The Techy Bit
With a certain amount of photographic knowledge and a common sense approach to scientific practice, most people can make a Gum Bichromate print.
Nobody needs to be a Nobel prize winning chemist or physicist to make a gum print, but an ability to previsualise, an enquiring mind and a willingness to experiment will certainly make life easier.
This piece on the techniques of Gum Bichromate printing is not intended to be an exhaustive treatise on the subject, rather a way to outline my own particular working methods and workflow. As such I make the disclaimer that what works for me, in my spare room and bathroom in Oxfordshire, UK. may not work so well for other practitioners around the world. Simply put, my methods work for me.
The Gum Bichromate process also uses several potentially hazardous chemicals, the handling of which the user should be acquainted with. No liability is accepted by the author for accidental poisoning or long term ill health due to careless handling of these chemicals.
Making a gum Print-the bare bones
Making a Gum Bichromate print (known simply as a Gum print) is probably one of the easiest methods of producing a photographic print. The workflow is simple: Coat your paper with a solution consisting of Gum Arabic, watercolour pigment and a chemical called Potassium Dichromate, allow to dry, place a large negative (the size of your final image) on top, expose to Ultra Violet light (the sun?), place paper in water where the parts of the emulsion not hardened by the UV light (through the denser parts of the negative) simply dissolve away. It’s a lot like ordinary Silver Gelatine darkroom printing except the thinner areas of the negative harden the emulsion and render it less soluble in water. You are left with a hardened emulsion of gum Arabic and watercolour pigment. The Potassium Dichromate dissolves away in the water.
The majority of gum printers will start out wanting to print on paper as it's perhaps the most accessible and convenient surface to print on. There are many different types of paper that can be printed on, some more successfully than others. The main requirement is that the paper be strong enough to withstand potentially many long soaks in a water bath without disintegrating. Some practitioners use very thin tissue paper for a delicate, transparent effect which obviously requires a certain amount of experience and skill to handle.
My personal favourite is Fabriano Artistico 300gsm hot pressed, on which I have produced virtually all my gum prints. It is a 100% cotton fibre watercolour paper with an internal and external sizing, which is relatively dimensionally stable and has a slightly warm off-white colour. I have on occasion used Bockingford, Daler Rowney Langton Prestige, Somerset Satin amongst others but the Artistico gives me what I want in the way of texture, sharpness and durability.
Pre-shrinking paper
The watercolour paper most Gum printers use is generally made from various combinations of cotton fibre and woodpulp. The more expensive papers tend to have a higher level of cotton, though some of the papers containing woodpulp do have their own character when it comes to making Gum Bichromate prints on them. Natural fibres such as cotton or wood pulp do tend to change their dimensions when they get wet. They contract somewhat, meaning that the paper will shrink to a greater or lesser degree. I have found that generally the higher the temperature of the water, the greater the shrinkage- much like accidentally shrinking a new cotton shirt in a very hot washing machine.
The paper changing dimensions is detrimental to the quality of a gum print as successive printing layers will never line up or register correctly, giving a fuzzy outline and ruining fine detail. Some Gum printers don’t seem to mind this and look upon it as just one of the quirks of the medium. I, however will either remove a layer immediately or simply rip up the print and start again if my required fine detail is not there. One of the myths about Gum printing is that fine detail is not possible. I have proved it not only to be possible but highly desireable. I want my Gum prints to have the highest amount of detail possible, taking into account the subject and lighting conditions. Poor registration of subsequent layers will lead to exposure and colour imbalances in the final print where deep midtone detail, for instance, is partially covered by shadow detail of perhaps a different colour.
I have always strived to produce prints based on my pre-visualisation of the original subject, and generally they end up as I wished them to be. This isn’t always the case, though, and serendipity does play an important part in the success of many a fine print. Registration errors are just sloppy technique.
Paper shrinking is easy, simply immerse the cut to size sheets. in a tray of water not warmer than you will develop in, soak for around 30 minutes turning over occasionally to avoid air bubbles and hang to dry. It’s prudent to only handle the paper by two corners and to avoid any diagonal lifting as this can stretch the paper out of shape so that it dries buckled.
Sizing
The majority of available papers have an internal sizing that helps with their wet strength and resistance to staining from the various pigments used by the artist.
The Gum printer will be giving the paper several soakings in water which can wash out some of this size (even the supposedly waterproof internal types found in some papers) and render the paper susceptible to staining. Some Gum printers do not feel the need to worry about this as they believe their technique precludes the possibility of any staining occurring. It is, of course possible to only use pigments of the non-staining type but I feel this cuts out many possibilities and limits the gamut available. I personally always size my paper unless I am going for a one coat print, which is rare.
Sizing can be carried out by either soaking in or brushing on a layer of high quality gelatine, a diluted solution of Pva glue, or possibly a rabbit skin size. The type and concentration of a size will have a direct influence on the contrast of a gum layer.
I personally just brush on a thin layer of 4% gelatine solution to which I have added a few drops of formaldehyde as a hardener which renders the gelatine insoluble in water. Experience is key here as getting a good size down will help your printing no end. It would be most preferable to not have to size at all and would certainly save a lot of time but I guess it’s part of the craft.
On the subject of types of size, I have recently been experimenting with various combinations of PVa and gelatine. The PVa I used is Winsor and Newton Acrylic Gesso Base which I have used in various combinations, with and without gelatine in an attempt to find a viable alternative to using formaldehyde as a hardener. I used to soak my gelatine sized prints in a tray of formaldehyde solution but after a particularly frenetic printing session lasting several months in 2009 I decided the risks to my health were not worth the effort, so I looked for an alternative.
I have to say the Gesso does work well and I have found none of the disagreeable side effects from its use such as the first layer working and subsequent layers peeling off in the wash. I have tried it in combination with gelatine and found that it becomes its own hardener, that is, the gelatine compounds with the Gesso and becomes totally insoluble. I did find, however, that the prints made from absolutely any combination involving Gesso had a sparkly finish to them that may look pretty but does not suit any kind of pictorial look I am after. My best compromise has been to simply add a few drops of formaldehyde to my neat gelatine as I coat it.
Pigments
The Gum printer will use watercolour pigments of Artist quality. Using cheap student grade or powder paint will compromise quality as these are often padded out with ineffectual fillers to make them go further (for which read cheaper). I have only ever used Winsor and Newton watercolour pigments as a: they are the most readily available and b: they have a full range and publish very detailed information on their constitution, longevity and staining/graining/transparency attributes.
All pigments will have several characteristics. The most important of these from a Gum printers perspective are; tincture or covering power,transparency/opacity, staining and graining.
A Gum Bichromate print is usually made from several layers of differing colours or intensities of pigment. If the objective is to only print in one colour, then that colour must be built up over several layers in order to achieve the full tonal range that the pigment is capable of.
If the gum print is to be made up from several layers of differing colour then it is important for the printer to know how each colour interacts with the previous colour. For instance I get a very nice blue mid-tone, without ever printing a blue pigment, by overlaying a cyan colour over a first coat of a pinkish colour. This is where it is important to know if a pigment is transparent or opaque as an opaque pigment will simply cover the previous layer. The delicacy and intricacy of Gum Bichromate printing is produced through the subtle choices of colour available and learning the characteristics of the various pigments is both time consuming and frustrating. Some Gum printers go to extraordinary lengths to learn about their pigments and have very detailed knowledge of their exact constitution, which is very helpful to them or to others seeking their advice. I tend to wing it and see what happens, as every failure is notched up to experience which I learn from.
It is important for the Gum print beginner to get to know a range of the most commonly used pigments, which may limit the colours they can use for a while but they will understand more about the process by limiting one of the many variables- such as trying to use every colour under the sun.
Watercolour pigments from any manufacturer are divided roughly into natural pigment and synthetic pigment types. Many colours have a mixture of both these types and the pigment name and international number is usually printed on the packaging or tube. Winsor and Newton publish a slim brochure outlining the exact constituents of their watercolours which I have found very useful on occasion when I need to know the opacity/transparency of a particular colour.
I keep a small swatch book into which I wet a finger and smear a blob of any new watercolour paint I buy. It gives a good idea of how the paint will look at various densities and allows me to see the effects of overlaying or mixing various pigments.
Wetting a finger and applying pigment to paper is probably not a good idea as some of the pigments used are toxic!
It is not my intention to delve too deeply into the science and mysteries of pigments and watercolour paint production other than to say an appreciation of colour and what the printer can do with it is fundamental to the variety and richness that Gum printing can produce.
Gum Arabic
This is the solution that binds the pigment that is coated onto the paper.
Gum Arabic is derived from The Gum Acacia tree which grows well in some of the hotter parts of Africa.
It is possible to buy many different varieties and colours of Gum Arabic, all of which will work well for Gum printing. The reason for the large scale differences is probably akin, as much as anything else, to the differences in fine wines. Some years are vintage years, regional climate and sub-species of Gum Acacia tree all help produce a different solution.
I tend to buy my Gum Arabic from a fine art printers supplies outlet who sell it for use in certain mechanical printing processes such as lithography. I buy a batch of 3 litres at a time which is currently enough to last me for several years- the amount used for each print is really quite small. This gives me a consistent batch that eliminates one variable from the process. It can sour if some kind of preservative is not used. I simply add 2 or 3 drops of neat formaldehyde to each new litre and nothing will grow in that!
There is plenty of reading matter to be seen on the web regarding the different types of gum- much of it far more detailed than I could be here.
The Dichromates
The other essential ingredient in Gum printing is the need to add something for the Gum Arabic to react with. There are three types of Dichromate, which are all salts of Chromium, that can be used in Gum printing: Potassium, Ammonium and Sodium.
Potassium and Ammonium Dichromate are usually used in Gum printing and are differentiated by the amount of each that can be dissolved in water. Potassium Dichromate will reach saturation point at around 13%- that is 13 grams can be dissolved in 100ml of water before no more will dissolve. Ammonium Dichromate will top out at around 25%.
The differences in the final result between the two types is negligible. What does count is the exposure that has to be given to make the print. Ammonium Dichromate has more strength as a saturated solution than Potassium Dichromate. It thus seems to act faster, and it’s true that the printer will need to give less exposure when using Ammonium Dichromate. What is actually happening is the extra Dichromate in the solution is able to harden more of the Gum in the emulsion for any given exposure than would be the case if Potassium Dichromate were being used. This has the effect of hardening more of the tonal range that the negative produces, resulting in a lowered contrast. It is in some ways analogous to silver halide printing in the darkroom where extra exposure will darken more of the mid-tones and highlights, only in Gum printing the emulsion is hardened and rendered insoluble further up the tonal range. The visual effect is the same- lowered contrast.
It should be apparent from reading this that the type and strength of Dichromate being used is a great way to control the contrast of the final print. I print almost exclusively with Potassium Dichromate mainly because its lower saturation means I can give a longer highlight exposure- which in turn means my shadow exposures are not too short. I also keep several bottles of lower saturation Potassium Dichromate to use when I need further control. It should be said that exposures stay constant for the shadow exposures when using the lower saturations as I just want less of the gum to be hardened in these areas.
The Negative
Gum Bichromate printing is a contact printing process where a negative the same size as the final print is required. It is not currently possible to print Gum through an enlarger as the Ultra Violet light required would be absorbed by the glass of the enlarging lens.
Many printers, myself included, produce their negatives using a desktop inkjet printer. It has the advantage that digital files from the cameras of most contemporary photographers can easily be converted to a negative form and output as a negative onto commercially available inkjet transparency film. This method offers a readily repeatable means of producing a large number of negatives from any original batch of digital photographic files without venturing into a darkroom. The materials are easily available and quite cheap at the moment.
Traditional darkroom methods of producing a large contact printing negative include making an interpositive from an original camera film neg, then printing that to produce a negative or direct printing from a positive transparency onto a sheet of panchromatic film material. Whatever the methods used, it was time consuming, expensive and only repeatable if working methods were exact and totally controlled. I have made many negatives in the darkroom during my career and can only praise the control and consistency that the digital approach has now given me.
The digital world is, however, far from perfect and I still believe that the finest negatives are produced using large format cameras and quality film and processing techniques.
My approach to photography sadly precludes lugging around a huge camera and its associated film holders and since I was forced to literally tear down my darkroom, I have no option but to use digital methods now.
The aim of negative making is easy; produce a suitable negative for the process you want to print. The Gum Bichromate process is traditionally a short scale process, that is it is not thought possible to print a full range of tones from any given negative. The process generally only prints 4 or 5 of the available grey tones that are possible from any negative, the full tonal range of the negative being produced through subsequent printings with different exposure and colour combinations.
My workflow for producing a negative is as follows; correct and retouch the original digital file in Photoshop, convert to black and white, apply suitable Photoshop curve to file, convert to negative, print onto OHP material.
Of course, this is a very simplified version of what actually happens, but over the years I have been able to automate most of the stages involved and have evolved curves to suit most subjects or lighting situations and to take into account most of the material variables I have come across. For the record, I presently use an Epson SP2400 printer with native K3 inkset, Lloyd Paton A3 Inkjet transparency Film and a curve that has evolved with my printing.
Light Source
The main requirements for a light source for Gum Bichromate printing is that it should emit a good level of Ultra Violet light, should be large enough to illuminate the largest print to be made and should be consistent in its output.
Some printers use sunlight which has a high portion of the invisible UV in the spectrum. Other printers use specialised plate burners or other such devices found within the reprographic industry. I use a simple domestic sunbed and have done for the past 11 years. I bought it second hand at a garage sale and only towards the end of 2009 did I have to replace the tubes. My sunbed is 5ft long and enables me to make prints up to 18 inches wide. It’s not very elegant but has always worked well for me.
Coating, Exposing, Developing
Coating a gum print can be one of the most frustrating aspects of the whole operation. It can simply go right or totally wrong and at the time there never seems to be a happy compromise. The secret with coating a gum print is to achieve as thin a layer of your emulsion as evenly applied as possible. Several factors will influence the successful coating operation; Type of paper, type of size on the paper, type of brush/applicator, room temperature and humidity, density of emulsion, experience, operator mood amongst others.
My working method is (for the very first coat) to tape down the paper onto a sheet of cheap but flat board using low tack masking tape. I use the type of board sold in artist supply shops. This board will rapidly age, become full of pinholes and soon be covered with streaks of different coloured emulsion. I love the effect that all this produces and am now on my third board in 11 years of gum printing. They almost become works of art themselves!
Once the paper is pinned to the board, which is literally on the floor, I can lay the negative on top and mark out the area I need to coat. I always produce a small border on my negatives and know that if I mark around the corner of the negative and coat up to these marks I have enough of a coating for the whole picture.
Mixing my emulsuion is done in a small white saucer. I first squeeze out a small worm of pigment from my watercolour tube, say around pea size- after many years I have gotten a feel for exactly how much pigment to use for each coating-onto which I pour a few drops from a measured volume of Gum. I blend this into the pigment using a very cheap childs paintbrush and continue , rather like making cheese sauce in the kitchen, until I have fully blended my pigment and Gum solution. I next add, in one go, the pre-measured volume of Dichromate and gently blend the whole lot with the same paintbrush. Using a flat white saucer enables me to see that the pigment, gum and dichromate are well blended. Some pigments can be rather stiff and swirling the lot around in the saucer lets me see whether there is anything left un-blended.
There are generally a few bubbles in the mix by this time so I set aside the saucer for a couple of minutes.
Coating is usually done with a Hake brush. These are made from goat hair and originate in Japan. I am led to believe that Hake means “Brush” in Japanese.
Working quickly, I wet my brush in tap water and shake off the excess. I then dip my brush in the emulsion, making sure it is well saturated and wipe off the excess on the edge of the saucer. I cover the area to just outside the marks on the paper with horizontal strokes and quickly cover the whole with vertical strokes. Smoothing the emulsion is done using a second Hake brush, working rhythmically and at some speed to smooth out the brush strokes from the coating brush. This is where the temperature and humidity of the working environment becomes important as high temperatures and low humidity will cause the emulsion to set faster than it can be coated and smoothed. This is very frustrating and has led to me declaring a “Gum Printing Season” outside of which I cannot print and instead spend the time taking pictures to print when the season comes round again. I could, of course, air condition my room but I’m an amateur printer (professional photographer) working from home and my wife would not be too happy at such modifications. I also tend to think that such limitations are a challenge to be relished. I did, for example, print through 2009, modifying my technique to suit the ambient conditions. There is really no substitute for a controlled environment, though.
So having coated my print, and I am implying that this is the first coat on the print, I dry the print with the cold air setting on my wifes hairdryer. I never use the hot setting as this causes the paper to dry unevenly and thus cockles up and refuses to lie flat. Once dried, and I have a feeling for how dry the paper is after many years of practice, I lay the paper once again on the board to rest for a couple of minutes. The reason why I dry my print with a hairdryer is that it gets the job done and stops the emulsion from staining the paper too much. The time between coating/drying a print is also a variable. I have previously left the paper to dry naturally in a dark place with more staining evident. Using a hot hairdryer can also cause staining, so gently does it!
Once the coating is adequately dry I lay it on the board and place the negative on top. I then use a piece of 20x16” pvc to hold it flat against the board, leaving enough room to pin through the negative and paper in the top two corners. I am very precise about the exact angle that the map pins go through the negative/paper sandwich and try to push them through as vertically as possible. When the map pins are in place I then tape the negative to the paper using scotch invisible tape and remove the map pins. Turning the paper though 180 degrees, the process is repeated again. The negative is now pin registered in four corners and without lifting, so I don’t disturb the registration, the combination is slid under the top sheet of plate glass under my sunbed. A quick wipe over of the glass to remove dust and check for anything that might have settled on the top sheet and I am ready to expose my print.
Exposing
I expose my coatings to print the maximum highlight detail first. This is going to be the longest exposure the print will receive and is calculated so that everything that is not to remain pure white in the final print will expose. It is possible to print for the deepest shadow detail first and build the rest up through subsequent coatings but doing it that way will invariably mean that the final deep mid-tone and shadow areas will probably turn out the wrong colour (unless the print is only going to be made in monochrome). There’s also the danger that deep shadow detail will be clouded by the subsequent coating regardless of the colours used.
My first exposure, using Potassium Dichromate and a pigment that will give a nice mid-tone rendering (ie not a delicate pink or yellow) and using my usual digital negative material is 5 minutes dead. I time this to the second purely to keep a standard time but 5mins and 5 seconds will make no difference.
Once exposed the paper is removed from the sunbed and the negative gently peeled away and hung up using the tape still stuck to it.
Development
I have previously prepared a 20x24” developing dish with tepid water deep enough to let the print float on the top. The print is placed in this face up and allowed to become totally soaked. At this point air bells are usually apparent on the surface of the print which are removed by swirling the water gently in the tray. The emulsion will not have softened enough at this point for this to be damaging. At the point that the paper becomes limp and no air bell are seen, I turn the paper over and lay it gently on the surface of the water. I gently grasp the furthest two corners of the print (print is landscape in the tray) and lift it out and flip it so that it is face up, immediately reversing this procedure so that I can lay the print face down again on the surface of the water.
This sound complicated but is accomplished in little over 20 seconds. If I see the yellowish dichromate start to dissolve from the print, I know development has started and I leave the print well alone for around 10 minutes.
At some point in the first 10 minutes I should start to see the unexposed emulsion streaming off the print and colouring the water closest to the paper. I know that my exposure has been correct and I will turn the print over and lay gently floating face up so that I can gauge its process. Here I am looking for signs that enough of the emulsion has dissolved from the brightest highlight areas- those I want to remain pure white. If I develop for too long then some of the brighter highlights, say just under the shoulder of the curve, will start to lose their density and my final print will have a truncated range of tones in the delicate upper values.
A useful gauge as to whether the print has been fully developed, although not a hard and fast rule, is to see if pigment is still streaming off the print. If so, then the print may well need a little extra development. At this point I often just slap the print face up on a sheet of 20x16” pvc and rinse the excess away under a gentle stream of water. This is where experience counts- get it wrong and you could rinse away the whole lot.
When I am satisfied that the development has finished, I prop the print up, still stuck to the pvc, in the bath and allow the excess water to drain off.
When all the surface water has drained off the print it is easy to see more of the remaining pigment on the surface of the paper. There is a nice glossy relief effect which if watched carefully and mentally noted can tell the printer a lot about how the print is going, especially with subsequent coatings.
The print is then laid mostly flat, still stuck to the pvc board to dry. I believe this allows the print to dry without the minute stretching that would occur if I hung the print up to dry.
Further coats
A Gum Bichromate print is built up from several coatings, roughly split up into highlight, mid-tone and shadow coats. In my working practice I am very keen on using colour to both differentiate areas of the prints tonal range and build up a useful working density without having to print many coats. I typically print four coatings on any one print, though some work with three and some need as many as five or six goes. I also attempt to decide at the outset how many coats a print is going to need and what my colour strategy will be. This is something I also attempt to achieve at the taking stage of the original photograph, though obviously there are many steps to be taken before the final print is produced and much can change.
Having made the first coating that contains all the tones I require other than those I wish to remain pure white, I have a very flat looking print with areas of pure white. The whole of the image can be seen but it effectively has a massively truncated tonal range. My next step is to start printing the lighter mid-tones. This is where the overall colour of the final print will become apparent and is where the skill in knowing how colours blend and change is important. A typical example, though a little rough and ready, is visible on the animated graphic on this web page.
The mid-tones of any print either photographic or reprographic are where shading and roundness of the subject are most readily apparent. The highlights give clarity and the shadow tones add depth but the mid-tones add form and feeling. It is very fitting, therefore that the Gum Bichromate process seems to handle these areas the best- beautifully rendered mid-tones are the forte of the process and allow the printer to go to great lengths using infinite colour combinations in these areas.
Subsequent coating is carried out exactly as the first coating with the difference that the exposure will usually be less (otherwise exactly the same tones will be covered) and registration problems can become apparent.
Registration
With the first coating I made four pin holes, using push pins or map pins to effectively register the negative to the paper. If all has gone well with the drying of the first coat, once the second coat has been applied and dried, all the pinholes will register perfectly. This is not usually the case, unfortunately. There will be factors out of the control of the printer that can affect registration such as differences in pigment density or minor paper production blemishes. Other factors are more controllable and include temperature and humidity of the coating room, drying of the coating, thickness and application of the coating, printer mood and attitude.
I try very hard to adopt a standard procedure for all the controllable mechanical variables as Gum printing is difficult enough to do well without giving myself more problems. I do find that a lot of registration problems are paper batch related, whereby with any particular batch of paper I find the same type of problem cropping up repeatedly. This is still probably better than random problems, though!
One of the biggest problems that will be encountered regarding registration is the likelihood that as the print builds up in density over several coatings, those very coatings can actually pull the paper out of shape so that with each successive printing the registration becomes more and more off. This can be helped by coating as thinly as possible, not coating huge areas of non image border and not making too many coats.
Workarounds for registration problems include; steaming the dried paper on the back to change its dimension, applying a damp sponge to the back of the print- my preferred method as it is more even than steaming and registration on a lightbox. Sometimes the map pin holes in the paper become enlarged, especially after too many coatings and the negative will probably never register again.
Some printers, especially those printing from digital negatives, have gone to the trouble of making negatives to match the current size of the paper to compensate for any shrinkage. I don’t know how this works or how they register the new neg. on the print but it seems like quite a lot of hard work to me. Possibly best to avoid registration problems in the first place.
Shadow coat
I will usually give two mid-tone coats of different exposure/colour combinations then go onto the shadow coating.
The shadow coating to my eyes brings out the sparkle in the mid-tones and boosts the overall contrast and is a crucial stage in my workflow. I keep the exposure the same as the second mid-tone coating but simply use a half strength Potassium Dichromate solution in the mix. I find this more effective than simply giving a reduced exposure with a full strength Dichromate as at this point the exposures are getting rather short and there is an increased risk of error. I also find that it simply works and rarely have to deviate from my usual practice.
It can sometimes be difficult when to decide the print is finished. If you have a pre-visualised idea of what you want then that makes it easier but often the print will simply tell you it is done and should be left at that.
Mounting
I simply cut a bevel edged window mount for my prints and put them in a portfolio box for protection. I never show the edge brush marks in my work as I really just want the photograph to represent my ideas. I don’t want to draw attention to the fact that it is a Gum Bichromate print. I use the medium because I have got control over colour, texture and a tactile quality that other print making methods lack and at the end of the day, the final picture is what counts, not how I arrived there.