Making Prussian Blue — The First Modern Synthetic Pigment (Berlin, 1704)

Prussian blue (iron(III) hexacyanoferrate(II), Fe₄[Fe(CN)₆]₃) was the first modern synthetic pigment — accidentally discovered in Berlin between 1704 and 1706. The paint maker Johann Jacob Diesbach was attempting to make a red lake pigment from cochineal, using a batch of potash (potassium carbonate) supplied by the alchemist Johann Konrad Dippel. Unknown to Diesbach, Dippel had previously used that potash in the distillation of animal oil — contaminating it with potassium ferrocyanide (prussiate of potash). When Diesbach mixed this contaminated potash with iron sulfate and cochineal, instead of the expected red lake, he produced a deep, vivid blue precipitate.

The discovery broke the centuries-old monopoly of ultramarine (from lapis lazuli) and smalt (from cobalt glass) as the only available blue pigments. Prussian blue was cheap, intensely coloured, and easy to manufacture — within decades it became the dominant blue pigment in European painting, textile printing, and wallpaper manufacture. Its intense tinting strength is extraordinary: a tiny amount of Prussian blue can colour a vast quantity of paint. It is transparent to semi-transparent, making it excellent for glazing techniques, and it produces beautiful dark blues, blue-blacks, and (when mixed with yellow) vivid greens that no previous pigment could match.

SAFETY WARNING: The synthesis involves potassium ferrocyanide, which is itself of low toxicity, but releases hydrogen cyanide (HCN) gas if heated above 300°C or treated with strong acid. The reaction also uses iron sulfate (mildly irritating) and dilute hydrochloric acid. All steps must be performed in a well-ventilated area, and the reagents must NEVER be heated beyond the temperatures specified. Wear gloves, goggles, and a respirator when handling the acid. Dispose of all waste solutions responsibly.