The photographer must prepare the platinum printing paper, selecting among of the finest and purest papers in the world. The choice is an important decision. The beautiful subliminal qualities of the paper will become intrinsic with the image. Any invisible impurities cause unpredictable results. The paper must be fine, consistent in quality, considering the expense of the platinum salts. The photographer must also become something of an organic chemist. Complex solutions are carefully mixed from scratch, and measured out in tiny amounts, to make the light-sensitive emulsion that will coat the paper. Solid platinum is dissolved with potassium and chlorine in a series of acid baths; the acid is replaced with water, and the solution is crystallized into a platinum salt. This is combined with light-sensitive iron salts (ferric oxalate). Modern platinum printers usually intermingle platinum with palladium or iridium salts, to produce subtle variations in “warmth” (soft brown color) and contrast.
The photographer must rely on his experience. Dissolved in pure water, the platinum and iron salts are the emulsion, which is hand-coated with a brush onto a sheet of paper. The paper is then dried. It is usually used immediately, so only a few pieces of prepared paper are made at a time. Throughout the process, the photographer is well advised to keep a detailed, exact record of every step; the paper used, the number of drops of the chemicals, the proportions of the mixtures, the humidity of the environment and the paper, etc., so that errors are not repeated. Mistakes are expensive and time-consuming! Even when all goes well, it is not uncommon for a platinum photographer to make between five and fifteen attempts, with various combinations, to achieve the final print.
The negative is placed in direct contact with the paper, held down under a sheet of heavy glass. Platinum is so stable, that even in a photosensitive salt, it is reluctant to change. To expose a platinum print requires hundreds of times more energy than a silver print. Special ultraviolet lights are used, or sunlight itself. The exposure time may vary from minutes to a full hour. The ultraviolet light “reduces” (purifies) the platinum salt to a darker, pure metallic state. A faint outline of the image appears on the paper when exposure is nearly complete. The fully exposed paper is then taken into the laboratory, and a special chemical bath is poured upon it. Development is instantaneous. Then, a series of mild acid baths remove any remaining traces of iron and other extraneous material from the paper. The final image is formed out of sub-microscopic particles. They Look So Beautiful, metals, embedded in the paper fiber.
