The Joy of POP #4: Controlling Humidity

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This is the fourth 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 (POP) works because the ammonium-based sensitiser traps enough water molecules for the image-forming reactions to occur under UV light. By contrast, the traditional developing-out process needs a developer to complete the reactions.

The amount of water trapped by the sensitiser (i.e. its humidity), is super-critical to the process. To control the process, and be able to make consistent fine prints, you must first learn how to control sensitiser humidity.

Failure to control humidity is the primary cause of the printing-out process’s reputation for being difficult. This is sad because all it takes to control humidity adequately is a fairly basic understanding of the topic and some simple working practices.

Principle One: Room temperature and ambient air humidity are important but not super-critical so long as they are not extreme, and so long as you are not using them to control sensitiser humidity. Aim to keep a stable ambient environment, and adapt your paper drying process to your environment.

Principle Two: The humidity of your sensitiser will slowly match the humidity of the air that it is in contact with. If the air is drier than the sensitiser, then the sensitiser humidity will drop. If the air has higher humidity, then the sensitiser will moisten.

Principle Three: Everything I said in my book about drying paper remains true. You want to dry the paper for sufficient time for the sensitiser to bond to the paper fibres, but not so much time that the sensitiser gets absorbed too deeply into it.

This is my approach to sensitiser humidity control. The instructions assume you are using Herschel paper, and you may need to adapt timings if you are using something else.

  1. I use a dehumidifier to keep my darkroom ambient humidity between about 56% and 64% RH. I prefer the lower end of this, but that’s difficult in the summer. The room temperature varies between 18°C in the winter to about 24°C in the summer.
  2. When I have finished coating my paper I leave it to rest for 10 minutes in the dark. This allows the sensitiser to bond properly with the paper fibres.
  3. What I do next depends upon the tones I want and whether I am using palladium or platinum.

I finish the drying with one of the following three approaches:

  • Use a cool hair drier to finish off the drying, and then re-humidify with a few minutes in a humidifying tank over water (see my book page 73, and summarised below) – this produces the coolest tones
  • Use a cool hair drier to finish off the drying, and then let the paper rest for another 20 minutes in the dark – this produces the warmest tones, but at the cost of a little Dmax
  • Allow the paper finish drying in the ambient air for another 20 minutes (total time = 30 minutes) – this produces slightly warm blacks and a good Dmax

The contrast of the sensitiser varies slightly with each of these so it is important to stay consistent. Palladium also tends to a wider range of tones than platinum – from very warm at low humidity to blue/black at high humidity.

My humidifying tank is simply a darkroom tray with a couple of centimetres of tap water in it. I attach the dry paper to a sheet of board and lay it over the top of the tray for three to ten minutes. The exact time depends upon the room temperature and whether I want a neutral black, warm black, or cold blue/black. More time leads to colder, bluer blacks and higher Dmax.

Once the paper is dried/humidified (whether by resting or in a tank) I use it immediately. Any delay will change the sensitiser humidity and mess things up.

That is all there is to humidity control. If you follow these steps then you should have consistent prints. Remember that you will have to adjust your process depending upon the paper used.

Your next step is developing and clearing, which I will cover in a future post.


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: