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Why are blueprints blue?

Engineers and architects use blueprints to illustrate project plans. But why are they always blue? See more architecture pictures.
Milos Jokic/Stockbyte/Getty Images

If you have a blueprint for success, you're not alone. At least when it comes to the "blueprint" part. The word "blueprint" has become part of a global lexicon used to symbolize a plan, strategy or framework. But what is a blueprint, really?

At its most basic, a blueprint is a reproduction of an image that already exists. Engineers or architects use these large-format prints to illustrate project plans using white lines and text on a backdrop of blue. And it's not just because they happen to really, really like blue.

A blueprint's signature hue is tied to a chemical process. In 1842, an English photographer, chemist and astronomer named John Herschel discovered that combining ferric ammonium citrate and potassium ferricyanide created a chemical reaction and a compound called blue ferric ferrocyanide (or Prussian blue). This photosensitive solution could be used to reproduce documents in a process similar to developing a photograph from a negative.

The process, called cyanotype, was adopted by early photographers and led to the first book illustrated by photography and then became the darling of architects and engineers. Here's how it works: First, you create a drawing and transfer it to vellum paper or tracing cloth, both of which are so thin they are practically see-through. Then, you saturate a piece of regular paper with an ammonium potassium mixture and let it dry. Place the drawing atop the paper coated in the chemical solution, expose the papers to a bright light and wait for a chemical reaction to take place. Within a matter of minutes, the chemical-coated paper will be transformed into blue ferric ferrocyanide -- with one important exception.

Wherever the light can't shine through the top paper (because of the lines from the original drawing), the blueprinting paper remains white. After rinsing the paper in cold water to halt the chemical reaction and allowing the paper to dry in the dark, you're left with a nearly identical duplicate of the original drawing. The reproduction is a negative image that appears white against a background turned a specific shade of indigo by the reaction of the chemical compound. Although the process requires several steps, it became a big hit with the pre-computer crowd of the 19th and 20th centuries; it was still faster and cheaper than creating large-scale drawings by hand [sources: Pendle, Soniak].