […] he had the pleasure of faking even precious stones of all types”. They had the same characteristics as real stones, from which they could “in no wise be distinguished”. Not only this. In the brief guide to the wonders of Palazzo Sansevero known as the Short note (1767), there is a reference to “some jewels, which by nature are pale and have little colour”, which the Prince is said to have treated in order to bring out their colour as much as possible (the amethysts took on “the strongest and most beautiful colour you could ever wish for in an amethyst”). Lastly, in 1767, di Sangro made “another curious discovery”, succeeding in “stripping the true lapis lazuli of its blue colour, turning it white”.

His ability to permeate glass with any colour must have seemed equally incredible. The French scientist de Lalande, who saw the results of these experiments, wrote in his travel diary: “The art of coloured glass seemed a long lost secret. The Prince of Sansevero has worked on it with success. He has pieces of white glass, where you can see the different colours, clear and transparent as if the glass had come out of the furnace with those same colours”. De Lalande deemed the procedure simply “perfect”.