An introduction to the gum bichromate process

Peter J. Blackburn gives us an introduction to the Gum bichromate process including a step-by-step to this alternative photographic process.

Writer and photography / Peter J. Blackburn

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Always be careful when handling chemicals. Read the health and safety instructions.

Expressive, delicate, and inspiring-many are the complimentary adjectives showered in praise of the gum bichromate print. Those experienced in the making of gum prints, however, might tend to add temperamental, tedious, and elusive to the descriptive repertoire. From Fox Talbot to Robert Demachy, from the Lumière brothers to Heinrich Kühn, the bichromate process has a long and varied history spanning well over a century. Each artist persevered through their own series of trials and failures. Eventually, they overcame, in varying degrees, the myriad of pitfalls encountered on the road to creating wonderful prints. The beginner gum bichromate printer would do well to study the works of those early masters.

After falling out of common use for an extended period of time, a resurgence in gum printing began again in the 1970’s through the writings and work of a new generation of artists. Recently, with the digital revolution firmly in place and technical advances in paper sizing, the popularity of all types of dichromate printing is once again building steam and gaining speed. Witness the galleries in this web site and those of gum artists peppered all over the worldwide web. Witness, too, the plethora of new publications devoted to the subject of gum printing and the increasing number of exhibitions featuring both monochrome and tricolor prints. One would think gum printing has, at last, come of age.

There are probably no two gum printers who approach their work in precisely the same manner. The information provided below is just one of many points of embarkment and offered to the reader as basic introductory material. Most of what is presented comes from twenty years of personal experience, and perhaps a bias or two picked up along the way. As you progress in your pursuit, feel free to explore, diverge, and “wander around deck.”  May your vision increase as you embark on this journey and discover firsthand the beauty of gum bichromate printing.

The gum bichromate process in brief

Gum bichromate (or dichromate) printing involves creating a working emulsion made of three components:

• Gum arabic
• A dichromate (usually ammonium or potassium)
• Pigment

The emulsion is spread on a support, such as paper, and allowed to dry. A negative or matrix is then laid over top the emulsion and exposed to a UV light source. Usually a contact printing device or a sheet of heavy glass to ensure even, constant contact is employed. The light source will harden the dichromate in proportion to the densities of your negative. After exposure, the paper is placed in a series of plain water baths and allowed to develop until the unhardened portions of the emulsion have dissipated.

 

Materials for the gum bichromate process

• Gum arabic
  A bottled, predissolved, commercial brand should work fine, especially for the beginning printer. Check your local art shop or graphic arts supplier. Later you may wish to make your own by dissolving 30 grams of powdered gum to 90ml (or less) of very warm distilled water. Refrigerate to extend its shelf life thereby reducing the need to add a preservative.
• a dichromate solution of either (choose one) ammonium or potassium
  To make a near saturated solution (29%) of ammonium dichromate, add 29 grams to 100ml of very warm distilled water.
  To make a near saturated solution (13%) of potassium dichromate, add 13 grams to 100ml of very warm distilled water.
  My preference favors a potassium solution. Potassium dichromate, widely used in other processes such as carbon printing and photogravure, provides reasonable exposure speeds while minimizing the tendency to leave an orange stain. Should you choose a very strong UV source, a weaker solution may offer better control of your exposures. I use a 5% solution of potassium dichromate. Either compound can be purchased in a bright orange crystal granular form from a chemical supplier. Handle with care and take proper safety precautions; keep out of reach of children; consult a Material Safety Data Sheet (MSDS) for further handling instructions.
• Water-soluble pigments
  Virtually any brand of watercolor will provide at least a few pigments that will be suitable for gum printing. Some pigments may stain your paper resulting in flat, low-contrast prints with ruined highlights. However, you may find the staining effect suitable for certain images. For your beginning work try cadmiums, cobalts, ultramarines, earth pigments, and lamp black. You might try a mixture of Prussian Blue with Burnt Sienna as an alternative to single black pigments.
• Brushes
  The gum bichromate emulsion is essentially a modified watercolor. As such, I recommend investing in a few good quality watercolor brushes (flats or wash brushes; 1″ and larger). Unless you enjoy plucking stray hairs out of your sticky emulsion, hake-style brushes might better be reserved for other artistic purposes.
• Paper

Again, the gum bichromate emulsion is essentially a modified watercolor. Therefore, begin experimenting with watercolor papers that can handle repeated washes. In my opinion, you have two basic choices.

First are watercolor papers that can be classified as “old school” in that they utilize tried-and-true gelatine as a key sizing agent. Size, a term used in paper manufacturing, is a paper additive designed to regulate how and to what degree moisture will interact with the paper’s fibers. Gelatine is a very effective size and one that many gum printers continue to use today with excellent results. Papers that are gelatine sized sometimes need additional size applied to the surface followed by an application of a weak formaldehyde or glyoxal solution (hardener) to help maintain paper quality throughout multiple print layers and many water baths. Again, consult a MSDS for handling instructions if using either hardening solutions.

Your second choice include “high tech” papers that incorporate an inert, synthetic size called alkyl ketene dimer (AKD).
These papers are quite robust, maintaining their sizing qualities through many washes. High quality prints are possible without any additional sizing. AKD-sized papers are my personal preference and have been for many years.

Most watercolor papers come in a variety of surface textures ranging from hot press (a very smooth paper) to cold press (a paper that features a lightly textured surface) to rough (a more pronounced texture). Many gum printers, including myself, prefer the hot press surface while others are drawn to the cold press varieties. Paper formulations can change, often without notice, greatly affecting the quality of your work. Consult a paper supplier or the manufacturer for current information.
The beginner gum bichromate printer would do well to further investigate and increase their knowledge of paper. It is an essential key to quality gum printing.

You will need to preshrink your paper if you plan to do multiple print layers. Let your paper soak in very warm water for at least 10 minutes (some papers may require more soaking time) and then dry allowing the paper to shrink. That procedure will help maintain the registration of your negative(s) as you progress from one print layer to the next.

• A UV light source (see Dr. Mike Ware’s page entitled, Preparation for Iron-based Printing)
• Plate glass with spring clamps or a contact printer (see Cyanotype process page)
• A few developing trays a bit larger than the dimensions of your paper
• Small chemical-resistant plastic or glass jars for mixing emulsion, stirring rods, two eye/medicine droppers, protective gloves, timer

 

Making the print using the gum bichromate process

1Gum printing is a contact printing process. A good print begins with a good negative. You must have a negative produced in the actual size of the finished print. It can be as simple as taking a digital image and using photo manipulating software to invert (and flip) the shot to a negative making an inkjet print on paper or transparency film. You might also consider using an imagesetter to make quality negatives. Try to start with negatives that have good (not necessarily high) contrast and density.

For many years (and occasionally still) I made negatives using the analogue method of enlarging medium format transparencies onto orthochromatic film. I developed the film in dilute Dektol (1:20).

Much has been written about producing digital negatives and using software for alternative processes. Indeed, a properly made negative is one more key, among the many keys, to creating technically and esthetically pleasing prints. I would advise you to research, experiment, and practice!

“… a  train of thought…”

2With your negative ready and under subdued light or a safelight, combine one part gum arabic, one part dichromate, and a small length of watercolor from a tube. As an example for a typical 8 x 10 print, combine 3 eyedroppers full of gum with 3 eyedroppers full of dichromate. Then add about 2-3cm of watercolor from a tube. Mix thoroughly! With experience, you will discover that some mixtures require more pigment, others less depending upon, among other things, brand and color. Remember-mix thoroughly and occasionally stir as you use it!

3After you have mixed thoroughly, brush the emulsion onto the paper over an area slightly larger than your negative. Brush as quickly and evenly as you can. You may need to switch to a dry brush to help smooth the emulsion. Try not to lay the emulsion on too thick or thin, with your primary aim keep it even. Here is where finesse plays an important part in the process.

4Allow the emulsion to dry. A fan or a blow dryer set on the cool mode will help to speed the process along.

 

5Place your negative on top of the dry emulsion taking care that the “emulsion” side of your negative is in contact with the gum emulsion. Insert into your contact printer or sandwich with your plate glass (see step three in the Cyanotype process).

6Now expose your image to a UV source. A sunlight exposure may take from 1 to several minutes depending upon many factors such as pigment choice, gum/sensitizer ratio, time of day/season of year (if using the sun), and negative density. As an arbitrary starting time for your very first print, set the timer for three minutes and adjust your next prints accordingly.

7When the timer goes off, remove your print from the contact printer or plate glass. Rinse the emulsion for a few seconds (5-10) under a very gentle stream of water to remove the least affected dichromate first. You will probably see some orange solution wash away almost immediately. Why let your paper soak unnecessarily in dichromate that otherwise can easily be removed by a brief initial rinse? After the rinse, place your paper face down in a tray of water for 5-15 minutes, then gently into another tray for an additional 5-15 minutes, then finally (if needed) into a third tray for the same amount of time. Your goal is to develop an acceptable print within 20 – 30 minutes. It is not unusual to use a small brush to help clear away small areas of unwanted pigment. However, if you find that the print is not clearing away in the allotted time or you need to scrub your emulsion to remove unwanted pigment, try exposing the next print for a shorter period of time. On the other hand, if the emulsion has swept off your paper or is flaking and fragmenting during the brief initial rinse time or in the first wash, try doubling or tripling your exposure time.

8Now you may hang your print to dry or place face up on a drying screen. Afterward, it is usually recommended that another coat be applied of the same color or perhaps another color over top of the first layer to help improve the tonal range and density of the print. Simply repeat steps 2 through 8. After the last coating sequence, when your paper has dried, some printers suggest to remove any residual dichromate (indicated by an orange stain especially visible in the highlights) by briefly soaking the paper once more in a 5% (or less) solution of potassium metabisulphite.

Managing the gum bichromate process

Successful gum printing can be achieved through careful management of the many variable factors associated with the process. Those factors include the following:

• Paper: preshrinking, adequate sizing
• Humidity: moisture content in the paper and working environment
• Exposure: UV sources; diffusion vs undiffused sources, time of day/month of the year (sun)
• Pigment: consistent quantity and quality; staining control
• Gum/Pigment Ratio: gum arabic – using more gives higher contrast, too much causes emulsion to flake in development
• Developing: water temperature, bath manipulations
• Technique: brushing style, measuring style, development style
• Negative: damage from heat under hot lights, registration issues, density and contrast

Consider the variables above (you will discover others) as control knobs on a console. As you begin printing in dichromate, try to standardize as many variables (preset the console) as you can such as paper, pigments, dichromate, gum, etc. Then, as you evaluate your progress, adjust only one or two of the knobs at a time. Often just a minor adjustment of one or two knobs will make a big difference. You will find that some variables, such as paper and pigment batches, change without warning requiring major adjustments to be made to other knobs on the console.

Producing quality negatives (ones that have adequate contrast and density) and paper sizing are perhaps the most important variables to master, especially as you begin. The process is more forgiving as those two variables are effectively grasped. Artists choosing to use the sun for their UV source can expect to see a greater degree of variation from print to print than those using a more consistent light source.

While we are the subject of constants and variables, I would encourage the reader to also consider gum’s related process, casein printing.  Substituting gum arabic with a casein binder, in my experience, has greatly helped to make the operation more forgiving especially when the sun (what I call the “constant variable”) is the selected UV source. The variables become more manageable while the image quality is virtually identical. More information on casein printing can be found here.

In closing, here are four helpful disciplines you may wish to include in your printing routine:

• Start with a good negative: Check for adequate density and contrast.
• Manage the variables: A poor print is usually the result of failing to control one or more variables. Evaluate your negative(s) first and then your sizing quality if you persist in achieving unsatisfactory results.
• Keep accurate records: Use the back of your prints to note exposure/pigment information and keep a log.
• Make a daily test print especially if you use the sun : Start your sessions exposing a small test print of a step wedge or a negative with long tonal range for reference to help keep time and material waste to a minimum.

Download this article as a printable pdf.

Peter J. Blackburn, MA, has worked principally in casein and gum bichromate printing for over 35 years. He has been represented by Afterimage Gallery in Dallas, Texas since 2004.