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By Terry King on

Introduction to gum bichromate printing

Terry King gives an overview of gum bichromate printing, along with a step-by-step guide to the process.

Fifteen years ago, I was in a similar state of mind to that of Robert Demarchy some ninety years before. I too needed something to enliven my personal photography. Someone suggested to Demarchy that he should go back to the experiments of the 1850s. This was when alternatives were being sought to the impermanent silver photograph. Demarchy found his inspiration in the gum print and went on to become one of the leaders of the Pictorial movement, a movement which later was to become both derided and degraded. I, for my part, went to a talk by a gentleman from Maidenhead called Steinbock.

He showed me my first gum print.

The subject matter was conventional; it was in the school of the 1930s salon photography known as ‘the little gem’. But the technique of this black and white print gave it a power, a hyper-reality that seemed to outshine so many of the silver gelatin prints I had seen. My immediate reaction was that, if he could do that in black and white, there must be a whole new world out there, in colours, that I could choose for myself.

Subsequent experience showed that after cyanotype, using a contrasty negative to make a gum print is the easiest way to make a photograph. And if that is the peak of your ambition, then fine. But, as with the cyanotype, if you want any subtlety, you will need to apply photographic expertise and experience. In the case of the gum print you will need it by the ton. Also, to rise to the challenge of the process, you will need to be bull-headed and persistent as there are so many variables and so much that can go wrong.

That was a health warning. But the end result of your persistence can be so exciting, and give you so much freedom from the parameters laid down by the chemists, that silver gelatin will seem to lack lustre, and the only two other photographic printing methods that will seem worthy of serious attention will be platinum and gravure.

One of the joys of the gum process is that there is no one right way of doing it. The method you establish will reflect your own personality and way of working. But do not try it if you are tense or in a bad mood.

If you are the kind of person who needs to be told precisely what to do, you have probably given up reading this already. What I intend to do is give the recipe, details of the materials and their sources, the basic working method and a few useful hints. It is then up to you to go away and make your gum print.


Process

This process, and much photomechanical printing, depends upon the reaction when a dichromate salt is mixed with an organic colloid, eg a gum or gelatin, and how that reaction is speeded up when the mixture is exposed to light. Put in terms that I can understand, the gum molecule is a flexible chain of hydrogen and carbon atoms with oxygen side spurs. The dichromate molecule can be excited by light at the right wavelength to throw one of its oxygen atoms which fills in the gaps between the spurs on the gum molecule chain. This stiffens the chain and the resulting stiff gum becomes insoluble in water. Increasing the levels of light or dichromate, increases the insolubility of the materials which consequently, lessens the contrast. After exposure and development, the insolubilised gum, which is porous, retains the solid particles of pigment, while any remaining dichromate in solution is washed away.

Well that was fairly simple; remembering these principles should enable you to answer most of the problems that the process throws up.


Method

For a monochrome or multicolour three exposure 16 x 12 print, to obtain a wide range of tones.

  1. Squeeze pigment about the size of a petit poi onto your tea plate, and add 5cc of gum and mix thoroughly using palette knife.
  2. Add 5cc of saturated dichromate solution and mix thoroughly.
  3. Tape down paper to a smooth surface and mark off area to be covered by negative. Coat paper with a lightly dampened brush using smooth caressing strokes, being sure not to abrade the surface. This may be done in subdued light. Your objective is an even surface.
  4. Place in the dark to dry for at least forty minutes until there is no trace of tackiness or shine on the surface. Remember that the chemical reaction is only speeded up by light. It continues in the dark to the extent that you should not be surprised if results from sensitised paper stored for more than twelve hours prove unsatisfactory. I am told that it will last longer if you store the paper in the freezer.
  5. Place the negative, emulsion side up, on the sensitised paper. Tape it down outside the picture area and make two holes through the margin of both the negative and the paper using the map pins. These holes will serve for registration for subsequent exposures.
  6. Exposures here assume a negative suitable for grade 2 silver gelatin paper with the work piece, coated with Gloy gum and ammonium dichromate, at 18” from an HPR 125 Mercury vapour lamp. Your first exposure should be between five and fifteen minutes depending on the negative. Tests are advisable. If you are using pigments other than strong blacks, the image should print out. If it does not look as if it is done enough, give it a bit longer.
  7. Develop in lukewarm water to wash away the uninsolubilised gum to the desired level of contrast. Floating the print upside down on the surface of the water is known as automatic development. You have to wait until you can see the pigment falling off. Using a wet soft brush gives you greater control as does directing streams of water at those parts of the print you wish to affect. Remember one extra minute under the lamp means ten extra minutes of development. You are developing for detail in the highlight areas; do not attempt to make this first exposure look like a finished print.
  8. Dry in a strong stream of warm air. If you have significant areas of highlight, size. Re-dry.
  9. Recoat, using more pigment, for the middle range of tones, then repeat steps 3 through 8, but the exposure should be one third of that at 6, or less.
  10. Repeat using more pigment and a shorter exposure for the shadow detail.
  11. Dry, fix in UV light for a short time, and wash for six hours. If you are impatient to see the final colours uncontaminated by the orange of the chrome salts, place the print in a clearing bath of potassium metabisulphite. The stronger the solution, the quicker the clearing. I just use water.

There are many other ways of making a gum print. Some are more complicated and achieve different aesthetic results. Others satisfy the need of the proponents for complication. Other methods depend upon cumulative misreadings and misunderstandings of writers on the subject for the past 150 years. I reinvented the process for myself to keep it simple and avoid deadly poisons such as mercuric chloride which some use to kill off the bugs in the gum. There seems to be as much chance of killing yourself. Make fresh as you go along.

For me, the picture is the objective, not the process.


Ingredients

Dichromates
HEALTH WARNING: The dichromates, formerly known as bichromates, are cumulative poisons that can be absorbed through the skin; the dust can eat away the mucous membranes, eg in the nose and lungs. They are cancer suspect agents. As with all chemicals, treat them with respect. Potassium dichromate (K2Cr207) can be dissolved to the extent of one part in eleven in water. Ammonium dichromate (NH4Cr207) can be dissolved to the extent of one part in three in water. This is the maximum amount that can be absorbed and is known as a saturated solution. The ammonium compound is three times as nasty and three times as fast.

Light
You need near ultra violet light (380 x 10-7m) available free from the sun but inconsistent. Alternatives are blacklight fluorescent tubes or mercury vapour UV graphic art lamps which offer greater flexibility. I use a Phillips HPR 125 MV lamp. UV health lamps also work.

Brushes
You will be spreading gum, which watercolourists use as a lacquer, onto watercolour paper. The recommended brush is a watercolour lacquer brush; Omega, series 40 with a code number that gives the width in mm. The brushes are described as ‘Pura Setola’.

Gum
Gum arabic comes from a tropical acacia tree. Winsor and Newton sell it in a prepared form that is expensive but is of the right consistency (170 Beaurme). It is good for your first experiments. You can buy a very expensive version in lumps, together with bits of tree, which is known as royal gum arabic. You have to dilute it 50/50 with water and filter it. White gum arabic powder, diluted 1:1 with cold water and left to dissolve, achieves much the same effect. Liquid gum sold as an etching resist is too thin. Stevens’ glue is gum arabic but its brown pigment degrades whites. Gloy gum is PVA with surfactants; it is three times as fast but not so easily controlled.

Paper
Most papers, and for that matter most surfaces that are not smooth and shiny, can be used for gum printing. Paper below 300gsm in weight will need stretching by taping it to a smooth surface around the edges of the paper and then soaking it so that as it dries, it shrinks and stretches. If the paper is not adequately sized, it will need coating with a 5% solution of rabbit skin size to preclude degradation of highlights. I avoid problems of sizing and lack of lateral stability by using papers that are adequately sized and of adequate weight. They also have a tooth to hold the gum to the surface during development (without a tooth, your picture will fall off the paper as the light acts upon the surface of the emulsion). Both papers are PH neutral and archivally sound. They are Bockingford 140 lb and Fabriano 5 (Not) 300gsm.

Pigment
Use strong permanent artist’s watercolour in tubes from Winsor and Newton. The more pigment you use the less chance the light has to do its work. A basic palette should include red, alizarin crimson; blue, cobalt and indigo; yellow, new gamboge; green, sap green; burnt sienna for brown and neutral tint and ivory black. Avoid pigments that stain such as the Hookers greens. You will be coating the whole of the paper and washing away the gum you do not want. A staining pigment will stain the whole of the image area.

Film
The gum emulsion is very slow and is only capable of accepting a density range of about 0.7 at a single exposure. You will need to make contact prints and give more than one exposure if you wish to obtain a wide range of tones with good gradation. You will need to have negatives the same size as your final print. If you are intending to make 10 x 8 inch prints there is no problem. Use medium speed camera film either direct from the camera or enlarged onto the baseboard from a transparency. For larger sizes, use graphic arts films. Lith and line films can be used but they should be exposed and developed for continuous tone unless you are looking for line results. I recommend heavy base films as the films do not maintain stability where more than one exposure is required. Paper negatives can be used to save cost for the larger sizes; the image on the paper negative will need to be laterally reserved; the exposure made emulsion to emulsion and increased by a factor of three.

Other tools
A 5cc spoon from the pharmacy, long bladed plastic or steel palette knives and a white ceramic tea plate as your palette, map pins and drafting or magic tape, a contact printing frame or two pieces of 5mm plate glass.

Other material
Rabbit skin size, potassium metabisulphite (from home made wine departments).


Sources

The following sources of materials are those that I have found give good and reliable service. They are not exclusive.

Falkiner Fine Papers Ltd | 76 Southampton Row, London WC1 4AR | 020 7831 1151

Hunter Penrose Supplies Ltd | 32 Southwark Street, London SE1 1TU | 020 7407 5051

Intaglio Printmaker | 62 Southwark Bridge Road, London SE1 0AS | 020 7928 2633

Keith Johnson Photographic Ltd | 175 Wardour Street, London W1V 3FB | 020 7439 8811

L. Cornelissen & Son | 105 Great Russell Street, London WC1B 3RY | 020 7636 1045

London Graphic Centre | 16-18 Shelton Street, London WC2H 9JJ | 020 7240 0095

Michael Putman Ltd | 151 Lavender Hill, London SW11 5QJ | 020 7207 3055

Process Supplies (London) Ltd | 13-25 Mount Pleasant, London WC1X 0AA | 020 7837 2179

Service Offset Supplies Ltd | Oakwood Hill Industrial Estate, Loughton, Essex IG10 3TZ | 020 8502 4291

Sigma Aldrich Ltd | The Old Brickyard, New Road, Gillingham, Dorset SP8 4XT | 0800 717181

Silverprint Ltd | 12 Valentine Place, London SE1 8QH | 020 7620 0844

Starna Ltd | 31/33 Station Road, Chadwell Heath, Essex RM6 4BL | 020 8599 5115

This information has been collated as a service for enquirers. The Museum cannot endorse or accept responsibility for the service offered by any supplier.

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