Blueprinting was developed in the mid-1800’s, when scientists found that ammonium iron citrate and potassium ferrocyanine created a solution that was sensitive to light (which can be used to reproduce documents). Someone would create a design on a piece of translucent paper or cloth, which was placed over a piece of blueprint paper that had been coated with this mixture. And when both were exposed to bright light, the chemicals reacted to form a blue compound called blue ferric ferrocyanide (also called “Prussian Blue”).