Answer: Prussian Blue

Prussian Blue is, as far as pigments go, quite famous. The intense blue was the first modern synthetic pigment, has been used in art (it’s the source of the rich blue you see in The Great Wave off Kanagawa by Hokusai), for military uniforms (that’s where it gets the name Prussian Blue, from the uniforms of the Prussian Army), and in machine shops to assist with carefully mating two surfaces (in the form of Engineer’s blue, a mixture of the pigment and oil).

In addition to serving as the world’s first source of light-fast rich blue dye and forever changing the art world and textile production, Prussian blue also happens to be a pretty serious medicine. The vibrant pigment is on the World Health Organization’s List of Essential Medicines because of how effective it is as an antidote for heavy metal poisoning.

Pigment therapy exploits the compound’s ion-exchange properties and high affinity for certain “soft” metal cations. In particular, it is very well suited for treating people who have ingested thallium or radioactive isotopes of caesium. Eating several grams of medical-grade Prussian blue pigment reduces internal residency time (and exposure) by about two-thirds and significantly decreases the risk of immediate or long-term health complications.