Rosemary Barton died from cyanide poisoning, nearly a year ago... Agatha Christie, Sparkling Cyanide, 1945.
Contrary to its name, cyanotype has nothing to do with actual cyanide, and it was not the suspect in the case of Rosmary Barton's murder (solved brilliantly by Colonel Ryce).
Cyanotype was invented in 1842, by sir John Herschel, English astronomer, chemist, and mathematician, who probably followed the advice of his friend, dr Alfred Smee, senior surgeon to the Royal General Dispensary - credited today as the conceptual father of modern computers.
Cyanotype is a simple, basic process for positive printing. To make a photosensitizing coating, ammonium ferratic citrate (in Sir John's times sold by pharmacists as "an iron tonic") is mixed with potassium ferricyanide. The exposure to sunlight triggers the reaction between ferric salt and ferricyanide, resulting in unsoluble blue dye, known as "Prussian blue" or (not always correctly) "cyan blue" - hence the name.
A positive image can be produced by contact printing, i.e. by attaching either a photo-negative or simply any kind of diaphanous, openwork, or even solid object directly to the sensitized surface. After the exposure to sunlight, a simple wash in water removes the highly soluble, unexposed sensitizer, producing the characteristic, monochrome image.
Sir John invented cyanotype mostly for the practical purpose of copying his notes and blueprints, but soon a family friend, Anna Atkins, one of the first female photographers, realized its full potential, and started what she called "autobotanography" - series of haunting, cyanotype impressions of algae, flowering plants and ferns.
Recently, cyanotypes enjoy a new wave of appreciation. Last year, the first ever "Cyanotype Day" was announced on March 16th - Anna Atkins’ birthday anniversary, and several cyanotype events have been presented all over the world; starting from "Roses, Cyanotypes and Grannies With Guns” exhibition at Photo London 2015, following with the creation of a world-record setting cyanotype in Goa, India, and concluding in "Cyanotypes: Photography's Blue Period", an impressive array of prints, from mid-19th century, until now, presented currently at the Worcester Art Museum, Massachusetts.
From the conservator's point of view, under the right conditions, cyanotypes are usually stable. Most problems involve mechanical damage of paper support by rolling and folding, especially when paper is brittle. The color can fade when object is placed in alkaline environment or when exposed to light - it is necessary to consult preservation specialist before any kind of exhibition.
For more information on cyanotype please check: Dusan C. Stulik & Art Kaplan, Cyanotype, The Getty Conservation Institute 2013.