If you ever wondered why blueprints were blue and not black or
red or white or brown or any other colour than, well, blue —
it’s not because architects really like the color but because
the technique in making blueprints caused the paper to turn
blue. Mental Floss delved into the history of blueprints and
discovered that the blueprint process was developed in the 1800s
when scientists found an easy way to reproduce documents by
using ammonium iron citrate and potassium ferrocyanide as some
sort of old school photocopy. How does that really work? Well,
as Mental Floss explains it: Someone creates a drawing on
translucent tracing paper or cloth. The drawing is placed over a
piece of blueprinting paper, which has been coated with a mix of
ammonium iron citrate and potassium ferrocyanide from an aqueous
solution and dried. When the two papers are exposed to a bright
light, the two chemicals react to form an insoluble blue
compound called blue ferric ferrocyanide (also known as Prussian
Blue), except where the blu