Written by Geraldine Granahan, Preservation Assistant for the McKim, Mead & White Architectural Record Collection.
Recently the staff of the library and conservation department spent a fun afternoon in our conservation laboratory attending a workshop on the process of making cyanotypes, or as they are more commonly known — blueprints (so called because they contain the pigment Prussian Blue).
The pigment itself has an interesting history, having been discovered by accident in or around 1704, when a dye maker in Berlin was trying to make the color red. He was mixing the required ingredients (crushed insects, iron sulphate, and potash) when he realized that he had run out of potash. So, he borrowed some, not realizing that it had been distilled with animal oil. To his surprise, he found that instead of turning red, as expected, the ingredients transformed into blue (although the chemistry was not understood at the time, we now know that the potash reacted with the animal oil, which contained blood, to create iron ferrocyanide). Blue being, at the time, a particularly difficult and expensive color to create, this accidental pigment — the first modern synthetic color — was soon in widespread use for paints, inks and dyes.
Prussian blue later played a part in the world of architecture and design, as well as photography. The world of architecture changed forever when in 1842, Sir John Herschel, who was a chemist, astronomer, and photographer, developed the process for blueprints. In a series of experiments, Herschel discovered that if he held a pattern drawn on tracing paper over photo-sensitive paper and shone a lamp over both, the parts that were not protected by dark lines would change their chemical formula in the light. The paper would then be dunked in a vat of potassium ferrocyandie, and the resulting image would be white lines on blue paper; and that is how a blue print got its name.
At first, the process was used mainly for botanical illustrations — most notably, by Anna Atkins, a botanist and arguably the first woman photographer. Her book British Algae: Cyanotype Impressions, privately published in 1843, was the first to be illustrated using a photographic process. Being relatively cheap and simple, by the late 19th century, the cyanotype process also gained popularity among the growing ranks of amateur photographers. Many beautiful examples, such as this one by Charles Gilbert Hine, can be found in the library’s Graphic Collections.
Cyanotypes gained the most widespread commercial use, however, as a cheap, easy and practical method of duplicating architectural drawings. Although architects began using other processes to reproduce drawings in the 1940′s, the term “blueprint” is still in use today.