Blue fog in cyanotype highlights

Owned by Break/Fix — the cyanotype that went wrong, whose first case works Ware’s algorithm in the order this entry lists it and adds the causes a bench supplies rather than a bottle: a working humidity near 55 per cent and the hour a coated sheet needs to dry in the dark; the fluorescent tube, whose ultraviolet output Ware tells you to avoid; and a first bath carrying iron from the prints before it, since iron(III) with hexacyanoferrate(III) is a powerfully oxidising pair whose product is insoluble. That page names this entry as carrying the full cause list and the hexametaphosphate rescue. It was written earlier from Ware’s monograph and the encyclopaedia entries built from it.

What you see

Tone where there should be none. The highlights of a cyanotype carry a pale blue that the negative did not put there, and in the worst cases the coated but unexposed border is blue as well.

Ware’s first instruction is about how to look, and it is the reason the fault is diagnosable at all: coat a margin, mask part of it during exposure, and leave a strip of paper uncoated. Then you have three references on one sheet — uncoated paper, coated-and-masked paper, and the image — and the comparison between the first two is the measurement.

Fog is not stain. Ware insists on the distinction and gives the test: fog is unwanted image substance and is grey-blue; stain is unwanted other residual chemicals, especially iron salts, and is yellow. Two different faults, two different colours, and the yellow one has its own entry.

Likely causes

Ware’s six, in his order:

1. The sensitiser is already decomposed.
2. There is a hostile chemical in the paper, reducing the sensitiser.
3. The safelighting is inadequate under the conditions of working.
4. The mask or negative is not dense enough in its maximum value.
5. The wet chemistry is faulty, chemically reducing the sensitiser.
6. The clearing procedure is inadequate, leaving residual iron — which is the yellow fault rather than this one, and is why the algorithm ends by asking about stain.

What is happening: the chemistry and physics

The cyanotype’s whole photochemistry is a reduction: light reduces iron(III) to iron(II), and iron(II) meets hexacyanoferrate to precipitate Prussian blue. Anything else that reduces iron(III) makes the same pigment in the same place, and light is not required.

That single sentence generates the whole cause list. A reducing impurity in the paper does it. A sensitiser that has already begun to decompose has done part of it in the bottle. A wet-processing bath carrying a reductant does it after exposure. And light that was not supposed to be there does it by the intended route at the wrong time.

Ware names one specific and common route and its remedy. Chemical fogging — “greening” — of the coating due to paper impurities may often be prevented by adding citric acid to the sensitiser before coating, to a final concentration of about 2 per cent: one drop of 0.05 cc of a 40 per cent w/v citric acid solution per cc of sensitiser. He adds the caveat that matters: do not add citric acid to the stock, because it shortens the shelf-life.

There is a second acid trap, and it runs the other way. In the wet processing, Classic and Simple cyanotypes can only tolerate a very weak acid — under 1 per cent citric — because stronger acids will cause blue chemical fog in the highlights. A mineral-acid developer is right for the New cyanotype and wrong for these.

And there is a fifth cause that lives in the tray rather than in the paper or the bottle. Ware records that iron(III) getting into the developing bath fogs the background blue, because iron(III) and hexacyanoferrate(III) together are a powerfully oxidising pair whose reduction product is insoluble — which is Ware’s algorithm’s cause five arriving by the commonest practical route. A first bath that has done a few prints is carrying exactly that iron, which is why the published capacities are small: a litre of 1 per cent citric acid is limited to two or three prints for the simple cyanotype, and the New Cyanotype’s acid bath to six to ten 10 × 8 prints per litre, not to be re-used. Those are limits rather than suggestions, and Part XXI’s break/fix page works the differential that separates this cause from the four above it.

!The confirming test: Ware's algorithm, which asks when the fog appeared

Compare the coated-but-masked border with the uncoated margin, and answer in sequence.

Question	If yes
Fog apparent on coating?	Sensitiser decomposed, or a very hostile chemical in the paper
Fog appears during drying? Then: fog if dried in total darkness?	Yes — hostile paper chemical. No — faulty safelight
Fog appears after a print-out exposure?	Mask or negative not dense enough
Fog after wet processing?	Sensitiser decomposed, faulty safelight, thin mask, or faulty wet chemistry
Stain of sensitiser after the wash?	The clearing process is inadequate
Stain in uncoated areas of the paper?	The wet chemistry is contaminated

The stage at which the tone first appears is the diagnosis. That is the whole design of the algorithm, and it is why the border and the margin have to be there before anything goes wrong.

Ware adds one refinement for the fourth cause: include a small area of high UV blocking — Rubylith — beside the negative, and compare it with the negative’s own maximum density. If the Rubylith stays clean and the negative’s shadow does not, the negative is the fault and not the chemistry.

And note his warning about development processes: fogging due to a faulty safelight may not be visible until wet processing is complete.

Diagnostic questions

• Is the tone blue-grey or yellow? Fog against stain, and they have different cures.
• What is the paper, and is it buffered? An alkaline-buffered paper is hostile to this chemistry twice over — see alkaline bleaching.
• How old is the mixed sensitiser? The mixed classic sensitiser has an inconveniently brief shelf-life, and the citrate stock grows mould within a week or two — see a sensitiser gone cloudy.
• What acid is in the developing bath, and at what strength? Under 1 per cent citric for Classic and Simple; a mineral acid belongs to the New process.
• Was the coating dried in the dark? Ware’s second branch turns on exactly this.

Corrective action

A fogged print can sometimes be cleared, and Ware gives the reagent. Because solutions of high ionic strength promote peptisation of Prussian blue, a treatment in sodium hexametaphosphate at 5 to 10 per cent can be used to remove the blue in the highlights of a fogged cyanotype. Sodium hexametaphosphate has an encyclopaedia entry.

It is a trade, not a repair. The same peptisation removes image substance from everywhere else as well, so the highlights clear at the cost of density in the tones you wanted — which is the peptisation fault applied deliberately.

The better answer is the citric acid, added to the working sensitiser and not to the stock, and a re-coat.

Prevention

Choose an unbuffered paper, and test it before committing to a box.

Add citric acid to the sensitiser at the point of use if the paper needs it, at Ware’s 2 per cent.

Mix the sensitiser fresh and dry the coating in the dark.

Build the references into every print: an uncoated margin, a coated masked border, and a scrap of Rubylith beside the negative. They cost nothing and they are the difference between a diagnosis and a guess.