A short, technical explanation...
The contemporary platinum printing process begins with taking a picture. It doesn't matter if that raw picture is capture on film or on a compact flash card in a digital camera.
If the negative is a digital image, it must be converted to an analog state, or the equivalent of a piece of film.
Unlike silver prints which can be made by shining light through an enlarger, the negative for a platinum print must be exactly the same size as the final print.
This is because the platinotype, or platinum method, is a contact printing method.
Imagine this. If you're final print will be 16x20, your negative must also be 16x20. This delicate and tedious process partially explains the cost of this art form.
Platinum printing is contact printing
Once the negative is ready, a master printer brushes a light-sensitive solution containing platinum and palladium salts (palladium is added to increase warm tones) onto the print paper, usually an all-cotton, totally acid-free archival rag.
When the solution dries, the master printer exposes the negative in contact to the paper to ultra-violet light.
To bring up the image, the printmaker dips the exposed paper into a developer to transform the metal salts back to a metallic state. The print then goes through a series of clearing baths.
The finished image that emerges from the final wash has particles of the precious metals permanently embedded in the fibers of the paper.
Importance of the Noble Metals Group
Platinum and palladium, both rare, precious metals, belong to the noble metal group. This stable, exclusive group resists corrosion and oxidation to the extent that prints made with them can live, unchanged, for thousands of years.