Van Dyke Brown is a similar process to the Cyanotype (both trialled by Herschel) but with the added step of fixing.
Van Dyke Brown Poppy
A 3 part solution is mixed and used to sensitise the paper. A contact print is made using UV light and the result are rinsed, fixed in dilute hypo, and given a final wash. The poppy photogram is from a flattened dried petal and the Savoy is from an acetate negative.
Van Dyke Brown Savoy Cafe, Breichin
The prints transform from a bright orange to a rich brown after fixing – similar to the paint pigment used in oils and watercolours since the seventeenth century.