Black Spots

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Close-up of a black spot

Mysterious black spots have plagued platinum printers from the early days of the medium. They tend to appear as small, dense spots surrounded by a corona or a sometimes comet-like shape. I have lost many prints to these horrible things.

After reading Mike Ware’s impressive and immensely detailed ebook on platinum/palladium printing, Platinomicon, I thought it would be good to publish the current state of my understanding of black spots.

What’s particularly galling about black spots is that they don’t appear consistently. Sometimes I have had lots of them, at other times almost none. And I think the same is true for other printers.

As with so many topics in platinum/palladium printing, there is not unanimous agreement about what causes them or how to solve them. Sometimes paper is blamed, sometimes use of metal tools, sometimes it’s the water, and sometimes other working practices.

The only area where there’s consensus is that the spots are caused by particles of ferrous iron – iron(ii) to be more specific. These particles oxidise when they come into contact with the platinum/palladium salts, reducing the salt and thereby depositing dense masses of pure platinum or palladium.

There’s also strong evidence that some sort of contamination in the paper can trigger black spots. I’ve spoken to far better printers than me who can talk at length about times they’ve received a new batch of paper and suddenly started getting black spots. Usually this is blamed on iron(ii) contamination, but I believe that it’s more likely to be a chemical contamination (see Mike Ware’s hypothesis below).

Ferric Oxalate is Part of the Problem

I have seen black spots produced by every combination of platinum/palladium chemistry, except for ammonium ferric oxalate. When I have made a sensitiser with ammonium ferric oxalate (for the printing-out process) rather than ferric oxalate (for the developing-out process), I have never seen any black spots.

Mike Ware has a plausible hypothesis for this. If the ammonium ferric oxalate is reduced before exposure, for example by a chemical contaminant, then the resulting iron(ii) complex will be soluble: it just washes away. By contrast, when conventional ferric oxalate is reduced, the iron(ii) complex is insoluble. It dissolves gradually in the developer, and produces the spot as it goes. The observable fact that the spots appear quite slowly in the developer supports this, as does their distinctive comet-like shape which is caused by the flow of developer over the dissolving spot.

If we accept this hypothesis then there could be any number of potential chemical contaminants in the paper that could trigger black spots (see Mike’s Platinomicon for further details).

But there are also likely to be issues with the ferric oxalate itself that are not related to the paper. For example, the ferric oxalate may contain trace amounts of ferrous oxalate – iron(ii) oxalate to be more specific. Dissolved iron(ii) oxalate would lead to slight fogging; but particles of iron(ii) oxalate would lead to black spots. Iron(ii) oxalate has quite low solubility in water so it’s feasible that contaminated ferric oxalate could contain insoluble particles as well as dissolved iron(ii) oxalate.

Just in case you’re wondering, black spots seem to appear with every manufacturer’s ferric oxalate; at least every one I’ve tested. And they don’t seem to be related to the age of the ferric oxalate. So it’s not that one manufacturer has a uniquely bad product.

But There’s More to it Than Just the Ferric Oxalate

I believe that all the ‘solutions’ for black spots have probably helped someone, somewhere, sometime. Many of these have become part of the folklore of platinum/palladium printing; others are definitely best practice.

For example, it’s often said that you must not use metal tools when making platinum/palladium prints. This may once have been a concern, but with modern stainless steel tools there is no reason to worry about this.

However other solutions, such as using a good water supply and keeping your tools clean, are definitely best practice. Likewise, if you find a particular batch of paper is prone to black spots, then it is sensible to abandon it and find better paper.

I have come to the conclusion that there are actually multiple root causes of black spots; to avoid them you have to tackle each of these causes.

I’m confident about three root causes:

  1. Particles of insoluble iron(ii) in the ferric oxalate (i.e. Mike Ware’s hypothesis)
  2. Particles of something in the developer (see below for my reasoning on this)
  3. Chemical contamination of the paper that creates particles of iron(ii) when
    the sensitiser is applied, dried and/or exposed

There may be more…

26/1/2018: Christer Törnkvist gave an example of a metal tool appearing to cause black spots. He found that using a rotary cutter to trim paper seemed to lead to an increase in spots. It makes sense that a rotary cutter, especially a self sharpening one, could shed particles of metal onto the paper.

Some Solutions

In my testing I have found four practices that have a marked affect on the black spots. The first is fairly obvious. The second and third are less so. When I discovered them, I thought I had solved the problem, only to later find the spots returned. So far the fourth is still working.

One: Avoid Prolonged Development Times. The spots develop over time, so a shorter development time will reduce their intensity, even if it doesn’t actually eliminate them. If I’m having a bad day then I develop for seconds rather than minutes. With a hot developer the effect on the shadows tones is minimal, but it can mean that the spots don’t fully form and can be etched off the finished print.

Two: Filter the Sensitiser Solution. My first ‘solution’ was filtering the mixed sensitiser solution using a syringe and disposable filters. This dramatically reduced the number of spots but didn’t eliminate them. However, the fact that filtering the sensitiser had at least a partial effect indicates that this is one of the causes of black spots.

Eventually I stopped doing this because the filters were cumbersome and expensive. Instead I now keep my sensitiser solutions slightly warm using a reptile heating pad. This should make them less likely to precipitate particles due to temperature changes. I also try to avoid drawing solution from the bottom of the bottles, where precipitate would most likely settle.

Three: Filter the Developer. A few years ago I realised that fresh developer tends to produce fewer black spots than old developer. I started filtering my developer and the frequency of spots dropped. I have incorporated this as a standard part of my working process: I clean my developer by filtering it through a coffee filter every time I pour it into or out of a storage bottle, and after every print.

Four: Add Oxalic Acid to the Sensitiser. So far, I’ve not been able to explain why this works, but it does.

I have found that simply adding a few drops of oxalic acid to my sensitiser eliminates the remaining spots. This surprises me because my ferric oxalate already has quite a lot of oxalic acid in it. There seems to be something special about adding it at the time of mixing the sensitiser.

Here’s my mixing sequence:

  1. Take a clean shot glass
  2. Add two drops of 25% Tween 20 for an 8×10 (if required)
  3. Add four drops of 10% oxalic acid
  4. Switch off the white light, and switch on a low power red LED
  5. Add 1ml of ferric oxalate
  6. Add 1ml of platinum solution
  7. Add a few drops of palladium solution depending on the warmth I want in the print
  8. Swish to mix, and then use

If I omit the oxalic acid then I’m likely to get some fine spots; if I use it then I get none. This works with ferric oxalate I’ve purchased and also the stuff that I’ve made myself, so I don’t think it’s specific to the ferric oxalate manufacturing process.

A Final Observation

I can’t work out whether this observation is real or imagined, so take it with a pinch of salt.

Sometimes it seems that black spots very much prefer to appear in highlights than in mid-tones or shadows. I have seen them in mid-tones and shadows, but nowhere near as many as I’ve seen in highights. This could be the nature of my prints, which tend to be high key; or perhaps a psychological bias. But it seems to happen way to frequently to be my imagination.

If this is a real pattern then it suggests that there is something about black spot formation, that is encouraged by high key areas. For example, obviously the highlights get significantly less UV exposure than the rest of the print. This means that there is a lot more iron(iii) in the highlights than iron(ii). And it also means that the platinum/palladium salts have nothing else to react with.

If you have any thoughts about this then please reach out to me at the email address below.