A method for printing photographs in these precious metals was first patented in 1855 by William Willis, an Englishman. The permanence of silver based prints was suspect, and photographers wanted a process where the image was sure to last. Platinum is an ideal candidate, as it is very permanent. By the turn of the 20th century, platinum had become the favored paper of fine art photographers. However, the metal is very expensive, and by the end of WWII platinum paper was no longer being commercially made.
Platinum is a hand-coated, UV light sensitive contact printing process. The final print will be the same size as the negative used. This is due to the low light sensitivity of the medium. Technically, platinum is one of the "iron" printing processes, as a compound called Ferric Oxalate is the light sensitive material. Print exposure times at noon on a clear summer day will be about 6 minutes! This may seem a disadvantage, but it does allow the photographer to make prints in subdued room light, without the need for a darkroom or an enlarger.
Why platinum/palladium? Platinum is the more permanent of the two, gives a cooler image tone and better separation in the highlights. Palladium is warmer, and gives better separation in the shadows of the print. Many printers use a combination of the two to reap the benefits of both. In any case, the prints exhibit a very long scale in the mid-tones, possess a feeling of depth (due the fact that the paper has no gelatin overcoat), and are completely archival without the use of fixer or long wash times. The process is very low contrast, meaning that subjects photographed with a seven or even an eight stop range may be printed with detail. In smaller film sizes (120 and 4x5), a negative which yields a full range silver print will work fine. In larger film sizes, the practitioner may want to give an extra stop of exposure, ensuring that the shadow areas of a scene have full detail in the negative. Too much over-exposure is a detriment, as it will cause longer and longer print exposure times. Use of the "Zone System" for exposure and development control is highly recommended.
Platinum remains the king of all photo process, possessing a beauty, depth, and permanence valued by photographers, collectors, and museums. Increasingly, contemporary photographers are turning to platinum to add further value to their work.
Historic platinum photographers:
Peter Henry Emerson
Frederick Evans (who supposedly stopped making photographs when platinum papers were no longer available,)
Adam Clark Vroman (Vroman's bookstore in Pasadena, formerly Vroman's book and photo supply, and whose platinum albums can be seen at the Pasadena Public Library.)
Leading contemporary practitioners: