What’s the Reason Behind Blueprints Being Blue?

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If you’ve ever had a blueprint for success, you’re in good company. The term "blueprint" has transcended its literal meaning to represent a plan, strategy, or framework worldwide. But what exactly defines a blueprint?

Fundamentally, a blueprint is a copy of an existing image. Engineers and architects utilize these oversized prints to showcase project designs, featuring white lines and text against a blue background. This choice isn’t merely due to a preference for the color blue.

The iconic color of blueprints is rooted in a chemical reaction. In 1842, John Herschel, an English photographer, chemist, and astronomer, found that mixing ferric ammonium citrate with potassium ferricyanide produced a compound known as blue ferric ferrocyanide (or Prussian blue). This light-sensitive solution could replicate documents in a manner akin to developing a photo from a negative.

Known as cyanotype, this method was embraced by early photographers, leading to the first photographically illustrated book. It later became a favorite among architects and engineers. Here’s the process: First, a drawing is made and transferred to translucent vellum paper or tracing cloth. Next, regular paper is coated with an ammonium potassium mixture and left to dry. The drawing is placed over the chemically treated paper, exposed to bright light, and a chemical reaction occurs. Within minutes, the paper turns into blue ferric ferrocyanide, with one key distinction.

Areas blocked by the original drawing’s lines remain white. After rinsing the paper in cold water to stop the reaction and drying it in the dark, a near-perfect replica of the original drawing is produced. The result is a negative image with white lines on an indigo background, created by the chemical reaction. Despite its multiple steps, this method became popular in the 19th and 20th centuries as a faster and more cost-effective alternative to hand-drawn large-scale designs [sources: Pendle, Soniak].

Who Put the 'Blue' in Blueprint?

The tale of Prussian blue's creation reads like a grim fairy tale. In 1704, an alchemist, Johann Konrad Dippel, and a dye-maker named Diesbach shared a lab in Berlin, Germany. Dippel aimed to craft a universal cure by boiling animal parts into a pungent potion, while Diesbach produced vibrant dyes. One day, while Diesbach was creating a deep red dye from insects, alum, iron, and sulfate, he added potash borrowed from Dippel’s concoction. The result was a stunning, deep blue dye.

After retracing the steps, Dippel discovered that the potash contained ox blood, which, when mixed with iron sulfate, triggered a chemical reaction, producing a brilliant blue. Unlike other blue dyes that faded easily, this one remained strikingly vivid.

Initially named Berlin blue after the city, the dye was later dubbed Prussian blue when it was used to color the uniforms of the Prussian army. The color became synonymous with both the army’s fierce reputation and their unexpected victories, such as at the Battle of Waterloo.

By the late 19th century, Prussian blue gained popularity among Impressionist artists and Japanese printmakers. In the 20th century, it became the color of newspaper ink, typewriter ribbons, and even eye shadow. Scientists also found that Prussian blue could act as an antidote to heavy metal poisoning, attracting and removing toxins from the bloodstream [source: Pendle].

Prussian blue eventually became valued for its practicality as much as its novelty. However, John Herschel did not live to see its widespread use. It was only five years after his death that blueprints were recognized as a cost-effective and straightforward method for reproducing architectural drawings [source: Granaham].