Platinum prints are characterized by their delicate surface and subtle tonal gradations. They are made by sensitizing paper with iron salts and exposing the paper in contact with a negative until a faint image has formed. The paper is then chemically developed in a process that replaces the iron salts with platinum and intensifies the image. Platinum prints were popular with art photographers around the turn of the twentieth century, but when World War I caused the price of platinum to rise, palladium (a related metallic element) was introduced as a more affordable substitute.
Video from the George Eastman Museum