Presented in France by the photographer and physicist Adolphe-Alexandre Martin (1824-1896) in 1852, the ferrotype is a positive proof on metal plate. A very thin white iron plate is coated with a dark varnish which, once dry, is covered with collodion (preparation obtained by dissolving nitrate and cellulose in a mixture of ether and alcohol) or, from the 20th century, silver gelatin-bromide. The plate is then sensitized to silver nitrate. After the image is taken, the plate is developed and fixed. It is then varnished to avoid abrasions.
If the ferrotype belongs to the positive category, the image produced is actually a negative. This effect results from the silver grains that form the image: lighter than the black varnish layer in front of which they are placed, they give the ferrotype the appearance of a positive.
Unlike the daguerreotype, the ferrotype is a fast, easy to use and cheap process. He was thus particularly appreciated by itinerant merchants for portraits; some consider him thus as the ancestor of the photomaton. Under the English name "tintype", ferrotype was widespread in the United States between 1856 and the end of the 19th century.
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