One day in around 1704, a Herr Diesbach was settling down to make carmine lake according to a tried and tested recipe – mixing ground up cochineal, alum and ferrous sulphate, then precipitating it all with an alkali – when he realised he had run out of alkali. He borrowed some from his boss, but did not realise it had been distilled with animal oil. Suddenly, to his amazement, he found blue instead of red in his flask. The clue is in the ‘animal’ element: the mixture had contained blood, which contains iron. Diesbach had unwittingly created iron ferrocyanide, which was dubbed ‘Prussian blue’ and was instantly popular, particularly as a house paint.
From ‘Travels through the Paintbox’ by Victoria Finlay.