A contact print is made with fine quality paper, which was coated with iron salt and potassium ferricynanide.
The image is produced by placing a negative or botanical specimen on the sensitised paper and exposing it to light. Where light reached the paper the area turned a stunning cobalt blue colour; where the light was blocked, it remained white.
This early process was invented in 1840 and is still in use today; it is commonly known as blue print and used for architectural drawings. Contemporary artists are also using this process to great effect.