Cyanotype is a method for capturing an image of an object. It is the technique used to create blueprints (copies of technical drawings), but one of its first uses was to capture images of plant material. Whereas photography creates an image of an object by capturing light on light-sensitive material, cyanotype creates an image from the shadow of an object. Areas of a cyanotype image that are exposed to UV light (eg from sunlight) turn blue, while areas in shadow stay white.

Developments

The invention of cyanotype is credited to Sir John Herschel, son of the astronomer William Herschel. In the 1830s, Herschel experimented with chemical compounds that changed colour when applied to paper and exposed to light. Compounds of silver produced paper very sensitive to light, but images needed treatment with sodium thiosulfate (hyposulphite of soda) to stop the chemical reaction. In 1842, Herschel developed a process using compounds of iron, which was easier and quicker. Paper coated with a mixture (green in colour) of ferric ammonium citrate and potassium ferricyanide turns brown in a few minutes when exposed to sunlight. Areas in shadow turn blue. When the paper is washed in water, the chemical reaction stops. Blue areas turn white, and the brown areas turn blue.

The technique was used in 1843 in a book by a botanical artist. Known as 'the first photographic illustrated book', Photographs of British Algae: Cyanotype Impressions by Anna Atkins features cyanotype silhouettes of numerous specimens, demonstrating the structures of the algae more accurately and quickly than a hand-drawn image could¹. Although the images were photographs, the text was handwritten - only 13 copies of the book were made, each featuring slightly different cyanotype images. Anna Atkins went on to publish Cyanotypes of British and Foreign Ferns in 1853 with fellow botanist Anne Dixon.

Do Try This At Home

You can buy cyanotype kits from craft stores. Some kits contain ferric ammonium citrate and potassium ferricyanide for you to mix and paint on to paper yourself. Other kits contain paper that has already been coated with chemicals. You can experiment with different objects such as plant material, paper cutouts, jewellery, or designs painted on to glass² to get different effects in your blue and white creations.