Why are blueprints blue?
The name stems from one of the first practical methods of copying images without simply tracing them by hand. John Herschel invented a way to do it with chemicals in 1842, and the process (called cyanotype) involved turning the paper blue.
You would do your drawing on a very thin piece of cloth or paper and lay it over a piece of regular paper that had been soaked in the key chemicals. Then, after being exposed to bright light, the paper would turn blue—except where the drawing was blocking the light from getting to the paper, leaving those lines white.
The process was extremely popular until Xerographic copies and computers started to take over, but the name stuck. Now, many blueprints are blue in name only.
It’s because of how those documents are made. The blueprinting process was developed in the mid-1800s, when scientists discovered that ammonium iron citrate and potassium ferrocyanide created a photosensitive solution that could be used for reproducing documents.
The process goes like this: 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 blueprinting paper was covered, and the light blocked, by the lines of the original drawing. After the paper is washed and dried to keep those lines from exposing, you’re left with a negative image of white (or whatever color the blueprint paper originally was) against a dark blue background.
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 blueprinting paper was covered, and the light blocked, by the lines of the original drawing.
Ammonia printers, large-format copy 湖北体彩app that were around before toner was a thing. Ammonia prints are characteristically blue.