Prussian blue (also called Berlin blue or Paris blue or Paris blue in paint) is a dark blue pigment produced by the oxidation of iron(II) ferrocyanide salts. It has the chemical formula FeIII4[FeII(CN)6]3. Turnbull’s blue is chemically identical but is made from different reagents and its slightly different color comes from different impurities and particle sizes. Prussian blue was the first modern synthetic pigment. It is produced as a very fine colloidal dispersion as the compound is not soluble in water. It contains varying amounts of other ions and its appearance strongly depends on the size of the colloidal particles. The pigment is used in paintings and is the traditional “blue” in designs and became prominent in 19th century Japanese Aizuri-e (藍摺り絵) prints. In medicine, Prussian blue is used orally as an antidote for certain types of heavy metal poisoning, such as thallium (I) and the radioactive isotopes of cesium. The therapy exploits the ion exchange properties of the compound and its high affinity for some “soft” metal cations. It is on the World Health Organization’s List of Essential Medicines, the most important medicines needed in a basic health system. Prussian blue gave its name to the hydrogen cyanide (hydrocyanic) obtained from it. In German, hydrogen cyanide is called hydrogen cyanide. French chemist Joseph Louis Gay-Lussac gave cyanide its name, derived from the ancient Greek word κύανος (kyanos, “blue”/“cyan”), after its Prussian blue color.