The Platinotype

By John Rudiak View Camera   January / February 1994 It is essential that the ferric oxalate is as pure as possible because this is the controlling agent in the printing process -- responsible for emulsion speed, contrast (maximum denisty) and fog level. In the past, it was easier to obtain ferric oxalate when it was used as a tanning agent in the leather industry, but it has since been replaced with less toxic compounds (all oxalates are poisonous). The only use for it today is in the iron based printing processes. Pure ferric oxalate is bright green, turning brown as it decomposes to ferrous oxalate. It's the only compound in the process that has a shelf life once it is dissolved in water for use. It will eventually cause fogged highlights at which point it should be discarded. I've had good results using ferric oxalate made by Richard Sullivan of Bostick & Sullivan in Van Nuys, California. The platinum compound that is used is a salt of the metal named potassium chloroplatinate, represented by the chemical formula K2PtCl4. The palladium salt is analogous-palladium chloropalladite, K2PdCl4. A saturated solution of these salts is mixed with an equal volume of an equal molar concentration of ferric oxalate, applied to the apper, dried and exposed to an ultraviolet light source. After exposure we have a mixture of compounds in the paper; unreacted ferric oxalate, exposed and reduced ferrous oxalate (the provisional image) and the platinum and/or palladium salts. Ferrous oxalate is a powerful reducing agent, capable of reducing the platinum salt to metallic platinum, but needs to be made soluble by the developer so that this reaction can take place. A concentrated solution of potassium oxalate, the classic platinum developer, answers these needs as suggested by the following equation: 2FeC2O4 + K2C2O4 + K2PtCl4 ------ Fe2(C2O4)3 + Pt + 4KCl (ferrous; pot. ox; plat salt; ferric metal salt) The developer in this case is never exhausted, and upon use becomes increasingly rich in platinum salts and ferric oxalate, a fact which caused old time platinum printers to believe that the older and more used the developer, the richer the prints obtained from its use. Because we want only platinum metal to remain in the print, we need to remove all iron salts. Traditionally, a weak solution of hydrochloric acid is used to convert the iron oxalates to ferric chloride, which is water soluble, and is then removed by washing. This completes a basic overview of the platinum process. Next, I would like to discuss the mechanics invloved in producing a platinum/palladium print using methoids which I have found to be the most satisfactory over the last 15 years. Mixing The Chemistries The chemistries invloved are generally not benign, however good chemical common sense and caution can and should be exercised to prevent problems resulting in physical alignments. Platinum and palladium are both heavy metals and should not be allowed to enter the body in powered form or in solution. All oxalates are poisonous and should be handled with due respect - the dust from the powdered ferric oxalate is particularly dangerous and should be handled outside the darkroom while wearing a dust mask. All solutions except the clearing bath should be mixed with distilled water. Pre-mixed solutions of all chemistries are available commercially, but the formulas will be different in a lot of cases than those described here. Do not use metal containers for mixing or storing solutions. The Platinum Solution A saturated solution of potassium chloroplatinite is prepared by dissolving 10.0 grams of the salt in 50 ml. of warm (80-90 degrees F) distilled water. Upon cooling, some of the salt will precipitate out of the solution, necessitating rewarming prior to use. This is stored in a brown bottle, and has an indeterminate shelf life. The Palladium Solution An equimolar solution of palladium is made relative to the platinum solution. The most economical method is to use palladium chloride (PdCl2) in combination with sodium chloride in solution. 3.5 grams of sodium chloride are dissolved in 55ml. of warm distilled water and then 5.0 grams of palladium chloride are added and stirred until dissolved. Stored in a brown glass bottle, it has a long shelf life. A mixture of platinum and palladium salts has advantages over either platinum or palladium used alone -- most notably a consistency of printing (contrast, speed, and color) over a range of relative humidities and temperatures. It is convenient at this time to mix equal amounts of platinum and palladium solutions to make a single solution that is added to an equal amount of ferric oxalate prior to coating on the paper. This will also prevent the platinum from crystallizing out of the solution as it cools. The ratio of platinum to palladium can be varied to affect print color, palladium being much warmer in tone than platinum. The palladium solution is also about 1/4 of the price of palladium and when used with an equal amount of platinum and developed in the developer described later results in slightly warm-black print color. The Ferric Oxalate Solution This is the light sensitive agent and hence the most delicate of the compounds that is used and should be protected from ultraviolet light and excess heat. It is also the most difficult to dissolve. To mix, 100ml. of distilled water is placed in a red glass flask and warmed to about 100 degrees F. In a separate room with good ventilation 27.0 grams of ferric oxalate are weighed out under incandescent light (wear your dust mask) and transferred to the the warm distilled water. Now you can go back into the darkroom and keep it warm while stirring until it is dissolved. Stored in a brown glass bottle it will keep for 3 or 4 months if prepared in this manner. Those familiar with platinum will notice the absence of several ingredients as compared to the classic recipes in normal use. Potassium chlorate, an oxidizer, is and has been used to to increase contrast in the platinum emulsion, but has several drawbacks -- instability and increased print graininess with increased contrast levels. It also necessitates the use of two oxalate solutions, one with and one without chlorate which are mixed in various proporations to control contrast, traditionally with the inherently inaccurate "drop" method of measurement. Also left out is the oxalic acid from the ferric oxalate solution which was believed to add to the richness of the blacks in the print. Contrast Solution 3% hydrogen peroxide, available at any drugstore in ready to use form. When added to the sensitizer solution it does not have the increasing grain problem associated with potassium chlorate. Developer Solution This is a variation on several of the traditional "cold tone" room temperature developers utilizing potassium oxalate and a phosphate, with the exception that this formula contains a buffer system to regulate pH. Into a liter of warm distilled water dissolve 140 grams of potassium oxalate, then 20 grams of dipotassium phosphate, followed with 5 grams of oxalic acid. Dilute with an equal amount of distilled water and use half for developing and the other half to top up your developer as it gets carried out with the prints. Clearing Bath This is simply a 0.3% solution of hydrochloric acid. You will save considerably by purchasing Muriatic Acid from a hardware or pool supply store-this is essentially a 30% hydrochloric acid and is diluted 1 to 99 for use (10ml. in a liter of water). In its 30% strength, the acid is very strong and will burn skin and clothing, so wear eye protection and gloves to make several gallons at once, adding acid to the water, (not the other way around) slowly, while stirring. At 0.3% dilution the acid is very weak and fairly harmless. Keep the large supply stoppered as hydrogen chloride will evaporate into the air and cause problems in the darkroom. While there are several alternative clearing baths currently recommended such as citric acid and EDTA (a chelating agent) hydrochloric acid has the advantages of working well with a wide variety of papers, and if made more concentrated can act as a print reducer if palladium is present in the emulsion. This can leave the paper acidic and brittle, however, if the paper is not sufficiently washed and neutralized. This is all that is needed to sensitize and process a platinum print once a paper is selected. The choice of paper has more effect on the final look of the print than any other single factor. Paper color, size, weight, and surface all influence the final look. For a paper to work well it needs to be sized sufficiently to keep the sensitizer from soaking into the paper too quickly- we need time to spread it around to get an even coating before it is absorbed into the paper. It also needs enough wet strength to stand soaking for half an hour or more, some of that time in the hydrochloric acid. Experimenting with different papers will most likely be frustrating before it is rewarding. If a paper will take a calligraphy pen and ink without bleeding, it is a likely candidate as long as it is 100% rag. Rising paper company's Gallery 100 is a good heavy weight paper and Strathmore 500 series drawing is a very good light weight paper, both used with the plate finish surface. Before actually printing, we need to discuss the kind of negative needed to make a platinum print. The platinum emulsion is very slow and UV sensi-tive, making it out of necessity a contact process. The negative has to be the same size as the print you wish to make. A large camera can be used to make the negative, or an enlarged negative can be generated. Platinum doesn't have a straight line response to light as some claim, but it does have a much gentler toe and shoulder than silver resulting in more luminosity in the print. It is also softer, requiring a negative with more contrast than a negative made for silver printing. The smoothest platinum prints are made from emulsions with no added con- trast agent so the ideal negative would be made with this in mind. While the paper type and sizing does affect the printing speed and contrast of the emulsion, most papers work well with a negative that has a contrast range of 1. 7 or slightly less. Pyro, the traditional developer for platinum printing negatives, works well but requires a negative with a density range that would work well with a grade Ilh to grade 2 paper, because of the self-masking nature of the Pyro's yellow stain in this blue light sensitive process. Printing times will be approximately twice as long over that of conventional negatives..

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