Chemistry
Platinum printing is based on the light-sensitivity of ferric oxalate. Ferric oxalate is reduced to ferrous oxalate by UV-light. The ferrous oxalate then reacts with platinum(II) or palladium(II) reducing it to elemental platinum (or palladium), which builds up the image.
By varying the amount of platinum vs palladium and the addition of oxidizing chemicals such as hydrogen peroxide and potassium dichromate or potassium chlorate, the contrast and "color" of the final image can be modified. Because of the non-uniformity of the coating and mixing phases of the process, no two prints are exactly the same, adding additional "cachet" to a platinum print.
The inherent low sensitivity of the process is because the ferric oxalate is sensitive to ultra-violet light only, thus specialized light sources must be used and exposure times are many times greater than those used in silver-based photographic processes.
Due to the unavailability of pre-coated sensitized paper, all platinum/palladium printing is done on paper coated by the printer. The light sensitive chemicals are mixed from powdered basic chemicals, or some commercially available solutions, then hand applied with a brush or a cylindrical "pusher".
Many artists achieve varying effects by choosing different papers for different surface characteristics, including vellum, rag, and rice, among others - even silk. On the collecting market, platinum prints often sell for many times what a similar silver-gelatin print would sell for.
Read more about this topic: Platinum Print
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