Common Use Dates: 1880-1930
Alternate Names: Platinotype, Palladiotype
Mistaken For: Matte Silver Gelatin DOP, Matte Collodion, Salted Paper, Kallitype
Process Family(s): Photographic, Iron, Developing Out, Printing Out (less common)
The platinum process is based on the light sensitivity of iron salts. The sensitizer is coated directly onto the paper support and the image forms within the paper fibers resulting in a matte surface and relatively soft image.
The process was patented in 1873 by William Willis with subsequent patents and improvements from the 1870s to the 1890s. Willis started the Platinotype Company in 1879 producing the first commercial platinum papers. Willis’ patents were vague and difficult to follow. As a result, in 1882 Joseph Pizzighelli and Arthur von Hübl published practical instructions on the platinum process based on their investigations into each of the chemicals mentioned in Willis’ patents and suggestions for improvements not under patent. These instructions allowed photographers to hand-coat their own papers, though many preferred to purchase commercially made pre-sensitized paper. Pizzighelli and Hübl also introduced a platinum printing out process, but it was never very popular as it was difficult to control and get reliable results.
As the platinum process gained in popularity in the 1880s, many other photographic paper manufacturers began producing platinum papers. By the early 1900s a variety of surface textures and paper thicknesses were available as well as papers that could render cool black tones or warm brown tones.
While platinum printing is relatively easy, the chemistry is relatively complex. There are also a variety of additives to the chemistry that can alter the final image tone producing prints that range from a warm “sepia” brown to neutral or purple black.
A plain piece of high-quality paper is coated with a mixture of iron and platinum salts, usually ferric oxalate and potassium tetrachloroplatinate (II). The chemistry of the platinum process requires two successive chemical reactions. The first reaction occurs during exposure. The sensitized paper is placed in contact with a negative and exposed under ultraviolet (sun) light. The iron salt reduces from iron (III) (ferric oxalate) to iron (II) (ferrous oxalate) forming a visible yellow-brown image most noticeable in the shadows. Iron (II) is a strong reducing agent for noble metals like platinum, but needs to be dissolved to react with platinum. Therefore, the second reaction happens during development. The print is “developed” in a bath containing potassium oxalate, which dissolves iron (II). The iron (II) oxidizes and reverts back to iron (III), which in turn reduces the platinum (II) to metallic platinum forming the final image within the fibers of the paper support. This is followed by three successive clearing baths of dilute hydrochloric acid which dissolves iron (III). Finally, a thorough wash in water removes the acid and dissolved iron (III) from the paper.
The second, development, reaction is fairly slow and the pertinent chemicals tend to migrate out of the paper and into the developing solution before the reaction is complete resulting in a weak image. To overcome this problem, Willis tried several approaches including adding additives to the sensitizer to enhance the reactivity of the platinum salts, adding extra platinum salts to the developer, using sizing agents in the paper to hold the exposed sensitizer in the paper fibers, and the use of glycerine coated on the paper before development to keep the sensitizer from washing out. The glycerine method was further developed by Alfred Stieglitz and Joseph Keiley. Finally he sped up the reaction by heating the developer. The use of a hot developer also created warmer, brown or brown-black image tones.
In 1890 Willis introduced Sepia paper, which produced warm brown tones through the use of mercury in the sensitizer. In 1892 Willis introduced a cold development process which rendered more neutral black tones. This paper possibly held the sensitizing chemicals in the paper using a sizing agent though the manufacturing specifications were a closely guarded secret and remain a secret today. In 1906 Willis introduced Japine paper, which had a semi-matte surface. The specifications on this paper are also unknown, though it is likely the paper was treated with sulfuric acid, parchmentizing it and creating a more reflective surface. These papers closely resemble early baryta-less silver gelatin developing out papers.
The decline of platinum papers was slow. In 1902 it was discovered that platinum can be used to produce nitric acid, which is necessary for making explosives, causing the cost of platinum to rise. To counterbalance the rising cost of platinum, Willis introduced Satista paper in 1913, a silver-platinum-iron paper that required less platinum. The onset of WWI in 1914 made platinum scarce and expensive and by the middle of WWI many photographic paper manufacturers began to discontinue platinum papers. Willis’ Satoid paper introduced in 1916 was similar to Satista but warmer in tone, possibly due to more silver than platinum in the sensitizer. In 1916 he patented palladium papers, which were warm in tone and based on the same chemistry as platinum. Also, silver gelatin developed out papers were produced by several companies that mimicked platinum papers. All commercial production of platinum and palladium papers ceased by the 1930s with the closing of the Platinotype Company in 1937.
In the 1960s and 1970s there was a resurgence in platinum and palladium printing among fine art photographers. Though a few small companies produced commercial papers, most photographers preferred to hand-coat using a mixture of platinum and palladiums salts. Mike Ware, a British chemist, and Pradip Malde, an artist, worked together beginning in the 1980s to better understand platinum chemistry and published instructions on a viable platinum printing out process.