Cyanotypes

Cyanotype is a photographic printing process that produces a cyan-blue print. Engineers used the process well into the 20th century as a simple and low-cost process to produce copies of drawings, referred to as blueprints. The process uses two chemicals: ammonium iron(III) citrate and potassium ferricyanide.

The English scientist and astronomer Sir John Herschel discovered the procedure in 1842.^[1] Though the process was developed by Herschel, he considered it as mainly a means of reproducing notes and diagrams, as in blueprints.^[2] 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.^[3] 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.^[4]

n 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.^[6]
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.^{[citation needed]} 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.^[6]
Toning is the process used to change the color of the iron in the print cyanotype.^[6] 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.^[6]