Straight out of the rinse, a fresh cyanotype often looks duller than it will become, because part of the colour only reveals itself as it dries, over the hours, as the air oxidises the pigment. Hydrogen peroxide does just one thing: it triggers, all at once, the oxidation that you would otherwise wait for air and time to bring about.
This page sticks to that specific technique: the hydrogen peroxide bath that intensifies the blue just after rinsing. It follows on from our guide to rinsing a cyanotype and stops where colour toning begins, which aims to change the hue rather than revive the original cyanotype blue.
Why a cyanotype darkens as it dries
To understand hydrogen peroxide, you need to go back for a moment to the chemistry of the process. Under the effect of the sun’s UV, the iron changes state: iron in oxidation state III is reduced to iron II, which then reacts with the ferricyanide to form Prussian blue, the insoluble pigment that draws the image.
Potassium ferricyanide has low toxicity at the amounts used in the process, but it can release cyanide on contact with an acid, or under UV light when in solution.
At the time of rinsing, this reaction is not quite complete. Part of the Prussian blue is still in a pale, partly reduced form, and it is oxygen in the air that converts it to its full, stable blue form, which explains why a print keeps darkening for several hours after drying, sometimes until the next day.
This slow oxidation is the natural course of things, and a print that has been rinsed and then dried in the shade will eventually reach its final colour without any intervention. Hydrogen peroxide therefore does not add any blue: it simply brings the print straight to the end of this oxidation.
What hydrogen peroxide does to the blue
Hydrogen peroxide is a mild oxidising agent that you can buy at a pharmacy. When the still-damp print is passed through a bath containing a little of it, its blue intensifies before your eyes within a few seconds, immediately reaching the density it would only have reached after a night of drying.
The effect is spectacular to watch, but you need to understand exactly what it does. The bath does not create a colour the print would never have reached: it only brings forward the final state, and a correctly exposed and rinsed cyanotype will give roughly the same blue once fully dry, with or without hydrogen peroxide.
Its value is nonetheless real. It lets you judge the result immediately when you are running test after test and do not want to wait until the next day to know whether an exposure was right, and it offers the satisfaction of seeing the deep blue appear all at once, a moment much enjoyed in workshops, especially with children.
What dilution of hydrogen peroxide to use
The bath is made up very dilute, because hydrogen peroxide acts quickly and strongly. The hydrogen peroxide (in French) sold in pharmacies is most often at 3 per cent, or 10 volumes, and it is this common form that serves as the basis for the dilution.
In practice, a tablespoon of this 3 per cent hydrogen peroxide in a litre of clean water is more than enough. Keep to modest proportions, in the order of a few millilitres per litre, because a bath that is too concentrated does not intensify the blue any further and only risks damaging the paper and the sensitive layer.
It is better to start weak, because the effect quickly levels off and hydrogen peroxide never does more than bring the pigment to its stable form.
The step-by-step method after rinsing
The hydrogen peroxide bath comes at the very end, once the print has already been rinsed, because rinsing in clean water remains the step that clears out the unexposed iron salts and actually fixes the image. Hydrogen peroxide does not replace it and only makes sense after it.
- First rinse the print in clean water until the water runs clear and no longer carries any yellow-green.
- Immerse it in the dilute hydrogen peroxide bath, for anything from a few seconds to a minute, to see the blue darken.
- Finish with a short rinse in clean water to remove any remaining peroxide.
- Leave it to dry flat, out of direct light.
The result is then very close to the final colour.
Handling an already rinsed print by its margin in a finishing bath. The colour shown is not a measure of intensification.
Limits and precautions of the bath
Hydrogen peroxide will not rescue a failed print. An underexposed cyanotype will stay pale and washed out even after the bath, because oxidation only reveals the blue actually formed during exposure. If your prints consistently come out light, the remedy is a longer exposure.
As for precautions, 3 per cent pharmacy hydrogen peroxide is a mild product at this dilution, but you should still work with gloves and in a well-ventilated room, as for the rest of the process. Avoid the higher concentrations sold for other uses, which are pointless here and harsher on both skin and paper.
A useful shortcut, not a secret
Hydrogen peroxide is a handy tool if you want to see straight away where you stand, and that is why it has its place in the cyanotype toolkit. It does not turn an ordinary print into an exceptional one: it only cuts short the wait for oxidation and gives you an immediate reading of the final blue.
Where hydrogen peroxide pushes the blue towards its full density, the opposite technique lightens a cyanotype that is too dark and whose shadows have blocked up, this time using a mild alkaline bath.
Above all, a deep blue remains the result of well-sensitised paper, accurate exposure and careful rinsing. To practise these techniques with the two salts and a suitable paper, the Sténoflex cyanotype kit brings together the ferricyanide, the ferric citrate, the watercolour paper and the pipettes for your first prints.
Éric Marais — Photographer, author of two books on pinhole photography (published by Pearson) and creator of the Sténoflex. He has run introductory workshops in alternative photography, from pinhole to cyanotype, from the very beginning.