Платиновые отпечатки: история, технология и долговечность

Platinum prints, also called platinotypes, are photographic prints made by a monochrome printing process involving platinum. Platinum tones range from warm black, to reddish brown, to expanded mid tone grays that are unobtainable in silver prints. Unlike the silver print process, platinum lies on the paper surface, while silver lies in a gelatin or albumen emulsion that coats the paper. As a result, since no gelatin emulsion is used, the final platinum image is absolutely matte with a deposit of platinum (and/or palladium, its sister element which is also used in most platinum photographs) absorbed slightly into the paper. Platinum prints are the most durable of all photographic processes. The platinum group metals are very stable against chemical reactions that might degrade the print—even more stable than gold. It is estimated that a platinum image, properly made, can last thousands of years. In 1832, Englishmen Sir John Herschel and Robert Hunt conducted their own experiments, further refining the chemistry of the process. In 1844, in his book Researches on Light, Hunt recorded the first known description of anyone employing platinum to make a photographic print. However, although he tried several different combinations of chemicals with platinum, none of them succeeded in producing any permanency in the image. All of his prints faded after several months. Over the next decade, Hunt noted that platinum prints he had left in the dark faded very slowly but gradually resumed their original density, and had also shifted from a negative to a positive image, eventually becoming permanent. Willis introduced the "hot bath" method where a mixture of ferric oxalate and potassium chloroplatinate are coated onto paper which is then exposed through a negative and developed in a warm solution of potassium oxalate.

While Willis had greatly advanced the chemistry of the platinum process, there was still no reliable method for the individual preparation of platinum paper by 1880. Two years, later two Austrian Army officers, Giuseppe Pizzighelli and Arthur Baron V. Hubl, published a dissertation describing a straightforward process for preparing the paper. They continued their research for several years, and in 1887, Pizzighelli patented a new process that made the commercial production of platinum paper viable for the first time. The new process was briefly known as a "Pizzitype" and was marketed under the name "Dr. Jacoby's Printing Out Paper." Willis quickly countered this advance by obtaining two more patents in 1888 for cold bath processes. By adding more platinum to the developing process, he produced prints that had dense brown black shadows rather than the lighter browns that were the best that previous processes could produce. While much more aesthetically pleasing, prints developed by this process were difficult to produce reliably. Four years later, Willis began manufacturing a platinum paper that was designed for the cold bath process, and this became the standard for the rest of the decade. The business he started in 1880, called the Platinotype Company, rapidly expanded, and soon he was selling his paper throughout Europe and in the United States. By 1906, his company had sales totaling US $273,715 ($ in 2009 dollars), a significant amount at that time.

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 versus 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. In 2002, working from research done by Howard Efner and Richard Sullivan, Dick Arentz formulated the methodology for using sodium chloroplatinate as a contrast control agent. Richard Sullivan coined the term Na2 and began to sell a 20% solution through Bostick and Sullivan. Arentz found that, since it is a platinum compound (sodium chloroplatinATE), it does not work with platinum prints (potassium chloroplatinITE). It does, however, shorten the scale of a pure palladium print. When minute quantities of sodium chloroplatinate are added to the palladium salt/ferric oxalate emulsion it produces the high contrast prints needed for thin negatives, but does not exhibit the granularity found when using traditional chlorates. A palladium print made with potassium chlorate will take on a warm, sepia tone. The same print using sodium chloroplatinate will have cooler tones similar to those of a platinum/palladium print. The inherent low sensitivity of the process occurs because the ferric oxalate is sensitive to ultraviolet 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, 100% cotton rag, silk, and rice, among others. On the collecting market, platinum prints often sell for many times what a similar silver gelatin print would bring.