Toning is a technique used to alter the overall color and contrast of a photographic print and can also be used to increase the stability of black-and-white prints made with silver salts. When a toning agent is applied, either by immersion in a bath or locally with a brush, the substance chemically replaces some of the metallic particles that form the image. A number of different metallic salts—such as gold, platinum, or selenium chloride—could be used, each giving a distinctive hue to the print. While not technically a toning agent, mercury chloride was often used in the developing bath to increase contrast in platinum and palladium prints.