Salt printing was one of the earliest photographic techniques to be invented; it dates from the 1830s and makes beautiful prints. The colour varies, according to how the print is made, but commonly is a rich brown/purple – sometimes referred to as sepia prints. The prints are made by coating good quality paper with solutions, firstly of salt and then of silver nitrate. The paper is then exposed, under a negative, to the sun, or more usually a sun lamp of some description, and developed by washing in water.
Platinum printing is the aristocracy of the early photographic processes. This picture by one of the most famous woman photographers, Julia Margaret Cameron, was made in 1865. This image is composed of very finely divided platinum and palladium metal. The technique was very popular in the forty years before the First World War because of its very delicate highlights and mid-tones.
As with the other printing processes, paper is coated with special solutions and, when dry, is exposed under a negative to the sun or sun lamp, the ultra-violet rays are the key here, before being developed in another chemical mix.
If, as a child, you ever put transfer stickers on you arms, you will have a good idea of the fundamentals of this process. A pigmented gelatine layer coated on top of paper is made sensitive to light by a chemical solution and then exposed under a negative to ultra-violet light. This carbon tissue, so-called because carbon was first used as the pigment to colour the gelatine, is then soaked and pressed flat against a second piece of paper. Finally, the sandwich is soaked in warm water and the gelatine backing paper peeled off, leaving a fine print behind.