Salted paper process

Historically the salted paper process is the first copying process based on the sensitivity of silver halide salts to light. Talbot had already used it in 1839 perfectly. His manner of photography known as calotype (= beautiful picture) was already a negative – positive process and had two phases: first, creation of a paper negative and, second, its re-copying into a paper a positive.

The negative was prepared by brushing the paper with a solution of silver nitrate and then dipping it in a solution of potassium iodide. This method created insoluble silver iodide on the paper. The paper was dried in light and then made sensitive by applying a solution of silver nitrate and gallic acid. Paper made sensitive in this way needed to be exposed in a photographic camera while still damp.

The latent picture was developed in a solution of silver nitrate and gallic acid and further accentuated with gallic acid. The photograph was fixed in a solution of sodium thiosulphate. The resulting picture on the paper was usually red-brown in colour. A paper negative acquired in this way was usually made transparent with wax and copied with the salted paper process, thus the technique we are going to consider in detail here.

The technology of the negative and positive phase differs in principle. Originally Talbot tried to create a negative by a simple salt process (sodium chloride + silver nitrate); however, the process was unusable because of its very low sensitivity. When afterwards by chance he discovered the possibility of developing that latent picture with gallic acid, exposure could be significantly reduced and the process was developed to its final form.

Where we mention calotype we mean the process as a whole, thus both the preparation of the negative and its re-copying by the salt paper process into a positive.

Where we mention the salt paper process we mean only the second phase of the whole process, thus the technique of copying a negative.

Calotype was used most in the years 1840 to 1855 concurrently with daguerreotype, over which it had the advantage of allowing more pictures from a negative. Until 1855 the salt process was the only technique by which it was possible to copy a positive from a negative (in 1855 the albumin process was put into practice). We can thus say with surety and that all photographs on paper made up until 1855 used the salt process.

Since the salt process is relatively easy, comprehensible, lucid and, at the same time, key from the historical point of view, at NTM we are planning one-day creative workshops for anyone interested from autumn 2007 (notice will be given on the NTM website).

Principle

The salt process is quite simple. A piece of paper is soaked or washed with a solution of sodium chloride and after drying is soaked or washed in a solution of silver nitrate. Silver nitrate reacts with sodium chloride to make silver chloride, which is adequately sensitive to light for the purposes of copying (silver nitrate is itself also sensitive, but much less). The paper must be treated with both substances progressively after drying. It is not possible to make them into one solution – insoluble silver chloride would immediately coagulate and sensitising with such a coagulant is not practical.

Colour of the picture

The emerging picture is usually brown in colour with a red tint. The actual tint depends on the paper used, the manner of its sizing and the formula variation.

The reddish brown colour of the silver forming the picture is typical for all direct copying (visible copying) processes. Particles of silver emerging from direct copying processes are much smaller, have a more regular form and are more finely diffuse on the flat surface than clusters of silver acquired by developing processes. The reddish-brown colour is caused by just that shape and structure of silver particles. Pictures acquired by developing (bromidic silver baryta paper) have a silver colour of almost black.

Permanence

The permanence of the picture depends on the shape and structure of the silver. If the particles of silver are fine and uniformly diffuse in the picture, they are also more affected by the effects of climate and the picture fades more quickly than with a photograph acquired by a developing process.

In first place permanence of the image depends on the quality of the execution. Most important is sufficient washing out in all phases of the process. If it is perfect, the picture will probably have long permanence, probably greater than those printed in today’s minilabs. Although many period photographs on salt papers are now quite faded, some photographs made in 1840 still look very vivid, as if they were done yesterday.

In our case, if we are using historical techniques for enjoyment, we probably won't require archive quality. If the fact that our pictures will outlive not only us, but also several generations of our descendants, makes us happy, we can tone our pictures with gold and platinum salts. With this method the silver in the picture is to a great extent replaced by gold or platinum, thus noble metal with almost unlimited durability.

Requirements:

Chemicals:

Sodium chloride – we can use kitchen salt, but to be on the safe side we should try and use salt without any additives. From our own experience we can recommend white (thus not grey) salt, which you can get from Country Life shops.

Potassium citrate – this is added to the salt solution to increase the density of the picture.

Silver nitrate – the only more expensive chemical in the whole process. At the current time we can buy 100 grams for about CZK 800 to 1 000. When we consider, however, that for 100 ml of sensitiser we need 12 g of silver nitrate, which is enough to sensitise 50 pieces of paper of 13 x 18 cm format, the price is acceptable.

We must be very careful when working with silver nitrate, best with protective glasses and rubber gloves. If we touch the sensitiser by mistake, our fingers will turn black and nothing will wash it off. We should definitely protect the eyes. The literature states that splashing silver nitrate into the eyes can even cause blindness.

Citric acid – it is added to the sensitiser of silver nitrate. Adding it makes the sensitiser durable for several weeks.

Sodium thiosulphate – a fixing substance

Sodium carbonate – an additive to the fixer

Other requirements:

A flat brush for sensitising, best a hake brush from an art shop.

Paper suitable for sensitising.

Rectangular dishes.

Thick glass with polished edges for sensitising. Plastic pegs or adhesive tape for fixing the sensitised paper to the glass.

Paper kitchen towels for absorbing excess sensitiser and drying the brush.

A 1 litre plastic measuring glass for preparing the solutions, a 50 ml graduated cylinder.

A copying frame, best with a separate base for monitoring the course of copying.

Preparation of the negative

We have already discussed preparation of the negative, so now we discuss it only in relation to the salt paper process. We will need a negative with large contrast. The sharpness of our process will not always be exactly the same. It will be affected by the type of paper we use and the method of sensitising. Luckily it is not necessary to maintain a certain precision in the sharpness of the negative as with a normal process with bromosilver papers, because here the actual mask of the picture is applied. Since we are working with direct (visible) copying processes, a clear picture emerges already in the course of copying. First the darkest parts of the picture will appear – the shadow areas. These darkened areas of the picture constrain the falling light and limit further exposure of those parts and thus further darkening. In practice this means that we can successfully copy even an unusually contrasting negative, if we expose for long enough. The negative for the salt process must have a certain minimum contrast, though if it is even greater, it will be possible to copy well with a longer exposure.

It is easiest to start with a normal positive on RC paper. We can recopy it by contact on a suitable negative material (see Photo Art No. 6). If we are preparing a negative for the first time, we will prepare it in two variations to be certain – one with a sharpness we estimate as adequate and the second with a sharpness visibly higher. In this way we will probably manage to achieve the required contrast.

Selection of suitable paper

We can’t go wrong by using Italian Fabriano Artistico 100% cotton paper or Fabriano 5 50% cotton paper.

From domestic production “Fotopapír” supplied by the company EMBA (Paseky nad Jizerou) is sufficient for the purposes of archiving photographic negatives and positives. So far it is only available in a weight of 80 to 90 g/m2. It is possible to work well even with paper that thin – the paper is well sized and remarkably firm, so there is no risk of it ripping in the wet process. If we are not going to adjust the ready pictures and we will store and view freely, thicker paper is more suitable.

A tried and tested domestic product readily available in art shops is paper known as “Grafický hlazený” [polished] (supplied by Excudit, priced CZK 26 per piece at A1 format).

Papers usually have different sides. We use the smoother side for a sensitising.

For increased picture density and greater brilliance, it is possible to size the paper afterwards. Gelatine or starch added to the salt solution is used for this. Papers sized with starch have a brown tint, those with gelatine a red tint.

Paper sensitising

1. Impregnation with a sodium chloride solution.

A solution as follows is used:

Solution A:
Sodium chloride	20 g
Potassium citrate	20 g
Water	to 1000 ml

Potassium citrate increases the density of the image and shifts the shade of the picture to red. We can substitute it with sodium citrate. If we don't have citrate, we can leave it out. The quality of the paper used and method of its sensitising will have a greater influence on the picture's appearance.

In most cases applying a solution of sodium chloride to the surface of one side of the paper is recommended. During the following application of the silver nitrate solution to the paper it can happen that for some types of paper the silver nitrate soaks through the paper to the other side, which can make stains in the picture. In that case it is necessary to apply the salt solution with a brush or soak the paper by laying it on the surface of the solution.

In our experience paper prepared in this way curls considerably, not only in the course of preparations, but also after drying. This is caused by it being wet only on one side. Since we can’t always manage wetting uniformly, it curls up irregularly and wrinkles. Wrinkled paper is difficult to apply with a solution of silver nitrate and cannot be coated uniformly enough.

We had more luck with the second possible method, which is dipping the whole paper into the salt solution. Important is the length of time. The literature states a time of up to 10 minutes. However, if we use a salt solution without an added sizing agent (gelatine), over such a long time all the paper’s own sizing agent will wash off; furthermore, the salt solution will become dirty (cloudy). In our experience, it is enough to dip the paper in the solution only for the time necessary for uniformly soaking each side and so that no bubbles remain on the surface. This can be achieved in about 20 to 30 seconds (according to the paper size).

This method worked – the picture was sufficiently uniform and no stains appeared. However, it is possible that some types of paper will behave differently.

After soaking in a salt solution, the paper is dried in a normal way in the light. It is not sensitive yet and we can keep it for some time.

2. Sensitising with a silver nitrate solution

For this process we prepare two stock solutions.

Solution B
Silver nitrate	12 g
Distilled water	to 50 ml

 

Solution C
Citric acid	6 g
Distilled water	to 50 ml

Distilled water is necessary here; if we used tap water, the chlorides contained in it would create a white precipitation with the silver nitrate.

Before use mix solutions B and C in a ratio of 1 : 1. We recommend mixing only enough of this solution for a few days use. With longer storage the citric acid crystallises from the solution.

Citric acid is added to increase the durability of sensitised paper. We can confirm its effectiveness from our own experience. Paper prepared without adding it darkened in a few hours (in the dark) and the copied pictures had a significant stain, whereas we successfully used paper prepared with citric acid added to the silver nitrate after two weeks.

The best method of sensitising with the silver nitrate solution is soaking on the surface of the solution, i.e. laying the paper on the surface: sensitising by floating. This method was used the most often in the 19th century. In those days photographers prepared paper in a large quantity and worked with this method on a daily basis. Under those conditions it was necessary to use a large amount of nitrate solution, the large amount of paper dirtied the solution and it was not possible to store the rest.

Today we therefore recommend a more economical method, coating the paper with a brush. For the salt process applying the sensitiser with a glass road is not recommended, since paper impregnated with a salt solution is never completely flat and the rod is not easy to use.

We fix the now dry paper, impregnated with the salt solution on thick glass either with clothes pegs or adhesive tape. We place the negative we are going to copy on it and indicate its position with a pencil. We will coat the indicated area.

We work under normal tungsten light. With a 60 W bulb we should work at a distance of at least 2 metres.

We use a flat brush with soft fibres, best a hake brush. A brush of this type is very soft and releases almost all the solution it absorbs. At the start we wet the brush in distilled water and squeeze out the excess water with a kitchen towel. We put just 1 or 2 millilitres of the sensitiser into the glass and wet the top of the brush. We coat the marked area first with horizontal strokes of the brush and then we cover the whole area with vertical strokes. We recommend always just wetting the tip of the brush, so that we don't absorb too much solution – there could then be an excess of sensitiser in some places – puddles would form. An excess amount of sensitiser causes whitish stains on the picture after copying. Their traces remain until after completion of the process.

If the solution is spread well, we let it soak in for a few minutes in a horizontal position and dry out and then we can hang the paper on a line to dry. For quicker drying we can use a normal hair dryer, though cold air is best. Of course we should dry under a very weak tungsten bulb only or preferably in the dark.

Exposure

We place the negative on the dry sensitised paper and place it in the copying frame. We copy outside in natural light. We can do this on the balcony or window ledge. Copying inside by the window is not good, since the window glass filters out a significant amount of UV rays to which the paper is sensitive.

Exposure in direct sunlight can take a few minutes and somewhat longer in the shade. During cloudy weather in winter exposure of a denser negative can take several hours (then it is better to get hold of an artificial source of UV light).

Contrast depends on the type of light. In the shade we can achieve pictures with greater contrast than in the sun.

It is good to have a segmented copying frame, so we can keep an eye on the process of copying by opening up one part of it.

We copy long enough until the picture is a little darker than we require, in the course of processing it will slightly weaken again (see picture). The amount it weakens depends on the type of process. We will find this out only by experience.

Developing

After exposure, the picture is developed in ordinary water. A great part of the unused silver chloride is washed out of the paper. The water will become clouded with a whitish precipitation during this. If we are not going to tone pictures before fixing, developing for about 1 to 2 minutes is sufficient; with longer developing the picture can weaken. If we want to tone before fixing (with salts of gold or platinum), we need to wash out for about 10 minutes, since residues of silver chloride could be a problem during toning.

Fixing

The fixer consists of:
Crystalline sodium thiosulphate	100 g
Anhydrous sodium carbonate	2 g
Water	to 1 litre

Sodium carbonate is added to minimise fading.

As soon as we put the picture in the fixer, its darker parts will weaken almost immediately. Fortunately after drying they darken again.

We shouldn’t fix for too long, 1 minute is enough. With a longer time of fixing the picture could fade too much.

Washing out and possibilities of its acceleration

The length of washing out depends on the paper thickness. Papers with thicknesses of 200 to 300 g/m2 require washing out for at least half an hour.

Accelerating washing out

We can accelerate washing out, if we include a sodium sulphite bath during it.

After fixing, we wash out for about 5 minutes and then dip the photograph for about 5 minutes into a 1% sodium sulphite solution. Then only 5 minutes washing out should be enough. The total time of washing out is thus shortened by half.

Sodium sulphite is used for accelerating washing out even with normal photographic processes and is supplied commercially, for example, as Kodak Hypo Clearing Agent.

Stock solution:

Anhydrous sodium sulphite	100 g
Water	to 1 litre

Before use we dilute 1:10 and after use we pour away.

Drying

We dry the pictures in the normal way, hung on a line or lying on fabric. The picture strengthens again with drying. The resulting density is somewhat reduced than just after copying.

In our case we have noted that the colour of the picture partially changes on the first days after drying. Immediately after drying the picture had a hint of yellow. This was gradually lost (after storing in the dark) and the final colour is brown (see Fig. 1). The literature does not mention this phenomenon. It is a clear example of how the salt paper process is always individual according to the specific conditions. Possibly the type of paper used had an influence (graphically glazed for artistic purposes).

Toning

With the normal salt paper process we acquire pictures of a reddish brown colour. This shade can be changed by toning with gold salts (or platinum and palladium). The literature gives a range of formulas by which we can acquire red, purple, blue brown or blue tones. Toning also increases the permanence of the picture. In the case of the salt paper process toning is usually performed before fixing.

A disadvantage of toning is the lack of availability and the high price of the chemicals required – gold chloride, potassium chloroplatinite or palladium chloride.

Conclusion

If we refine the salt paper process enough, we will be capable of creating pictures of gallery quality, in character similar to other, more difficult and expensive processes.

We can also be happy that we needn’t worry about classical photographic papers disappearing. If it were to happen, we would know how to make them ourselves.

Each picture created by the salt process is a unique original. Possibly even more advantageous is that we are working with the oldest and the original technique which Talbot used to copy his pictures 170 years ago.

Literature:
http://www.alternativephotography.com
http://www.unblinkingeye.com