Around 1704, a German dye maker named Diesbach was mixing up a batch of red dye from potash and iron sulfate. But unknown to him, his supplier, a man named Dippel, had sold him potash that had been contaminated with animal oil. Instead of the red he expected, Diesbach wound up with a pale pink. When he tried to concentrate the dye to produce a deeper red, to his surprise he got a deep blue.
Chemistry in that day was too primitive to explain what happened. Today we know that the potash (an alkali) reacted with the animal oil to produce potassium ferrocyanide, which in turn reacted with the iron sulfate to produce Prussian blue–the first of the modern artificial pigments.
Even without knowing the chemistry, Prussian blue was easy and cheap to make, and took the art world by storm. Never before had artists had a stable, satisfactory blue pigment. The color quickly became popular throughout Europe.
The compound proved to be very versatile. It found use in ink, paint, typewriter ribbons, carbon paper, blueprints and an early kind of photograph called a cyanotype. Prussian blue was even used to treat heavy-metal poisoning. Used as an antidote, the molecule grabbed metal atoms like a claw, allowing them to be flushed from the body.
Eventually chemists found that other metal atoms could be substituted for one or both of the iron atoms in the original molecule, thus creating an entire class of compounds known collectively as Prussian blues. Stiegman created his superparamagnetic particles from a Prussian blue containing cobalt and iron. —D.W.