Cyanotype

Cyanotype is a simple copying technique with great possibilities. It was invented by John Herschel in 1842, thus just three years after the discovery of photography. With the new technique he created impressive photograms of bird feathers. In the 1850s photographers used cyanotype to make contact copies of negatives made with a wet collodion process. This simple technique allowed them to easily make contacts in the field. The British photographer Anna Atkins used cyanotype in 1843 to illustrate her book on algae. It was the first book of all with photographic illustrations and both the pictures (photograms of algae) and text were made with cyanotype.

Several American firms made paper for cyanotype commercially from about 1870 to 1930. Despite this, cyanotype never became widespread for creative photographic purposes. The reason was probably its bright, even garish blue colour, which was suitable for only a few motifs. Above all, it was not suitable for portraits and landscapes, then the most widespread themes.

The colour shade is of course relatively easy to modify by the selection of the shade of paper on which we are copying (for example, chamois). Generally, however, we can change the colour into a quite pleasant tonality using tea or coffee. Pictures toned with strong tea are already much less obtrusive and suitable for most motifs.

Herschel himself saw a use for cyanotype in the copying of mathematical tables. In fact, in the end the principle of cyanotype was used most for copying purposes. For a long time construction and engineering drawings were made in this way. They were blue in colour and therefore known as “blueprints”.

Have the advantage of cyanotype is a simple process of processing copies after exposure. Development, fixing and washing out is done in one single task: washing the copy in water.

Since the technique was easy and self-explanatory, it was formerly popularly used in teaching programs for children. Favourites were photograms of various leaves, bird feathers and everyday objects. Anyone meeting the principle of a photographic image face-to-face for the first time is almost always fascinated by how the light little by little forms a visible image of objects on clear paper and then how the image turns bright blue in ordinary water.

Thanks to its simplicity and low-cost, cyanotype can be a good starter for those deciding to try a historical photographic techniques. Experience gained through sensitising paper, exposure and processing will be useful later.

Principle

Cyanotype is based on the sensitivity of ferrous salts to light. The sensitive layer contains iron ammonium citrate and potassium hexacyanoferrate(II)). Daylight contains a certain portion of UV radiation and this reduces the iron(III) ions in these compounds to iron(II) ions. This produces compounds of blue – on the one hand Prussian blue – iron(III) hexacyanoferrate(II), and on the other Turnbull's blue – ferrous ferricyanide. Both these colours are bright blue and the picture is made up of their combination in the given ratio.

Durability and archiving

Both Prussian and Turnbull’s blue are very permanent and thanks to them cyanotype is very durable. They are even longer lasting than normal photographs and in this respect perhaps only platinotype is better. Herschel’s original cyanotypes look like they were made just yesterday.

It’s true that if we leave cyanotypes permanently in the light, the blue colour partially fades. If we then put the picture in the dark for several weeks, the shade returns to almost the same intensity as before. The explanation is that in the light the action of UV radiation causes reduction of iron(III) ions from the blue colour to iron(II) ions, making iron(II) hexacyanoferrate(II), which is white. If we stop the light falling, further reduction is limited and we thus allow oxidation by airborne oxygen, which oxidises iron(II) ions back to iron(III) ions, once again causing blue.

We must be of course careful with materials in which we place the pictures. Cyanotype cannot stand an alkaline environment (pH over 7.5), the picture will break down. We cannot use papers with an alkaline reaction for storage – i.e. paper with an alkaline buffer, labelled as "with an alkaline reserve".

Working procedure

Stock solutions:

If our water contains more iron (or rust from the water pipe), it’s better to use distilled water. Iron can cause stains during paper sensitisation.

Iron ammonium citrate exists in both brown and green form. The green form is more suitable for photographic purposes, since it is more sensitive.

Potassium sodium hexacyanoferrate (blood-lye salt) is not classified as poison (the cyanide group is bound in a complex compound), it is, however, a substance damaging to health.

However, be warned: if potassium sodium hexacyanoferrate gets into contact with strong acids, extremely poisonous cyanogen is released! If, for example, we have sulphuric, hydrochloric or acetic acid in our laboratory, we should keep them safely separated to avoid mixing with hexacyanoferrate in the event of the glass vessels breaking. The main acidic solution we use in photographic work is the acid stop bath. Though it is somewhat less concentrated it should not come into contact with blood-lye salt.

A stock solution of hexacyanoferrate (B) has unlimited durability in a closed the dark bottle. A stock solution of iron ammonium citrate (A) is durable in a bottle with limitation: if stored at normal temperature a layer of mould will form on its surface after several weeks. If, however, we remove it carefully (for example, with a piece of cotton wool on a stick), the solution will continue to work without change.

Working sensitising solution:

We mix together the same part of solution A and B. The ready working solution will be good for about a week, though there is no reason to prepare a larger amount for stock purposes.

We should may be working solution about a day after preparing the stock solutions, since the stock solutions need several hours to "mature".

Paper for sensitising

For test, presentation or training purposes when we are exposing mostly photograms and we don’t need completely clear and lasting colours, almost any paper will do. Since the paper is sensitised and exposed quite easily, we can easily repeat any unsuccessful pictures.

If we are more demanding or we want to have exhibition quality photographs, we’ll need to choose paper more carefully. Most critical are papers to which alkaline materials have been added in the process of production (buffers). It has already been said that the picture breaks up with the action of alkalis, so cyanotype made on alkali paper will be less durable. Alkaline substances (usually calcium carbonate) are today added to most normal papers intended for creative purposes – in order to neutralise residual acidity of the paper.

Cheaper papers intended for copying or packaging purposes do not contain alkaline substances and may even be acidic. An acidic environment will probably not affect the durability of the picture, but cheaper papers may contain various other additives or impurities (particularly if recycled) which may in time react with the blue colour. As a result of impurities some papers produce an ugly grainy image immediately after copying.

The best are papers with neutral pH (6.5 to 7.5) without additives. They can be made of cotton or cloth, but also from high purity wood pulp.

Suitable paper for cyanotype should also be sufficiently sized so as not to soak up solution too quickly during sensitisation. If the paper is not sized enough it quickly soaks up applied solution and we have to use more. The colour also easily washes out when developing on such paper.

At the National Technical Museum in Prague we tried various types of paper at hand. Almost all types were used for photograms.

We achieved the purest and deepest blue on paper supplied by Emba for photography archives (see PhotoArt No. 5). See Fig. 3. This paper is very well sized.

Pictures on quarto drawing paper have a less intensive picture and require more exposure.

Pictures on drawing card were somewhat more distinctive.

We even tried normal A4 office copying paper. The shade was very good (Fig. 2 ), but the paper was very fragile, in the course of processing it became soaked and when we pegged it out to dry it ripped under its own weight.

We even tried two types of yellow packing paper. They were quite good for photograms, but the blue had a slightly more dirty tinge, and since the paper was not sized, the colour was partially washed away during processing.

We achieved very nice results with chamois coloured paper (card intended for creative purposes – Fig. 4).

Paper sensitising

Simple application with a brush as described in PhotoArt No. 5 suffices for cyanotype. The brush should be as soft as possible, so it need not be wet much and greater force is not needed – a hake brush is very good, made of goat hair.

Since the sensitising solution is very sensitive to traces of iron, it’s important for the fibres not to be fixed in a metal holder.

A foam plastic brush is quite sufficient for some papers.

Since the sensitising solution is quite cheap, there’s no reason to use a glass rod for sensitising.

With some papers we can achieve a more intensive shade, if we apply the solution twice. We use very thin coats, applying the second after the first is completely dry.

We work under the weak light of a normal bulb, we can accelerate drying with a flow of hot air from a hair dryer.

The layer on the dried paper should be light yellow (with just a slight hint of green) and uniform in the light.

The sensitised paper keeps for several hours or even days, according to the atmospheric conditions. The worst thing is damp.

Exposure

Cyanotype is a relatively sensitive process, in direct summer sunlight exposure of a normally covered negative takes several minutes. Exposure times are comparable with the times needed for the salt paper process and just a bit longer than for the Van Dyke Process.

We can expose in the manner described in PhotoArt No. 7, best in a segmented copying frame, which allows us to check the growing density of the image.

If the correctly exposed copy should have a full scale of luminosity, just after exposure the highlights will be olive grey-brown, the middle tones will be dirty blue and the shadows pale blue-green. Since there will be partial solarisation in the shadows, tones could even be reversed here.

For determining the moment of correct copying most important is the density in the highlights. Since during subsequent processing in water the density in the highlights will weaken, after exposure they should be clearly darker than we want them in the final copy. Overall the correctly copied picture should appear just after exposure as significantly overexposed. It's worth doing a test on a strip of paper of the same type and sensitised in the same way as the paper on which will copy the definitive picture. We should always write the exposure time with a pencil on the back of the test and ready copy.

Processing

We process in normal water at normal temperature. Areas of unexposed coating are washed away, likewise with unexposed areas of the highest brightness, though here a weak blue delineation appears. The medium tones become bright blue. The dark areas have not so far achieved the correct densities – here the picture is partially formed of white iron(II) hexacyanoferrate(II), which forms a blue colour with subsequent oxidation by oxygen present in water and later in the air while drying.

We process the copy in water for about 10 - 20 minutes (according to the type of paper), in the interim we change the water at least five times. It is important that the released ferrous salts do not dirty the highlights. Washing-out must be sufficient for all the remaining ferrous salts to be washed out. Excess washing out may cause weakening of the picture. This can occur especially in the case where the water is alkaline. We know that Turnbull and Prussian blue break down in an alkaline environment. We therefore recommend acidifying the washing water with a few drops of hydrochloric acid or a teaspoon of citric acid.

Here at the National Technical Museum we have slightly alkaline tap water. Therefore we process the copy in water acidified by a few drops of hydrochloric acid. Thanks to that the blue colour is much more intensive than in water which is not acidified (citric or acetic acid can be used).

After processing the damp copy is still somewhat pale, the definitive correct density is achieved after drying, sometimes even after several days.

Drying

We dry the pictures in the normal way, hung up, fragile papers are better laid on cloth.

The picture attains its definitive form due to oxidation in airborne oxygen after drying.

There is a method of acceleration with hydrogen peroxide, where we can see the picture in almost definitive form at the start of processing (add a little 3 % hydrogen peroxide to the developing water).

Reducing an overexposed picture

If the picture is too dark, we can reduce it in 0.5 - 1 % sodium carbonate solution. We should interrupt reducing earlier rather than later; if necessary, we can repeat it after drying.

Toning

We can achieve a quite pleasant shade by toning with strong black tea or instant coffee. Tea and coffee contain tannin (tannic acid) which changes blue to blue-green and simultaneously colours white paper to yellow. The picture is overall much nicer after toning (see Fig. 5). We can use tannin directly, instead of tea or coffee (see Fig. 6).

A range of methods have been invented for toning cyanotype to black, brown, yellow, red or violet. Mostly this concerns a two-stage process. First the picture is bleached with an alkali (e.g. ammonium hydroxide or sodium hydroxide) and then toned with tannin. The reverse process is possible – first a tannin bath and then the alkali. There are also recipes where the picture is transferred alternately from bath to bath. Thanks to the many options we can achieve many variants of various shades. For example, here is one of the basic processes for toning to brown (used in the Fig. 7):

First we bleach the photograph in solution I, afterwards we wash out for several minutes and dip in solution II, in which it browns. Finally we wash out for several minutes.