Cyanotype and toning

AbstractIn this guide you will discover how to transform the classic Prussian blue of cyanotype into warm, cool and neutral tones through tannic-acid toning. You will see, step by step, the bleaching phase, how to modulate the colour between brown and black, the final wash and the alternatives to tannins. The explanations come from someone who works with these compounds professionally: you will understand why each reaction works, not just how to repeat it.

The Prussian blue of classic, traditional cyanotype is a very elegant and refined colour, but not everyone likes it spread across the whole image.

That is why ways have been sought to change that typical cyanotype colour: toning.

Toning is dictated mostly by the artistic taste of the author or the audience, but there are also cases where it is used out of the choice or need to protect the print for longer preservation (as with silver-based techniques).

I have worked with toning using tannic acid, one of the many polyphenols that, when combined with iron, create coloured compounds.

The interaction between iron and tannic acid has been known for centuries. Medieval scribes already used it when they copied sacred texts by hand — texts known to everyone, some still well preserved today.

The ink they used was made from an extract of oak galls, a source very rich in tannins, plus ferrous sulfate, gum arabic and a drop of alcohol to preserve it all. The result was a very dark colour between brown and black.

This is the same principle I use, but applied to cyanotype.

Toning requires a first bleaching phase with a basic/alkaline compound, followed by treatment with tannic acid. The different colours you see on this page depend on how this bleaching step was carried out.

The bleaching phase

This phase is essential because our tannic acid will not react with Prussian blue. I have not understood exactly why — perhaps because the iron’s binding sites are already taken up by the cyanide groups. Or perhaps because Prussian blue, having a cubic crystalline structure, prevents the bulky tannic-acid molecule from binding the iron atoms correctly. Either way, you must destroy the Prussian blue and turn it into another iron compound.

Once you have your blue image, dried naturally or treated with hydrogen peroxide, you must treat it with a strongly alkaline solution. What happens is the destruction of the Prussian blue to generate iron hydroxide Fe(OH)₃. Iron hydroxide is strongly insoluble in water, so it stays trapped in the paper’s mesh and takes on a soft yellowish-orange colour. The compounds used to achieve the result vary depending on the colour you want. We’ll see this in the next sections.

Toning toward warm brown

In this first toning we treat with sodium hydroxide 0.5–2% (NaOH) or with sodium carbonate at 4–6% (Na₂CO₃, absolutely not to be confused with sodium bicarbonate). The result is identical in both cases. Note that with sodium hydroxide a lower concentration is used, since it is a much stronger base than sodium carbonate. At these concentrations the print bleaches to a soft yellow in about 30–60 seconds. You can then finally treat with tannic acid at a concentration between 1–5%. I have seen crazy tannic-acid concentrations used on YouTube: I can assure you that beyond this concentration it serves no purpose. To this end I recommend buying pure, good-quality tannic acid. Generally, for toning you should soak your print in tannic acid for 5 to 10 minutes; beyond that your image will not improve. In this kind of toning, with complete bleaching to a soft yellow, you get a warm light brown. Despite what is written online, you will lose a little contrast compared with the blue version.

Toning toward cool black/violet

If you want to improve the image’s contrast and obtain more neutral tones, you need a trick. As we saw earlier, Prussian blue does not react with tannic acid. The trick is to convert only part of the blue into iron hydroxide. Initially, when we treat with tannic acid, we’ll have two overlapping colours: warm brown and Prussian blue. The result will be a more or less cool black depending on how much iron hydroxide you generated. To do this you cannot use the compounds I mentioned above, because they are too strong and the reaction is too fast to be modulated accurately. Sodium bicarbonate is a weaker base than sodium hydroxide and sodium carbonate, and the reaction will be much slower. What you should do is a wash of a few minutes with bicarbonate (1–5%) without fully bleaching the image. After a short rinse in water, add tannic acid at the same concentration and times as above.

Clearly, depending on when you stop the bicarbonate reaction, you can modulate the colour, making it more or less cool.

Washing after toning

Tannic acid has a nasty flaw: it tends to yellow the paper. So you must wash very well in water — and even then it will never be quite enough. I personally wash in running water for a few minutes and then with an automatic agitator for another 30 minutes.

Alternatives to tannic acid

As I said at the start, this interaction with iron is not unique to tannic acid: more or less all polyphenols produce coloured compounds with iron. If you want pure compounds you can buy, gallic acid is certainly a valid alternative. Chemically speaking, tannic acid is nothing but several gallic-acid molecules joined together. I have not tested it personally, but being a smaller molecule you would need much higher concentrations than those given above to obtain the same effect. Online you’ll also find various recipes with homemade polyphenol extracts; I have never looked into it, since you’ll hardly get reproducible results with such extracts.

In the photos below I have tried to be as faithful as possible in reproducing the colours, so you can understand the differences you can obtain. This method is just one of many historical procedures that ennoble the image; you can explore the full picture in my section on Alternative Processes.