Discovered in 1842 by the English scientist and astronomer Sir John Herschel, cyanotypes are a negative photographic printing process using a mixture of potassium ferricyanide and ferric ammonium citrate applied to paper, fabric or other materials and exposed in sunlight or UV lights.
Originally used to create blueprints as copies, Anna Atkins later used the process to document ferns and other plant life in cyanotype contact prints using the actual plants to create photograms.
The solution is mixed just prior to coating paper or cloth in a darkened room and allowed to dry away from the light. Placing all the elements on my sensitised paper in low light I cover with glass and expose in the sunlight for 1-30 mins.
Once exposed, I wash off the unreacted iron solution (where the sun did not touch) in water. Blue prints can be toned in a water bath of coffee, tea, wine and even cat urine to produce different variations of colour in browns, yellows and purples.