Platinum/palladium printing was invented in the 19th century and still has the widest tonal range and is the most permanent of any photographic process using chemical development. To make the print, cotton rag paper is brushed with a mixture of platinum and palladium metals combined with a sensitizer. The negative is placed in contact with the dried emulsion and exposed to ultraviolet (UV) light, then processed in a chemical developer.
"I think I like this printing method best of all the alternative processes because of the rich warm color and the quality of the image surface. Unlike the silver gelatin process, the metals lie directly on the paper and not in a gelatin layer. This gives the print a soft matte surface that has very rich shadows and delicate highlights. Changing the ratio of the chemicals involved can also effect subtle changes. Because of the non-uniformity of the mixing/developing process, no two prints are exactly alike."