Chemical keeping properties?



I’m fairly new to PT/PD printing and would like to know the capacities and keeping properties of some of the chemicals used in both coating the paper and developing the print. The two chemicals I’m seeking capacity and keeping property for is ferric oxalate sensitizer and potassium oxalate developer. Besides the numbers of prints that can be coated and processed (square inches) are there things to look for regarding exhaustion of these chemicals and are there steps to be taken (such as filtering) to maintain the solutions? I’ve looked at a great deal of the available literature but most of it is fairly vague on this issue (unlike traditional film processing information). I’ve also noted when watching some of the many YouTube videos that some practitioners are using print developers that appear almost black when poured? Any advice or knowledge you can share would be appreciated.


Potassium Oxalate developer does not exhaust, unless it gets contaminated. It should be replenished regularly with fresh developer by topping off the storage bottle as needed. It gets darker with use as it becomes loaded with metal. Periodically filter it through a coffee filter to remove larger particles and any other suspended debris that may have accumulated. You may find greenish crystals in the bottom of the bottle. This is a sign that the solution is saturated. They are not a problem, but if you want to get rid of them pour the developer into another container temporarily and fill the bottle with water. The crystals should dissolve after a while. Then dump it, or maybe see if it still works as a developer.

Ferric Oxalate sensitizer, at least the B&S kind which is what most of us use, has a fairly short shelf life, typically 6 weeks to 6 months in liquid form. As it ages, ferric ions become ferrous which is the most common source of chemical fog (as opposed to light fog) in the Pt/Pd process. Because ferric oxalate is an “ill-defined compound”, it’s behavior can be a little unpredictable. If you are buying it in liquid form, buy 25ml bottles, and only what you need. If you are mixing from powder, unless you are doing a lot of printing, only mix up 25ml at a time. Unless your printing volume is fairly high, my recommendation is to buy it dry pre-measured in 25ml bottles.

As for coating volume, I can give you some rules-of-thumb but don’t be surprised if you need to adjust them a bit for your conditions. The volume of solution needed to coat - let’s say an 8x10 image area (~ 80square inches) - is highly dependent on the absorbency of the paper. In general, papers with a hard smooth surface and heavy sizing (Hahnemuhle Platinum Rag is a good example) require less solution than those with a softer and/or more textured surface, or less sizing. I find most papers require 1.5 - 2ml of solution for this image size. In drops this translates to somewhere between 12 and 16 drops each of ferric oxalate and metal salts. Drops are an imprecise way of measuring which I abandoned a long time ago.

There is some information you may find useful on my website.



Terrific response - very helpful answers to my questions. There is one question I have that was not included in the original post and that regards the keeping properties of the clearing bath. I’m using a standard EDTA bath (1 gallon water, 220 g EDTA and 100 ml Permawash). What is the capacity of this bath - how do I know when to toss it and mix a fresh batch? Thanks.


When it gets yellowish (after 10-15 sheets or so) cycle it in as your “first” clear tray and keep fresher clear in your second and (if required) third trays. Then just keep the cycle going.






I’ll expand on Walker’s answer a bit.

You should use 3 consecutive trays of clearing bath. I know there are some who think 2 is sufficient, but not all of what is happening in the clearing process is visible. If you want to be sure you are getting full clearing, the best practice is to use 3 baths. As soon as 1 print has been in the first bath it will appear slightly yellow. More prints, more yellow. Eventually the 2nd bath will start to show some yellow. At that point dump the 1st, the 2nd then becomes the 1st, and make a fresh 3rd. The 3rd should never have any trace of yellow. For most papers 5 minutes in each bath is good. Some papers clear slower than others so it may be necessary to extend the times a bit.

I’m a bit wary of Perma Wash as a reliable clearing bath since it is a secret proprietary formula. It may work but I don’t like not knowing what is in it. I have heard that the key ingredient may be ammonium sulfite. Kodak Hypo Clear is composed primarily of sodium sulfite and sodium metabisulfite. With palladium, EDTA alone is insufficient. Sodium sulfite swells the fibers (ammonium sulfite probably does the same), but you still need an acid for thorough clearing. Here is the formula I have been using for many years with good results:

  • 1 liter water
  • 1 tablespoon EDTA
  • 1 tablespoon Citric Acid
  • 1 tablespoon Sodium Sulfite
Mix the ingredients in the order shown.

I use 2 liters in 16x20 trays, and rotate after 4-6 prints. If you use more solution you can probably do more prints per rotation.