The Joy of POP #3: The Sensitiser and How to Control Contrast

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This is the third part of my ‘Joy of POP’ series about how to get the best out of the platinum/palladium printing-out process. Here is a link to the whole series.

The platinum/palladium printing-out process uses three sensitiser chemicals:

  • Ammonium Iron(III) Oxalate (also known as Ammonium Ferric Oxalate, or just AFO)
  • Ammonium Tetrachloroplatinate(II) (APt)
  • Ammonium Tetrachloropalladate(II) (APd)

All three of these are readily available from chemical suppliers and through specialist photography retailers.

The Ziatype variant uses a single ‘Lithium Palladium’ solution instead of the APt and APd, but there’s no benefit to this unless you want a pre-made kit. (Actually I feel that Lithium palladium makes the process harder to control because it is so hygroscopic, and it also requires a palladium-only sensitiser. But it works and many people like it.)

The sensitiser is mixed as follows:

  • Half the sensitiser is AFO
  • The other half is a mixture of APt and APd. I tend to use either 100% APd, or 85–90% APt plus 10–15% APd. Between them these offer a wide range of options for tone

It is possible to achieve some degree of contrast control by varying the ratio of APt to APd. More platinum in the mixture increases contrast (necessary for thin negatives); more palladium decreases contrast (necessary for dense negatives). Unfortunately this has the downside of varying tone and Dmax too, so I do something different.

It is also possible to vary contrast by changing sensitiser humidity. But this also changes the print’s tone, and is also hard to do, so I try to keep humidity consistent from print to print (unless I actually want a different tone).

In the Ziatype variant, Ammonium Dichromate restrainer is added to the sensitiser to control contrast. This works with other POP variants of course. I use Potassium Dichromate because that’s what I can buy most easily.

Generally only a small amount of restrainer is needed to increase contrast and match the sensitiser to your negative. For example, with a sensitiser mix of 10 drops AFO + 9 drops APt + 1 drop APd I get an exposure range of 9.5 stops (see The Platinum Printing Workshop page 46 for how this is calculated). Adding a single drop of 10% Potassium Dichromate is so powerful that it knocks five stops off the exposure range (ER); although it also turns the print blotchy so this is clearly way too much for normal printing. A drop of 5% Potassium Dichromate is much better: this reduces the ER by about three stops, which is a considerable change, but also produces good print quality. A drop of 2% Potassium Dichromate reduces the ER by about tw0 stops, and a drop of 1% reduces the ER by about one stop.

The effect of the restrainer is proportional to the sensitiser volume. In other words, a drop of 1% Potassium Dichromate in 10 drops of AFO has the same effect as a drop of 2% Potassium Dichromate in 20 drops of AFO, or two drops of 1% of course.

It is worth noting that adding restrainer has two other effects, one good and one bad. The first (good) is that a little bit of restrainer eliminates the slight stain that that is sometimes seen in platinum/palladium prints. The second effect, the bad one, is that restrainer reduces the Dmax somewhat – although this is only marginal in small quantities.

I keep several several different restrainer strengths at hand: for example 1%, 2%, 5% and 10%. Most of my prints are 8″x10″ or larger so the higher strengths are quite useful. Intermediate strengths are easy to mix if they are needed.

As usual, the effect of the restrainer may vary depending upon your paper and exact printing process, so some testing may be helpful to you – or just dive in there and test through printing.

So there you are: the secret of POP contrast control is simple. Keep a consistent sensitiser humidity when printing, a consistent ratio of APt to APd, and then use restrainer to match your negative’s contrast.

Note: In my original post I wrote about mixing differing amounts of restrainer into Ammonium Ferric Oxalate stock solutions. I have since determined that while this works, the solutions are not stable and they lose their effect after some months. So this is not recommended. 

 

If you find this series valuable, then please consider buying The Platinum Printing Workshop. Here are links to the downloadable ebook and the printed version.

Updated links for the third edition: