Cyanotype: a chemist’s complete guide

AbstractIn this guide to cyanotype you will find everything you need to make your first prints with Herschel’s classic technique: the right reagents, the choice of paper, exposure control and development. The chemical explanations are written by someone who works with these compounds professionally — you won’t find ready-made recipes, but you will truly understand why each step works (or doesn’t).

Cyanotype was invented by a “mad” chemist named Sir John Herschel in 1842. As the name suggests, this technique produces blue photographs (Prussian blue). As we will see in other articles, it can later be toned toward other colours.

It is said to be the easiest of the historical printing techniques to learn. The chemical method itself is not particularly complex but, if you want images whose contrast and tonal relationships resemble what you see on your screen, you will find it is not as easy as it seems.

This technique has since been modified several times in recent years; the best-known modification (not the only one) is the one made by the British chemist Mike Ware. That variant uses potassium dichromate and oxalate salts, whose toxicity profiles are ill-suited to a home laboratory. For this reason I wanted to start from Sir John Herschel’s original technique, which instead uses substances that pose no health risks.

The reagents used in cyanotype

The reagents used are ammonium ferric citrate (C₆H₈O₇·xFe(III)·yNH₃) and potassium ferricyanide (K₃[Fe(III)(CN)₆]).

Ammonium ferric citrate is the photosensitive salt that makes the reaction work. It is also used in other historical prints such as the Van Dyke; in others it is replaced by ammonium ferric oxalate because it penetrates the paper fibres more easily. Oxalate salts, however, are more toxic than the corresponding citrate salts, which is why I recommend sticking with the original salt.

Potassium ferricyanide may worry you because of its name; in truth the cyanide group is strongly bound to the iron atom, making it barely toxic. If dissolved in strong acids it could release hydrogen cyanide, so be careful. This reaction does not occur with weak, dilute acids or in a neutral environment, so you can relax. Obviously, do not even think of doing all this in your kitchen, as I have seen in some videos on YouTube.

Preparing the emulsion

In the original cyanotype the two salts are dissolved in water separately and mixed in equal parts only just before use, because together they are unstable. Online you will often find recipes for making as much as 100 ml of solution: this is done for “mathematical” convenience, but it will be a big waste of money if you follow it to the letter.

These solutions tend to grow mould and should be stored in amber jars. To give you an idea, to print a 20×30 cm photo you will need between 1.5 and 2 ml of the mixed solution. I recommend preparing small quantities: I personally prepare 10 ml of each. When mixing, using a dedicated graduated plastic pipette, I mix 1 ml of each solution in a third container.

Using Herschel’s proportions exactly can be tricky, since at the time a “brown” variant of ammonium ferric citrate was used. Since the late 2000s a “green” variant has been produced that allows faster development times. The green ammonium ferric citrate is generally used at a concentration of 20 to 26% w/v; potassium ferricyanide at 12 to 16%. You generally work in a large excess — we will understand why in the next chapter.

The chemistry of the cyanotype reaction

The reactions behind palladium or platinum prints are chemically predictable: the iron salt reduces the metal by redox into its water-insoluble form, which stays trapped in the paper. The reaction that takes place in cyanotype, however, is not obvious. At first glance you have two iron salts that should not react with each other. I think — though I am not certain — that Herschel was very lucky: it is no coincidence that we speak not of an invention but of a discovery.

The Prussian blue molecule combines two different oxidation states of iron (an uncommon thing) and, above all, is insoluble in water, which lets it deposit in the paper fibres and form the image. To be honest it is not the only reaction that occurs: a soluble version also forms, KFe(III)[Fe(II)(CN)₆]. This is why you work in a large excess of reagents, because part of it turns into this soluble salt that is washed away in the later stages.

The paper

“Change the other parameters and you will see small differences; change the paper and everything changes.”

If I had to choose the variable that most affects the final result, I would choose the paper. You cannot use ordinary card: the paper must withstand long washes in water and be thick enough to absorb at least part of the emulsion. This is why watercolour papers work very well. I personally use Fabriano Artistico 100% cotton, with a substantial weight of 300 g/m².

There are also very expensive special papers, designed for historical techniques such as platinotype, that work well for cyanotype too: I have run several tests on Hahnemühle Platinum Rag. Comparing the two papers, with the same negative and process, I get very different results. The Hahnemühle is much lower in contrast and the whites are always veiled with a light blue; the Fabriano stands out for its strong contrast and demands care not to burn out the highlights. There is no better or worse: it depends on your taste.

You will often find papers marked “acid free”: useful for ordinary inkjet printing, but pointless for cyanotype. Prussian blue is stable at acidic pH and, on the contrary, extremely unstable at basic/alkaline pH.

The wetting agent: a blessing and a curse

A wetting agent is simply a substance that, brushed onto the paper, relaxes the fibres and lets the emulsion penetrate more deeply. For those of us who use ammonium ferric citrate instead of oxalate it becomes fairly essential. The challenge is determining the exact amount: too high a concentration will ruin the print, too low will do nothing. It is generally used at between 0.05 and 0.3% w/v.

The most widely used wetting agent is Tween 20 (polysorbate 20), a non-ionic surfactant you might find in some soaps or cosmetics. Other detergents can be used too, as long as they are non-ionic: a common anionic detergent such as sodium dodecyl sulfate could interact with the iron atoms in solution and produce unwanted results.

Exposure

Cyanotype began by exposing works to sunlight in a printing frame. As science advanced, it became clear that only UV light activates the reaction, as with all historical prints. If you want reproducible results, buying a UV lamp will be essential — you do not need to spend a fortune.

The exposure time depends on several factors:

• The lamp’s power in watts (I use a 100 W lamp)
• The distance at which you place your work from the light
• Use of a light box
• Use of a wetting agent (it can cut exposure time by up to 20–30%)
• The type of paper

Determining the time is essential, although you will soon realise you will never find the perfect time to get the print exactly as you see it on screen. At some point you will have to resort to a calibrated digital negative. The emulsified paper must be in tight contact with the negative: if you see sharpness problems, it is probably due to poor adhesion — I assure you a few clips will not be enough.

Development

Once exposure is finished, you will need to remove all traces of unreacted reagents and the soluble Prussian blue salt with water washes. I recommend an acidic solution: acetic, citric, hydrochloric acid… in truth they are all the same, only the concentrations change. I recommend citric acid because it is cheap, easy to find, not dangerous and does not smell.

The acid preserves the highlights by veiling the whites with light blue: the higher the concentration, the more the highlights are preserved, but the more contrast is reduced. For citric acid, concentrations range from 0.1% to 1%. I advise against tap water: its pH is highly variable, in some areas even alkaline, and as we have seen a basic pH damages the image. A wash in acidic water for about 5 minutes, changing the water often, will be more than enough.

Hydrogen peroxide

Once the washes are done you will see your image in a pale blue, not yet Prussian blue. To form, it needs oxidation, which happens naturally with the oxygen in the air in about 24 hours. If you are impatient, you can take some supermarket hydrogen peroxide (10–12 volumes) and dilute it 1:10 in water: oxidation will happen immediately. The final result will not change, you will only have sped up the process.

All that is left now is to let your work dry for at least a day and enjoy the result of your own hands.