Platinum is an early photographic process in which an image is formed by platinum metal, unlike in conventional photographs where it is formed by silver. The process was popular in the late 19th and early 20th century, when commercially produced platinum paper became available. It was the choice of great masters like Alfred Steiglitz, Edward Steichen and Paul Strand.
There are two essential qualities in a platinum paper: First, the paper should have a smooth (but not a plate) surface with good wet strength (internally & externally sized) so that after the many "baths" the paper will dry perfectly flat.
Second, the paper should be without any buffering agents (such as calcium carbonate) or alkaline chemicals so as to not upset the chemistry and possibly affect the performance and quality of the final print.
Platinum emulsion has a unique response to light resulting in the most beautiful tonal range in black & white photography. It also happens to be the only true archival photographic process. The platinum print will remain unchanged as long as the paper it is printed on exists, thus making the image virtually immortal.