Paper sensitising

For historical copying techniques we need to prepare and sensitive paper for copying the negative ourselves. We sensitise the selected paper with sensitising solution. For this we can either use a suitable brush of soft fibres, a foam plastic brush or glass rod. For some processes (for example for the albumin process) sensitisation by floating is used so that the sensitised paper is placed on the surface of the given solution whereby the surface of the paper is soaked.

Sensitising paper is relatively easy when we know how. A certain amount of practice is necessary since it is a delicate process which has a big effect on the results. The quantity of sensitising solution must be chosen well, the layer must be sufficient and uniform. That’s why this phase is considered as key, as the critical part of the whole process.

Experiences from creative workshops show that even when students use the same resources for sensitising paper and the same solutions, the results are essentially different in terms of the density and uniform spread of the picture. The amount of applied solution, the period and manner of mulling, the amount of pressure all have an effect.

Application of the sensitising solution with a brush

• The whole process of sensitising, including drying, is made under reduced tungsten light (of 40 W bulb from a distance of about 3 m) to which the salts used for the historical processes are not sensitive.
• We take the selected paper, which is several centimetres larger than a negative which we intend to copy.
• We fix it to glass. For this purpose we need glass of a format of about 24 x 30 cm, 3-4 mm thick. We fix the paper to the glass by either clothes pegs (plastic, since they are easy to wash), or sellotape. Usually we combine both methods – we fix the two outer borders to the edges of the glass with pegs, and if the glass is a lot larger the paper we will be left with one corner free which we can fix to the glass with sellotape.
• We lay the negative we are going to copy on the paper and, using an ordinary soft pencil, we mark out its position – the corners are sufficient. During the process in the aqueous solutions the pencil marks will be washed away.
• We wash the brush in distilled water and using a paper towel (we can use kitchen roll or toilet roll) we squeeze it and dry it.
• We take a beaker, glass or cup slightly larger than the diameter of the brush and using a syringe, for example, we squirt about 2 ml of sensitising solution into it – according to the size of the surface we are going to cover. That quantity of sensitising solution should be enough for a surface of about 9 x 12 cm.
• We wet the brush in the glass with solution and cover the whole indicated area first with horizontal brushstrokes. When it is completed covered, we continue with vertical brushstrokes. If you've ever seen a varnisher working on a door, you’ll know he starts in that way. If the brush is dry and there is no solution left in the glass, we add about another 1 ml of sensitising solution until the whole area is covered.
• We try to cover the whole surface uniformly, so that there are no half-dry places and so there is no excess of solution anywhere that has not soaked in. If there are any patches of unabsorbed solution they could make stains of a different colour and density on the resulting picture.
• If any puddles form anyway, we can draw them off with a paper towel.
• We leave the treated paper in a horizontal position to dry for a few minutes and dry it using a hair dryer – with cold air only. If we’re not in a hurry we can hang it on a washing line with pegs and let it dry. Of course, in the dark or under weak tungsten light.
• In order to have reproducible results, we should expose the sensitised paper on the same day. It usually lasts a lot longer, but we will have to try it out specifically.
• It is good to practice the whole process in advance using black tea or coffee on ordinary paper. Finally, we can try a piece of paper which we will actually sensitise, since every paper reacts differently – its absorption depends on the paper sizing rate.
• Efforts made will bring rewards. It’s better to make the first attempts with a cheaper technique, for example, cyanotype or VanDyke Brownprint. When we master sensitising of paper, we’ll be able to use it successfully with more expensive techniques, such as platinotype, without quantities of spoiled pictures.

Choice of a good brush

A brush for applying the sensitising solution should be made of soft natural fibre, which should not be fixed in a metal holder. Metal could react with the sensitising solution, change its composition and corrode. Favourites for this purpose are the Japanese flat brush known in slang as a hake brush – see image. They are made of various bristles, but sufficient for us is a brush made of goat hairs, which are the lowest price. The fibres are tied by a thread and a thin copper wire. The hake brush is particularly suitable because the fibres are soft, pliable, absorb solution well and release almost all the solution during application.

Brushes often have one bad characteristic: hairs can drop off them during work. If there aren’t too many hairs falling off, it doesn’t matter much and after drying the paper they are easy to wipe off. Despite this, it is better not to have hairs falling off your brush: we hold the brush vertically with the bristles pointing upwards and put a little instant glue around the whole circumference just above the point where the bristles are held in the holder. This will fix the bristles at the point where they are gripped (see picture).

We use a special brush for each process. In theory it would be possible to use one brush for several processes after thorough washing; in practice we are not always able to ensure such thorough washing. Aside from that, we wash in distilled water, and its consumption would be significant. If traces of chemicals from another process remain in the brush, they can cause various unexpected blotches on the pictures.

Foam plastic brush

For some stronger, well-sized types of paper a foam plastic brush from a drugstore may suffice – see picture. The disadvantage is that smaller-sized papers can quickly abrade when using a foam plastic brush.

Glass rod

To save sensitising solution a glass rod is often used for application. With it we can use all the sensitising solution without loss, which is necessary for the platino-palladiotype process, for which 30 ml of the given solution costs about $150, which is rather substantial. Savings are manifested particularly in the case where we are preparing sensitive paper only occasionally and in small quantities. Glass rods are available in the USA ready-made – for example, from Bostick-Sullivan. They supply them in various forms and lengths. The rod should have a length somewhat larger than the longer side of the paper we are going to paint.

We can make a simple suitable glass rod for the application ourselves by cutting off a piece of laboratory glass rod with a diameter of about 5 mm and sticking a glass holder cut from an eye glass to it with instant glue (see picture). We can even use a small piece of plastic for this.

When painting with a glass rod we work so that we first make a trail of sensitising solution over the whole length of the painted surface with a syringe. Using the glass rod we then push the solution to the bottom. At the bottom we lift up the rod and push the remainder of the solution back up again. We repeat this drawing up and down about five or six times (see picture). If heavier layers of solution are left on areas of the paper, we draw them off with a paper towel. During this, the paper is fixed with pegs and sellotape on thick glass as in the case of painting with a brush.

Sensitising by soaking the paper on the surface of the solution

This process was used earlier for the salt and albumin process. Paper should be carefully chosen for this purpose, since some roll up too much. It is best to work with thin papers – since old photographs were also made on albumin paper just as thin and had to be glued to a thick backing.

Since sensitising solution is relatively quickly contaminated by the paper (the solution of silver nitrate clouds), this method is suitable only if we work regularly and prepare paper in a large quantity.

We pour the sensitising solution into a bowl, droop the paper above the surface in the middle and gently lower it onto the surface. We let it become saturated and carefully remove it by one corner with pincers (picture).

Selection of paper for sensitisation

Another important element in the process is the paper for sensitisation. The type of paper used affects both the appearance of the image and its durability. The highest quality paper is manufactured from old cloth; quality paper is usually made of cotton or flax. Today most is made from wood pulp. Lower quality paper often contains residues of lignin from the wood material and yellows in time. An example is newspaper paper. With thorough cleaning of the paper material during manufacturing it is today possible to manufacture paper of the same quality from wood pulp as that from cotton (with a high content of alpha-cellulose).

If the paper pulp is not cleaned well, it will contain residues of acidic substances which will degrade the paper in time. That’s why paper for artists and archiving is neutralised by alkaline reagents (buffers), for example, calcium carbonate (chalk). Paper is modified so that its pH is above 7 and is designated as “paper with an alkaline reserve”.

For historical processes paper should, if possible, be neutral – thus neither acidic nor alkaline. On acidic paper the image does not last long. If alkali material is added it can react with the sensitising solution in an undesirable way (at the very least it will make it alkali). Paper with alkaline buffer is not at all suitable for cyanotype, since and alkaline environment breaks down the blue colour from which the picture is made. Aside from alkaline buffers other additives are added during manufacturing: extenders (for example, kaolin), bleaches, optical brighteners and colours. For our purposes paper should not contain these additives.

An important characteristic of paper is its sizing rate. Paper is sized to reduce its absorbability. A sizing agent can be added directly to the paper pulp during manufacturing or applied subsequently on the ready paper. Paper which is not sized behaves like blotting paper. Traditionally gelatine, starch or alum is used for sizing during manufacturing. Paper sized with alum (usually cheaper) is not suitable for our processes because the alum may react with the chemicals used. Today most paper mills use synthetic materials of the Aquapel type for sizing, which usually do not have a negative affect on the quality of the image in historical processes.

The greater the sizing rate, the more the image remains on the surface of the paper; the picture has better contrast and brilliance. We can recognise the sizing rate when we paint the paper with a brush. If, during complete application with the brush (that is with both vertical and horizontal strokes) almost all the solution is absorbed, the paper sizing is optimum. If the solution is absorbed too quickly, the paper is insufficiently sized for our purposes and the picture will be too deep (sunken) in the paper. If, on the other hand, the solution remains too long on the surface of the paper, the image will not be sufficiently anchored between the fibres of the paper and during processing it may even be partially wash away.

If the paper is not sufficiently sized, we can size it ourselves subsequently. For this purpose we can use gelatine, starch, dextrin or arrowroot used as a food raw material, which is sago or tapioca starch. For a single-level process papers sized during manufacturing will usually suffice. We are more likely to need subsequent home sizing if we try, for example, gum printing, which requires multilevel processing in wet solutions, since during each processing while wet, the original sizing agent is partially or fully washed away.

Also possibly important for us during multilevel processing is the extent to which the dimension’s of the paper change after wetting and drying. If its dimensions change (most often in one direction), it may be difficult to copy the same negative on the image again. Handmade papers are better in this regard.

As concerns surface, we will usually select smooth paper, rough or paper may also be nice for larger picture dimensions.

Another aspect is paper thickness, so-called paper weight. It is stated as surface density, in grams per metre of paper – in other words, how much one square metre of paper weighs. For our purposes papers of 150-200 g/m2 will suffice. Thinner paper may not be adequately stiff and may tear while wet. Also during sensitisation it can roll up unequally and pucker. Thicker papers are, however, unsuitable since the chemical substances absorbed during the process are more difficult to wash from them (for example, the fixing agent).

Today it is no longer a problem to buy quality paper from foreign manufacturers here. In specialised art shops we can find 100% cotton paper, for example, Artistico made by the company Fabriano, which we successfully tried in NTM. Equally as good should be the slightly cheaper Fabriano 5, which is manufactured from 50 % cotton and 50% high-quality wood pulp.

Since foreign paper is costly for artistic purposes, we have investigated whether paper which could be used for historical processes is manufactured anywhere here. In the end, we discovered that in Štětí they manufacture paper intended for archiving photographs and negatives which, although manufactured from wood pulp, is cleaned very well, therefore it is not acidic and does not contain lignin. Aside from sizing agents based on Aquapel no other additives are added, so the paper is almost pure cellulose. We have achieved excellent results with this paper in the salt process, VanDyke Brownprint and cyanotype. It is likely that it will also be suitable for platinotype and other more complex processes. The paper is supplied by the company EMBA (Paseky nad Jizerou) which, aside from regular packaging materials, also provides materials for archiving photography and photographic negatives. Normally so far they only have paper with a weight of 90 available. The paper is very strong and we haven’t had any problems with it being too thin.