Prussian blue was discovered in around 1704 in Berlin, by the chemist Johann Jacob Diesbach. Prior to this, blues obtained from woad and indigo were susceptible to fading in bright light, and were unstable as pigments, so could not be used reliably for painting or printing. Prussian blue is derived from an iron salt, and gives an intense, slightly red-caste blue.
In 1842, Sir John Herschel invented the cyanotype process, whereby Prussian blue pigment is fixed photographically by exposure to sun light through a negative.This technique was originally used by engineers as a way of accurately reproducing finely detailed technical drawings (‘blue-prints’) but was also adopted as a quicker printing process for photographs, as it did not require a darkroom. Many early postcards and family portraits were printed as cyanotypes.