Photographic Prints Using
Water Colours and Acrylics
(Gum Bichromate Printing)
How To

St Margaret's on Thames

THE PRACTICAL FOUNDATION

My early experiments 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 your of ambition, then fine. But like 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, 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 and the purists, that digital may seem lack luster, and only two other photographic printing methods seem worthy of serious attention, platinum and gravure. But even then great enjoyment can be got from the other processes requiring contact negatives, such as kallitype, salt printing and cyanotype. These lead on to pigment printing and bromoil, carbon and photo-etching. And then for good aesthetic reasons you can go on to combine them.platinumover gum or bromoil for example.

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, although the cross fertilisation of ideas that one gets on a workshop can save a great deal of effort. If you are the kind of person who can paint a front door with gloss paint, or who can bake a Christmas cake, there may be some hope for you.

INGREDIENTS

Potassium and Ammonium Dichromate

HEALTH & RESOURCE WARNING
I have been using dichromates for thirty-five years.

As with all chemicals, treat them with respect.

Wear gloves.
  • The dichromates, formerly known as bichromates, are cumulative poisons that can be absorbed through the skin.
  • The dust can eat away the mucous membranes, e.g. in the nose and lungs.
  • Dichromates are cancer suspect agents.

Until the 1980s most pictures seen in newpapers and magazines and other forms of publishing were produced using dichromated colloids. In the printing industry this involved the use of many tons of dichromates a year. When used in these huge quantities there were health risks. Photographers using alternative processes are unlikely to use more than a few grammes. Legislators, however, take blanket decisions without taking into account minor exceptions. This means that the availability of the dichromates is likely to be reduced. Some suppliers have stocks and other sources and methods are likely to be found.

Potassium dichromate (K2Cr2O7) can be dissolved to the extent of one part in eleven in water.
Ammonium dichromate ((NH4) Cr2O7) 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 fast.

Light
You need ultra violet light which you can get free from the sun but it is not as consistent as one might wish. The most effective wavelength is in the near UV range a little below 380nm, a wavelength which is not blocked by standard plate glass. Visible violet light starts at around 400nm. You can use a bank of backlight fluorescent tubes in a light table but mercury vapour UV graphic arts lamps give an even spread of light from a single bulb. I use a Phillips HPR 125 MV lamp which is designed to give a series of peaks for four different graphic arts processes. UV ‘health’ lamps and sun beds also work well.

Brushes
You will using brushes both to apply your gum/bichromate mixture and to develop the print. Remember that this is a ‘control’ process. ‘Control’ means doing something !. So called ‘automatic development’ ,leaving the print to soak to remove the less insoluble parts of the exposed negativce , rather denies the purpose of the exercise. I started with watercolour lacquer brushes for spreading the gum/bichromate/pigment mixtures onto watercolour paper, Watercolorists use these brushes to laquer their finished painting. .Try Omega, series 40 with a code number which gives the width in mm. The brushes are described as ‘Pura Setola’. Sponge brushes may also be used. Hake brushes can be used for spreading the gum and for selective removal of the gum in the course of the development process. ‘Hake’ means brush in Chinese and Japanese..

Gum
Gum arabic comes from a tropical acacia tree. Winsor and Newton sell prepared gum arabic which is expensive but is of the right consistency (17° Baume, a measure of specific gravity for heavy liquids, which for gum corresponds to its viscosity). It is good for your first experiments. You can buy it in lump form, together with bits of tree, as’ royal gum arabic’. You have to dilute it 50/50 with cold 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 arabic, sold as an etching resist also works. Stevens' Glue is gum arabic but its brown pigment degrades whites.

Gloy gum,was a PVA version available in Europe but not in the USA. It was a mixture of PVA and PVC with surfactants; it was three times as fast but gives a slightly shiny surface giving a different tactility to the final print, The manufacturers now make a replacement, Priit Glue which a clear glue which works but is expensive..enough to justify making your own. make your own by dissolving PVA powder in water and adding glycerine as a plasticiser.. Try a 20% solution of the Poly Vinyl Alcohol together with 5 cc of of glycerine per 250 ml. Put in the microwave on full power until it has come to rthe boil. Whwen it has cooled it is ready for use.
Albumen, white of egg, also works but it has to be ‘denaturised’ by beating it until you can hold the bowl upside down. You then leave it in the fridge overnight until the albumen has settled out again. You can also use casein derived from milk. I knew someone who used her own.

Paper
Most papers, and for that matter many other surfaces, that are not smooth and shiny, can be used for gum printing.

Some paper will need stretching by taping it to a smooth surface around the edges of the paper and then dampening the surface, you cam use an old sponge, so that as it dries, it tries to shrink but is stretched by the brown sticky tape holding it in place. Heavier papers , for example 140 lb per quire papers or 300 gsm papers are stable enough not to need stretching..

Use a 300 gsm or 140 lb paper such as Bockingford 140 lb and Fabriano 5 Not (‘Not’ means not pressed smooth).Both these papers have a ‘tooth’ which some think facilitates the adhesion of the various coats of gum to the paper surface (see ‘Sizing’).

Sizing
If the paper is not already adequately sized, it will need coating with a 2% solution of hard (160 bloom ) gelatine size to prevent the pigments sinking into and thus discolouring the surface of the paper. Given the frustration of finding after four coats that the fifth has stained the paper as a result of inadequate sizing, I now make it standard practice to size first whenever I make a gum print.

It is also good practice to give the sized paper a wash with 4% dichromate which should be dried in the dark and then given a thirty seconds exposure to UV before washing away the dichromate. The size will rthen stand up to many coatings and washings.. This hardening of the gelatine size is essential if you use watercolour papers.

It is also possible to prepare a hardened surface using titanium white acrylic as a substrate.. Mix the white pigment with ‘gum’ and dichromate, dry in the dark and then expose to light for thirty seconds and then wash and dry,

Pigment
Use strong permanent artists’ water-colour in tubes from Winsor and Newton. The more pigment you use the less chance the light has to do its work. Strong pigments need less pigment to achieve the same effect as weaker pigments. .

A basic palette should include
red, alizarin crimson and permanent rose
blue, cobalt and indigo;
yellow, new gamboges, transparent yellow, naples yellow
green, sap green, (avoid hookers green as it stains the whole surface
brown burnt sienna
black neutral tint, ivory black, (lamp black, which is kerosene soot, tends to oil its way into the paper. I prefer to use neutral tint or, if a very dense black is needed, I mix a powder pigment directly into the gum).

Avoid pigments that stain such as the Hookers greens. You will be coating the whole of the paper before each exposure and then washing away the gum that has not been insolubilised, a staining colour will stain the whole of the paper.

Film Negatives
The gum emulsion is very slow.If fine gradation is needed the density needed for a single exposure is about 0.7. That is why you to have a number of exposures 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 10x8 prints there is no problem, use medium speed camera film either direct from the camera or enlarge onto it on 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 thin films do not maintain stability where more than one exposure is required.

Paper Negatives
As one is printing emulsion to emulsion a paper negative made on
A paper with minimum grain, for example resin coated paper, will give as fine a print as one made from a film negative.

Paper negatives save cost for the larger sizes, tTe image on the paper negative will need to be laterally reversed; the exposure onto the gum print made emulsion to emulsion and increased by a factor of three compared with film. It is possible to use more than one negative to account for images with a large range of tone. This cuts down exposure times. A very dense negative will be needed for the shadow detail, a ‘normal’ negative for the middle tones and a ‘thin’ one for the shadows.

Digital Paper Negatives
Ink jet negatives are perfectly adequate either on acetate or on standard ink jet paper or light weight watercolour paper such as as Fabriano 5 liscia. A 100gsm paper works beautifully.

Just make your image on screen, if it is colour desaturate it, flip it horizontally, invert it and print to whatever size paper you need for your gum print.

Remember that as you are printing ‘emulsion’ to ‘emulsion the paper will serve to diffuse the light to give a sharp image. There are those who claim that this will necessarily give a fuzzy image. A little more thought will show that although this is not so, controlled fuzziness can contribute much to an image.

Registration
As this method was good enough for Rembrandt it should be good enough for us. Use Magic Tape, a low tack translucent adhesive tape, on the rebate of the image and the negative to hold the two together. Use a map pin to pierce holes through the rebates of both. Use these holes for registration but remember to take the pins out before exposing !

Other tools
a 5cc spoon from the pharmacy.
a long bladed plastic or steel palette knives
a white ceramic tea plate or a white translucent Pyrex Plate (Corning Ware) as your palette,
map pins
Magic tape
a contact printing frame or two pieces of 5mm float glass.

Other Materials
size (Gelatine).
potassium metabisulphite (Camden tablets will do).


PROCESS
This process, and much photomechanical printing, depends upon the reaction when a dichromate salt is mixed with an organic colloid, e.g. a gum or gelatine, and how that reaction is speeded up when the mixture is exposed to light.

In terms that I can understand:

gum has a long chain molecule of is a flexible chain of hydrogen and carbon atoms with oxygen side

the dichromate molecule when excited by the UV throws one of its oxygen atoms which fills in the gaps between the spurs on the gum molecule chain.

this stiffens the chain making the gum insoluble in water.

the more the light and dichromate, the greater the insolubility and the less the contrast.
the less the light and dichromate the greater the solubility and the greater the contrast
similarly the more the pigment there is to block the light, preventing the reaction, the greater the solubility and the greater the contrast

After exposure and development the insolubilised (hardened) gum. which is porous, retains the solid particles of pigment, while any remaining dichromate in solution is washed away,

the gum under the less dense areas of the negative will be thicker and retain more pigment and will thus be more dense

Well that was fairly simple.

Remembering these principles should enable you to answer most of the problems you will meet.

METHOD
These instructions are for a for a monochrome, e.g. black and white, or multicolour three exposure 16x12 print with a wide range of tones.

If you are using a digital or film negative of a scene with a wide range of tones and those tones are properly contained in the negative,that negative can be used for the highlights, middle tones and shadow detail.

There can be advantages in relation to the length of exposure if you make separate paper negatives for the highlight middle and shadow tones.

As in the rest of photography, if you get the negative right, everything else falls into place.
  1. Squeeze a small amount of pigment onto your palette, add 5cc of gum and mix thoroughly using palette knife. If, while mixing, you hold the knife at a very acute angle to the plate you should be able to see how strong the pigment will appear when it is in the paper. If it is not as trong as you would wish you can add more pigment. (Some people use gum ready mixed with colours to particular densities which they store using toxic preservatives; this is an unnecessary complication which not only restricts freedom of operation it is potentially dead making).
  2. Add 5 cc of a saturated solution of potassium or ammonium 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 without brush marks (unless you want brush marks).
  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. You should not be surprised if results from sensitised paper stored for more than twelve hours prove unsatisfactory. It will last longer if you store the paper in the freezer.
  5. Place the negative on the sensitised paper. Clearly this should be so that the image is right reading . If your sensitised paper is still not dry it will stick to your negative. Avoid contamination by putting a thin sheet of acetate between the negative and the sensitized paper.
  6. Tape the negative to the paper outside the picture area. Make two holes through the margin of both using the map pins. These holes will serve for registration.
  7. Your first exposure should be between 5 and 15 minutes depending on the negative. Tests are advisable. If you are using pigments other than strong blacks , the image will appear, If it does not look as if it's done enough, give it a bit longer. My current practice is to give a long first exposure which is for the highlights, 15 minutes for a film negative and half an hour for a paper negative. The middle tones exposure should be about third of the first exposure. The shadow detail exposure could be as short as 30 seconds for a film negative or less than two minutes for a paper negative.
  8. Develop in lukewarm water to wash away the uninsolubilised gum to the desired level of contrast. Do not delay development as the reaction is continuing even after the light has gone out. 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 that 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.
  9. Dry in a strong stream of warm air. If you have significant areas of highlight, and you have not applied a hardened base, resize between coats.
  10. Redry.
  11. Re-coat, using more pigment, for the middle range of tones, then repeat 3 through 8, but the exposure should be one third of that at 6, or less. More pigment means more ‘black’ or more of a colour suitable for the middle range of tones.
  12. Repeat using more pigment and a shorter exposure for the shadow detail.
  13. Dry, fix in UV light for a short time.
  14. Trickle wash wash for five hours to clear the appearance of the dichromate. If you want to apply further processes on top of the gum print washing the print for eight will make it chemically clean.. (If you are impatient to see the final colours quickly, 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.
There are many other ways of making a gum print. Some are more complicated to achieve different aesthetic results; others satisfy the need of the proponents of complication. Yet other methods depend upon cumulative misreadings and misunderstandings of writers on the subject for the past one hundred and fifty years. As I delved back in my own researches, I found the same mistakes repeated each time there was a revival of interest. It became clear that those writing the articles had never made a gum print. 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 their 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.

Base of Saxon Cross on Yorkshire Moors

SOURCES

Two organizations which have devoted themselves to the needs of the artist photographer for many years are:

In the UK

Silverprint Ltd
12 Valentine Place
London SE1 8QH
0207 620 0844

www.silverprint.co.uk
sales@silverprint.co.uk

and in the US

Bostick & Sullivan
PO box 16639
Santa Fe, NM 87592-6639
Factory address
1541 Center Drive
Santa Fe, NM 87507-9743
phone 505-474-0890
if calling from the UK, local London phone 0207-078-4187


For paper

Falkiner Fine Papers Ltd
76 Southampton Row
London WC1 4AR
0207 240 2339
www.falkiners.com

John Purcell Paper.
15, Rumsey Rd, London, SW9 0TR
Tel: 020 7737 5199.
www.johnpurcell.net

R K Burt & CO Ltd
57 Union St,
London SE1 1SG
www.rkburt.co.uk
+44 (0)20 7407 6474


General Art Supplies

Atlantis European Ltd
7-9 Plumbers Row
London EC1 1EQ
020 7377 8855
www.atlantisart.co.uk/

L Cornelissen & Son
105 Great Russell Street
London WC1B 3RY
0207 636 1045
www.cornelissen.com

Winsor & Newton
www.winsornewton.com/


Gum

Intaglio Printmaker
Southwark Bridge Road
London, SE 1
0207 704 6780
www.intaglioprintmaker.com


Dichromates

Sigma-Aldrich
The Old Brickyard New Road
Gillingham Dorset
01747 824414 or 0800 717181
www.sigmaaldrich.com/
They only sell to companies


Brushes

Atlantis

Cornellissen

Guang Hua Trading Company
7 Newport Place
London WC2