Out of this world 2: Photo printing process

Cyanotype is a photographic printing process that gives a cyan-blue print. The process was popular in engineering circles well into the 20th century. The simple and low-cost process enabled them to produce large-scale copies of their work, referred to as blueprints. Two chemicals are used in the process: ammonium iron(III) citrate, potassium ferricyanide.

A blueprint is a reproduction of a technical drawing, documenting an architecture or an engineering design, using a contact print process on light-sensitive sheets. Invented in the 19th century, the process allowed rapid and accurate reproduction of documents used in construction and industry. The blue-print process was characterized by light colored lines on a blue background, a negative of the original. The process was unable to reproduce color or shades of grey.

The English scientist and astronomer Sir John Herschel discovered this procedure in 1842. Though the process was developed by Herschel, he considered it as mainly a means of reproducing notes and diagrams, as in blueprints. It was Anna Atkins who brought this to photography. She created a limited series of cyanotype books that documented ferns and other plant life from her extensive seaweed collection. Atkins placed specimens directly onto coated paper, allowing the action of light to create a silhouette effect. By using this photogram process, Anna Atkins is regarded as the first female photographer.

In a cyanotype a blue is usually the desired color, however there are a variety of effects that can be achieved. These fall into three categories: reducing, intensifying and toning.

Reducing is the process of reducing the intensity of the blue. Sodium carbonate, ammonia, Clorox, TSP, borax, Dektol and other reagents can be used to do this. A good easily obtained reducer is Sunlight laundry detergent. When using a reducer it is important to pull the cyanotype out of the weak solution and put the cyanotype into a water bath to arrest the bleaching process.

Intensifying is the strengthening of the blue effect. These reagents can also be used to expedite the oxidation process the cyanotype undergoes. These reagents are Hydrogen Peroxide, Citric Acid, Lemon Juice, and Vinegar.

Toning is the process used to change the color of the iron in the print cyanotype. The colour change varies with the reagent used. There are a variety of elements that can be used, including tannic acid, oolong tea, wine, cat urine, and pyrogallic acid.

http://en.wikipedia.org/wiki/Cyanotype