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.