CHEMISTRY AND CONSERVATION OF PLATINUM AND PALLADIUM PHOTOGRAPHS
2 PLATINUM AND PALLADIUM PHOTOGRAPHIC PROCESSES
2.1 HISTORY OF PLATINUM AND PALLADIUM PHOTOGRAPHY
Platinum is first mentioned for use in photography in 1856, palladium in 1859, both as intensifying agents for silver prints (Dancer 1856; Photographic Notes 1856; Gwenthlian 1859). Old sources often use the terms “toner” and “intensifier” interchangeably. Platinum and palladium treatment of iron, mercury, silver, and uranium prints is more properly viewed as intensification. William Willis is widely credited with having created the platinum printing process. However, the first mention in the literature of a sensitizer based on platinum cites the experiments of a Mr. Merget, in which paper is coated with platinic acid, ferric chloride, and tartaric acid, then exposed, and developed using mercury vapor (British Journal of Photography 1873; Abney and Clark 1898).
From this departure point, William Willis designed and patented the first successful platinum printing process using a ferric oxalate–potassium tetrachloroplatinite sensitizer. The oxidation-reduction chemistry of these two ingredients, whereby iron is reduced upon exposure to ultraviolet light, then oxidized by a developer with concomitant reduction of platinum, and finally cleared using hydrochloric acid, remains the basis of platinum and palladium printing. The terms “fixing” and “clearing” are used interchangeably in old and current sources alike. “Clearing” is used to distinguish platinum and palladium processes from silver printing. Whereas many firms, including Willis Platinotype Company, mass-produced platinum printing papers, the process survives today as it was created, with hand-coated papers.
Features of Willis's initial process that have since been changed include the use of the oxidizing agents lead nitrate and silver nitrate in the sensitizer to prevent ferric oxalate from being reduced prior to exposure and to produce a finer deposit of platinum; the need to heat the potassium oxalate developer (hence the name, hot-bath platinum printing”); and the use of a clearing agent consisting of ammonia, ammonium chloride, oxalic acid, sodium chloride, and sodium hyposulfite.
Willis himself was responsible for many of these changes, starting in 1878 with the elimination of silver nitrate from the sensitizer, addition of potassium tetrachloroplatinite to the developer, and replacement of the hyposulfite-based clearing agent with hydrochloric acid, and followed in 1880 by the replacement of lead nitrate in the sensitizer with ferric chlorate.
In 1882, Joseph Pizzighelli and Arthur von H�bl proposed two further modifications—replacement of ferric chlorate in the sensitizer with potassium chlorate and removal of potassium tetrachloroplatinite from the developer, which left the hot-bath process largely in its modern form. In contrast, the printing-out platinum process introduced by Pizzighelli in 1887 was short-lived, owing to its inconsistency. Potassium oxalate was incorporated into the ferric oxalate–potassium tetrachloroplatinite sensitizer; thus prints developed as they were being exposed, and processing required only clearing with hydrochloric acid.
Coincident with Pizzighelli's printing-out experiments, William Willis introduced a cold-bath platinum printing process, named after the tone of the image rather than the temperature of the developer, using potassium tetrachloroplatinite in the developer instead of in the sensitizer. The results of this process, too, were inconsistent, and in 1892 Willis replaced it by a true room-temperature–developer (cold-developer) platinum printing method. The chemistry of the sensitizer was never disclosed, but Abney and Clark (1898) indicate that a much speculated-upon secret ingredient was likely a dichromate salt.
Methods for toning (replacing the image-forming platinum deposit with gold, mercury, or uranium) and intensifying (proportionally adding silver, palladium, or platinum to the image-forming platinum deposit) finished platinum prints also proliferated in the mid-1880s. The main deterent to their continued practice is the cost and toxicity of the chemicals.
Palladium printing is first mentioned in 1874. The process was less popular than platinum printing, chiefly because palladium was more expensive at the time. The greater inertness and catalytic activity of platinum make it more useful in industry and prompted its price to rise sharply during the World War I. Both of these properties are relevant to printing and conservation: unlike platinum, palladium can be dissolved by acid, thus images can be bleached upon clearing and possibly upon storage in acidic environments; of concern to conservation, platinum's enhanced catalytic activity accelerates paper embrittlement and is responsible for image transfer (Reilly 1986).
Palladium was discovered after platinum and was initially more expensive. In part for these reasons, palladium was sometimes viewed with interest by photographers as the finer alternative. The warmer tones palladium produced were variously preferred and disfavored relative to those of platinum. A consideration in palladium printing's favor is that a hot developer is not required.
Palladium printing is rarely mentioned in the literature before 1895 and was not widespread until the late 1910s. As platinum is now 10 times more expensive than palladium, the latter is the decided preference of most amateur printers today. Nonetheless, most manuals continue to focus explicitly on platinum printing, treating palladium printing as a minor offshoot. Historically, both platinum and palladium prints have the reputation of being permanent, due to the stability of the metal that constitutes the final image. More recently, the stability of the paper supports for both platinum and palladium prints has come into question, as described by Reilly (1986) and addressed at the Stieglitz Colloquy. The internal and external sources for the discoloration are of interest, as well as the means of treating and avoiding the discoloration by proper handling, storage, and exhibition.
2.2 CHEMICALS USED IN PLATINUM AND PALLADIUM PHOTOGRAPHY: A REVIEW OF THE LITERATURE TO 1940
The periodical liteature of Great Britain, the United States, Germany, France, Austria, and Belgium between 1839 and 1940 was studied to ascertain the full range of chemicals that were experimented with for platinum and palladium printing and intensification in the 19th and early 20th centuries, and to expand both on the well-known manuals of Abney and Clark (1898), Horsley-Hinton (1898), and Pizzighelli and von H�bl (1886) and on latter-day manuals by Steinberg (1992), Sullivan (1992), Nadeau (1986), Shillea (1986), Hafey and Shillea (1979), and Rexroth (1977). Fifteen journals were chosen, mostly from the History of Photography microfilm series, and every article mentioning platinum or palladium printing or intensification was reviewed. In all, 391 articles in publications such as the British Journal of Photography, the Philadelphia Photographer, L'Amateur photographe, and the Jahrbuch f�r Photographie und Reproduktionstechnik were surveyed. The result was a list of chemicals mentioned for use in sizing, sensitizing, developing, clearing, toning, intensifying, reclearing, neutralizing, varnishing, and mounting platinum and palladium prints; in intensifying iron, mercury, silver, and uranium prints with platinum and palladium; and in preparing chemicals for platinum and palladium printing and intensification. This list is available from the author and at the library of the Department of Chemistry at Princeton University (Gottlieb 1993)