About the Platinum Process
The platinum-palladium (pt/pd) process was developed in the 1800s, well before silver gelatin became the dominant technology of black and white photography.
Working under subdued lighting, the printer mixes up a light-sensitive liquid pt/pd emulsion. It is painted or rolled onto heavy rag art paper. Once the emulsion has dried (two coats in my case) a large negative the size of the print-to-be is placed face down on the coated paper.
This negative+platinum sandwich is covered by a heavy piece of glass and exposed to an ultraviolet light source, either the sun or high-UV fluorescent tubes in a dedicated lightbox. Exposures can run anywhere from a few minutes to an hour or more. Gradually a faint image appears. Then the glass, paper and negative are separated, and the image is developed to completion in potassium oxalate.
"Platinotype" photographs produced via the platinum process were highly regarded by Alfred Stieglitz, arguably the most important figure in the history of American visual arts, along with many other pictorial photographers of the early twentieth century. The prints are renowned for their subtlety, tactility and visual depth.