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

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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.

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.

In the Ziatype variant, Ammonium Dichromate restrainer is added to the sensitiser to control contrast. Generally only a few drops are needed to increase contrast and match the sensitiser to your negative. Personally I find the approach of adding drops of restrainer to the sensitiser too cumbersome. For example, scaling between prints of different sizes is tricky: if a 10”x8” print needs 3 drops, then a 12”x16” would need 7.2 drops all other things being equal. This is particularly important if you are using digital negatives and want to proof with small prints.

My ‘eureka moment’ was when I realised that it should be possible to add the restrainer directly to a stock AFO solution, and by varying the amount of restrainer I could make different ‘grades’ each with its own consistent contrast. And that’s exactly what I have done:

Grade 0 (stock) AFO:

  • 30g AFO
  • Made up to 50ml with distilled water.

Grade 1:

  • 30g AFO
  • 2ml of 5% Potassium Dichromate
  • Made up to 50ml with distilled water

Grade 2:

  • As Grade 1 but 4ml of 5% Potassium Dichromate

Grade 3:

  • As Grade 1 but 8ml of 5% Potassium Dichromate

I buy a pre-mixed 1/6 MOL Potassium Dichromate solution, which is approximately 5%. I prefer this because not having to work with dry Potassium Dichromate makes it is a bit safer to handle.

This approach has the huge advantage that if I vary print size I only need to vary the volume of sensitiser. There is no need to calculate how many drops of restrainer to add because it is already there. If I need to tweak contrast for a particular print then I can always mix up an intermediate grade or just add an extra drop of restrainer to the sensitiser.

I use Potassium Dichromate because that’s what I can buy most easily. I expect that both Ammonium Dichromate and Potassium Chlorate will work too but I have not tested these.

I have seen no problems with the shelf life of these graded AFO mixtures. One of my graded AFO solutions has lasted six months with no problems.

So there you are: the secret of POP contrast control is simple. Keep a consistent ratio of APt to APd, and use graded AFO solutions to match your negative’s contrast.

 

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.