In 1842, the astronomer and chemist John Herschel worked out a way to make a photographic print that required no camera.
You coat a sheet of paper with ammonium ferric citrate and let it dry in the dark. You lay whatever you want to copy on top of it — a drawing on translucent paper, a leaf, a lace curtain, a photographic negative — and you put the whole thing in sunlight. Ultraviolet light reduces the iron from its ferric to its ferrous state in the exposed areas. Then you wash the sheet in a solution of potassium ferricyanide. Where the iron was reduced, an insoluble pigment forms: Prussian blue. Where the light did not reach, the paper stays white.
The result is a blue ground with white lines. A negative, in blue.
Herschel did not patent the process. A surprising number of online summaries claim he did; he did not. He published it, and other people used it. The most famous early use was by Anna Atkins, who in 1843 produced British Algae, a book of cyanotype images of seaweed that is generally described as the first photographically illustrated book.
Two properties kept cyanotype in drafting rooms for the better part of a century. The first is that the chemistry is cheap and the process is slow only in the way that sunlight is slow: you expose, you wash, you are finished. The second, and the more important one, is that the result is a contact print at full scale. You could not enlarge it and you did not need to, because the drawing on the translucent paper and the print on the blue sheet were the same size — which is exactly what a working drawing has to be. Engineers did not adopt cyanotype because blue was attractive. They adopted it because it produced, in minutes and with no camera, a copy that could not lie about scale, could be folded into a pocket, marked up on site, and still be read years later in a drawer.
If you have ever referred to a detailed plan as a blueprint, you have been using the name of this process, and almost certainly without ever having handled the object. That is the interesting part. The word outlived the thing by decades.
The displacement happened in stages, and it is worth getting the order right, because the tidy version — computers replaced blueprints — is wrong. The first replacement, in the early 1940s, had nothing to do with computers. It was diazo printing, the whiteprint process. Diazo reversed the polarity: instead of white lines on blue, you got dark lines on white, which is far easier to read and far easier to annotate. It also eliminated the need for a darkroom, because diazo paper is developed by exposure to ammonia vapour rather than by washing.
That is where the smell comes in. Diazo development required pressurised ammonia gas, and anyone who worked in a drafting room in the middle of the twentieth century remembers it, because you could not not remember it. It is sharp, it catches in the back of the throat, and it did not clear from the room. It also raised genuine health concerns, and unlike cyanotype it needed no fixing step, which meant the machine sat there in the open, venting all day.
Diazo itself was displaced, and again not by a single event. Large-format xerographic copiers arrived around 1975, and through the 1980s diazo printing largely retreated. Then computer-aided design finished the job: once a drawing lived as a file, the print became an output rather than an original, and the drafting room — with its flat files, its ammonia, its particular blue — closed.
Which leaves the strange afterlife of the word. Blueprint now means any plan, especially a political or institutional one, and it will go on meaning that long after the last person who could identify a cyanotype by sight is gone. Language is very good at this. It keeps the word and discards the referent, and the referent takes the sensory memory with it. You cannot scan a smell. The blue is reproducible on any screen, more accurately than the original ever was. The ammonia is gone, and with it the one piece of the drafting room that no archive can hold.
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