A Non-Silver Manual: Gum bichromate

The chapter called “Gum bichromate” of Sarah Van Keuren’s book “A Non-Silver Manual: Cyanotype, Vandyke Brown, Palladium & Gum Bichromate with instructions for making light-resists including pinhole photography”.

Writer / Sarah van Keuren

Read the previous section of this book.

Gum bichromate is the most versatile and the most labor intensive process described in this manual. A gum bichromate print can be made in almost any color or combination of colors. Once a gum print has dried after development, it is tough and stable.

The gum process is a fusion of painting, printmaking, and photography. A light-resist, the size of the final image, is usually needed for contact-printing against gum emulsion. The resist functions as a printmaker’s matrix or a photographer’s negative.

Gum emulsion is a combination of pigment, gum arabic, and orange light-sensitive chromium salts in aqueous suspension. Usually a gum print is built up in layers of transparent or semi-transparent watercolor pigment. A layer of pigmented emulsion is hardened onto a surface by the action of actinic light.

Since layers of gum emulsion are applied by hand with brushes of various sorts, gesture can be subtly (or not so subtly) incorporated into the image. Right after a layer of gum emulsion has been developed in water, it can be manipulated subtractively with a stream of water, or with brushes, or by other means. The surface of a completed, dried gum print, with slightly raised matte highlights and retracted glossy shadows, is almost sculpted in a fine bas-relief.

Approaches to Building an Image in Gum

Digital negatives print well in gum. Nowadays gum workers of ordinary means can produce digital CMYK negatives which can be used to render watercolor approximations of the printing industry’s process colors. In this way you can experience first hand the phenomenon of weak individual layers of emulsion accumulating to emerge as a full-bodied gum print with a fairly realistic color approximation of the original.

CMYK separations are by no means the only way to make digital negatives. Different principles, derived from painting and printmaking, can govern the creation of separations and the choice of pigments. In the mode of the printmaker, a layer in Photoshop could be created for each gum layer, and a rough preview of the final gum print with its transparent layers could be viewed on the monitor. Adjustments can be made by creating new digital negatives or masks in response to how the gum print is evolving.

Using just one high-contrast negative, you can produce a gum bichromate image with a range of color by printing in successive layers of color from the same negative and brushing away parts of each layer to reveal underlying colors. For instance your first layer could be blue, parts of which you brush away. If red is the next layer, it would print red where the blue was brushed away and as brown on top of the blue; and blue would show in places where you brushed away the red. A third layer of color manipulated this way, would dramatically extend the palette of the print. And if you did not register the layers perfectly, ribbons of color could wrap around your hard-edged forms.

If a 21-step film scale of densities from to clear to opaque is contact-printed against a patch of gum bichromate emulsion, you will see, after development, that there is gradation or separation of watercolor values in only about 5 of the 2l steps. Below the most saturated of those 5 steps there may appear to be a stepwise darkening but that is a printing-out effect that is caused by the overexposure of dichromate showing through the adhered pigmented gum. The amount of pigment in the gum is finite and must be transparent enough to allow actinic light to penetrate and attach it to paper. The pigmented emulsion in highlights, above what hardened enough to adhere to the paper, may have hardened somewhat on the surface but not enough to hold onto the paper so it sloughs off entirely during development in water.

Using just one continuous-tone (as opposed to high-contrast) negative, one dark color such as lamp black can be printed repeatedly, each layer receiving a different amount of exposure, to produce a monochromatic image with a long tonal range. Each layer of black will print in the shadows creating a black of unsurpassed depth, a few layers will receive enough exposure to print into the mid-tones and only one or two layers will be exposed long enough to print into the highlight areas.

Another approach is to ‘place’ different colors by varying exposure times through the continuous-tone negative. One color is given only enough exposure to adhere in shadow areas; another color gets enough exposure to adhere in the mid-tones; still another color gets exposed all the way into the highlight densities. A potential drawback of this approach is that every color adheres in the shadow areas while it is being exposed to print into mid-tones and highlights. For instance a blue layer exposed just enough to land in shadow areas will change to green if a yellow layer is then applied and exposed to print into highlight areas. This problem can be solved by masking which will be discussed later in this chapter.

For those who like to draw, handmade light-resists, such as drawings and washes on tracing paper or cliché-verre, print well in gum, especially if additions and deletions are made on different layers of emulsion.

Photograms work well in gum. Objects can be 3-dimensional or flat, opaque or translucent. Photogram objects can be added, taken away or moved during the exposure of each layer of emulsion.

If you are at heart a painter, you can do away with light-resists, partially or altogether, and simply paint with the sensitized colors. When the paper is exposed to actinic light, washed, and dried, that layer will not be disturbed as new layers are painted on top. Sensitized watercolor can be handled in this way like glazes of oil paint.

Brief History

Chromium, a relatively rare gray metal, was not identified as an element until 1797. It was given the name for color, ‘chroma’, because the varied colors of its compounds were used for dyes and pigments. The light-sensitive properties of dichromates, including their ability to harden pigment mixed in colloidal gum arabic onto paper, were known by the 1850s but the gum bichromate process was not taken up in the early years of photography when the main role of the camera was to record and document. It wasn’t until the 1890s that gum, with its atmospheric, autographic qualities, was used seriously by photographers who sought to bring the medium into the realm of fine art. A show of gum prints in the 1894 London Photographic Salon by the French photographer, Robert DeMachy, and the English photographer, Alfred Maskell, influenced the revival of the gum bichromate process. DeMachy’s gum prints in red evoked sanguine drawings by the Old Masters and his gum prints in black on laid paper seemed like drawings by Toulouse-Lautrec or Georges Seurat. Almost immediately the gum process was taken up by Edward Steichen, and Gertrude Käsebier, both Americans, and by Heinrich Kühn, an Austrian physician— to name just three of the artists.

Steichen, a native of Michigan, was traveling to Paris in 1894 to study painting. He stopped in London on the way and was deeply influenced by DeMachy’s and Maskell’s gum prints. Steichen changed his first name from Edward to Edouard when he arrived in Paris, took up photography, and found the expressivity of painting within the gum process. He deepened shadows and imparted atmospheric color to some of his platinum prints with the addition of a layer or two of gum. Other prints he built up entirely of gum manipulated with visible brushstrokes, as in his series on the sculptor Rodin. Profoundly dark masses are in thrilling contrast to sculpted highlights. Explicit detail would be distracting in these prints.

Gertrude Käsebier made gum prints in black on laid paper that look like spontaneous sketches in charcoal. In the collection at the George Eastman House in Rochester, NY, there is a brown gum print of a woman in an interior. The windows are ‘blown out’ and she didn’t hesitate to loosely draw, in brown pencil, the sheer curtains that didn’t print.

Heinrich Kühn and other Austrian and German photographers made large gum prints on rough watercolor paper. Their handling seemed to derive from the lithographic tradition. Indeed, many lithographers (including myself much later on) turned from the lithographic stone to the gum bichromate process not just to escape solvent fumes but to enjoy the plasticity of the medium. Kühn took advantage of this to suit his compositional needs. I discovered his alteration by lifting the mat of a gum print of his at Eastman House and seeing that treetops were taller in the image area that was hidden by the mat than in the part of the image that was meant to be seen. Either he opaqued his negative (as Steichen often did) or brushed away the gum rendition of treetops. By whatever means, he bent reality for his artistic purposes.

World War I ended the golden age of pictorialism in which photographers saw themselves as fine artists, and gum printing fell out of favor. Edouard Steichen became Edward again and abandoned gum to work as a Naval photographer. After the war he became a famous fashion photographer and later, after World War II, he curated the Family of Man exhibition of photographs at the Museum of Modern Art. This exhibit influenced many young photographers in the 1950s (including myself at age twelve — it was the first photography book that I purchased).

Kodak reigned supreme until the late 1960s and early 70s when photographers in the Rochester area, tiring of manufactured photographic papers and longing for the mark of the human hand, began researching 19th century non-silver processes at Eastman House where the notebooks of earlier gum printers were available for study. Betty Hahn, Bea Nettles, Robert Fichter, Judith Harold Steinhauser and Patricia Dreher were some of the pioneers in the revival of non-silver processes in upstate New York.

Meanwhile, Lois M. Johnson, then a graduate student in Printmaking at the University of Wisconsin in Madison, saw a cliché-verre gum bichromate print by Camille Corot and was inspired to research and experiment with the gum process. Lois came East to join the faculty of the Printmaking Department of the Philadelphia College of Art (later to become The University of the Arts). She taught a class called ‘PhotoMedia’ in 1973 that introduced gum bichromate as well as cyanotype along with photo-screenprinting, photo-etching and photo-lithography. Patricia Dreher had moved recently from Buffalo to the Philadelphia area and we signed up together for Lois’s innovative class. Patricia taught me the vandyke brown process in the mid-70s. Later, Jerome Kaplan, chairman of Printmaking at PCA back then, invented a brownish-black light-resist that he brushed onto vellum and scratched into. He then printed from the vellum in gum and vandyke, processes that he too had learned from Lois Johnson and Patricia Dreher.

During the same time artist Phil Simkin had us all working to assemble pinhole cameras for a project at the Philadelphia Museum of Art (see ‘Pinhole Photography’). This initiated my interest in pinhole photography which was further stimulated when I met Eric Renner, pinhole guru and founder of the Pinhole Journal at a conference of the Society for Photographic Education in Baltimore in the early 80s. Eric and I taught together at Tyler School of Art one summer in the late 80s. He taught the pinhole part and I taught how to print from pinhole negatives in cyanotype and gum.

Also in the early 80s Tom Davies, co-founder of The Photography Place in Philadelphia, gave an influential workshop on daguerreotype called ‘Time Warp’. A few months later, Tom Shillea, recently arrived in Philadelphia from RIT, demonstrated platinum/palladium printing at The Photography Place and later at PCA.

Judith Harold Steinhauser became an influence in the Philadelphia area when she arrived to teach at Moore College of Art. Her gum prints seemed to deconstruct the medium. Her loose brushstrokes revealed how one layer of color applied over another created a third color.

Martha Madigan of Tyler School of Art made huge cyanotype photograms on rolls of BFK Rives paper. Later she worked with photograms on silver chloride printing-out-paper which is about as light-sensitive as gum bichromate emulsion. Catherine Jansen of Bucks County Community College created an entire room out of figures and objects printed in cyanotype on cloth. Book artist, Enid Mark, learned cyanotype and vandyke and incorporated those processes into her books. Although these artists did not print in gum, their work had some of the same roots.

Alida Fish, known already for her gum bichromate prints, came to the Philadelphia College of Art in the late 70s and after a while Jeannie Pearce, also known for her work in gum, arrived at PCA. Twenty years later, both Jeannie and Alida were pioneers in digital imaging at what was by then UArts.

In the 80s and 90s, newspaper photographer Don Camp created enormous casein portraits from lith film using soil as pigment mixed with milk protein and dichromate. He taught casein printing to Rosae Reeder who was at the time a graduate student in Book Arts/Printmaking at UArts (see Rosae’s chapter on ‘Casein Printing’).

Absorbing these influences, I began printing from pinhole negatives in cyanotype and gum bichromate. I combined the two processes after seeing a show of cyanotype/gum prints by Tony Romano, now deceased, at the Philadelphia Art Alliance around 1980. I liked the way the crisp intense blue of cyanotype in shadow areas looked under layers of black and red gum. For 20 years I printed this way from pinhole negatives.

In recent years I have been printing, still in cyanotype and multiple gum layers, from desktop negatives created by scanning and composing in layers from a single black-and-white pinhole negative, or collaging in layers from multiple pinhole negatives. More recently, I have been using small files captured with a digital camera and printing somewhat fragile CMYK separations onto cheap inkjet acetate. When I get a new printer with archival inks I will produce more desktop negatives printed onto better inkjet films and vellums. Meantime I’m using a Scitex imagesetter at UArts that produces smooth lith film digital negatives (in half-tones only), up to 18˝ x 24˝ in size, that cost less than what a commercial lab would charge. At 200 lines per inch, the halftone pattern is scarcely visible to the naked eye in one printed layer of gum and quite invisible if 4 CMYK layers are printed on top from 4 differently angled separations.

Ernestine Ruben, already well known for her platinum and pulp paper prints, is now working in gum and extending the possibilities of the medium in a painterly manner. For several years she has been a guest critic for the advanced non-silver students and a special mentor for a number of them. She told me about the online Alt Process List (see “Resources“) which I joined and participated in for nearly two years. For me it was a graduate school in non-silver processes from which I gained almost more information than I could absorb. It was great to read the emails of participants from all over the world who brought their expertise from incredibly diverse backgrounds to bear on alternative processes of all sorts. I had to leave the List, at least for now, to center on my own experience of gum and other processes, and also simply to find the time to rewrite and expand this manual, but I recommend it as a forum on alternative processes.

A new generation of gum printers from the Philadelphia area includes Mira Adornetto, Clare Amarakoon, Christopher Dardaris, Sandra Davis, Karen Fiorito, Rebecca Gilbert, Stuart Goldstein, Melissa Good, Matt Hollerbush, Jahjehan Bath Ives, Joe Ives, John Joyce, Karen Lefkovitz, Dana Leight, Martin Lennon, Dave McKenzie, Scott McMahon, Laurie Beck Peterson, Rosae Reeder, Suzanne Solis, Lori Spencer and Evan Woldow. These artists, and others that I apologize for neglecting to name, sustain and extend the revival of non-silver processes, and in particular of gum bichromate.

How it Works

In the gum bichromate process, watercolor pigment is blended with viscous gum arabic, sap of the acacia tree. The resulting pigmented gum is combined, usually 1:1, with an orange solution of light-sensitive chromium salts. This emulsion can be brushed onto the surface of paper that has been sized (see ‘To Size or Not to Size’ later in chapter), without soaking into its fibers. This is in contrast to watery chemical solutions, like cyanotype, vandyke, and palladium, that soak into paper or cloth and must lie in contact with organic fibers to reveal their full tonal range and intensity. Gum emulsion can be applied to non-organic synthetic surfaces as long as the surface provides enough ‘tooth’ for it to adhere in image areas. When an exposed gum print is submerged in water, the emulsion including the orange chromium solution releases in highlight areas that were not exposed to actinic light and remains attached where exposed to actinic light.

Minimally exposed areas, that received just enough actinic light to attach pigmented gum to the support, are soft and vulnerable to marring until the print dries. The orange dichromate solution mostly washes out and these passages can be deleted if desired with a spray of water or gentle brushing.

If any more than the minimum exposure is given through the negative, a printing out of the chromium salts, in tones ranging from yellow/beige to a fairly dark brown, will begin to occur beneath or within the pigmented image. For some reason this aspect of the gum process is seldom discussed. It is like an ochre under-painting and adds warmth and depth to the gum image while skewing the watercolor hue. When exposed through a negative with a wide range of densities, a single layer of gum can produce multiple hues in the developed print. For instance, a layer of emulsion containing blue watercolor can range in hue from pure blue in areas that received just enough exposure to adhere the emulsion to more heavily exposed areas in which the yellow/beige/brown printing-out of chromium salts turns the blue into various shades of green.

While the print is still wet from development, the blue passages in the least exposed areas can be deleted, if desired, with a spray of water. Mid-tone passages, in which the blue looks light green after development, received more than the minimum exposure. Pigment in the mid-tones can be brushed off usually, but the embedded chromium salts, that received exposure beyond the minimum needed to adhere the pigmented gum, will print out in shades of golden tan that cannot be deleted manually. These tans can later be removed in a bath of sodium bisulfite, after the print has dried and hardened overnight.

Shadow areas, that received the greatest amounts of actinic light, adhere, locking pigment onto the support, and printing out, along with the pigment, to produce greenish brown hues. It is difficult to lift or manipulate gum colors that have received this much exposure. The darker tans, that print out in these open areas of the negative, can be changed from brown to light gray/green during clearing in sodium bisulfite but do not vanish entirely.

A full-bodied image in gum is created, usually, with multiple layers of gum bichromate in varying hues and concentrations that are exposed for varying amounts of time and developed with varying amounts of manipulation. Unless the negative consists of entirely clear and entirely opaque film which can be given just enough exposure to adhere the pigmented gum, there will be at least some printing-out of chromium salts in the more open parts of the negative for each layer of emulsion.

Chromium Salts: Ammonium versus Potassium

A note of clarification: the words dichromate and bichromate are interchangeable. In this manual the nineteenth century ‘bi’ prefix refers to the gum bichromate process in deference to the artists who first used the medium, but the ‘di’ prefix is used to refer to the chromium salt by itself and is what you will find in chemical catalogs today.

Ammonium dichromate and potassium dichromate both exhibit two important printmaking characteristics already described: l) with exposure to actinic light they harden gum arabic with its cargo of pigment onto a support; 2) with further exposure both chromium salts print-out in shades of tan within the adhered emulsion.

But there are some differences between these chromium salts. Ammonium dichromate is more soluble in water than potassium dichromate which means that with a more concentrated solution exposures are quicker and more prints can be made within a finite period of time. For that reason we have been using ammonium dichromate in class. However, if you are working at home and using the sun as an actinic light source, the difference between a one minute exposure in the sun using ammonium dichromate and a two minute exposure using potassium dichromate may be insignificant.

Ammonium dichromate, the more expensive of the two salts, is combustible and is shipped as a hazardous chemical for an extra $25.00 within the U.S.; potassium dichromate (which was used more commonly in the past) is not combustible and can be shipped in the conventional manner — although some suppliers don’t seem to know this.
As I said, we have been using ammonium dichromate. However, now there are additional actinic light sources in Printmaking, so we will begin to use up an old 5 lb. jar of potassium dichromate powder that was given to us. I used it last summer and although it may be 30 years old it worked fine.

Taking Precautions and Safeguarding Your Health

According to Overexposure: Health Hazards in Photography by Susan Shaw (published by The Friends of Photography, 1983), “Ammonium dichromate is moderately toxic by skin contact and by ingestion; it is highly toxic by inhalation. Dichromates are also suspected carcinogens. Skin contact can cause irritation, allergies, and possible ulceration…” Since 1983 it has been shown that dichromates are indeed carcinogenic. The liver cannot eliminate heavy metals such as chromium. Chronic exposure to chromium salts through skin contact, inhalation or ingestion could eventually cause liver cancer. Short of liver cancer, chronic exposure can induce asthma, other allergic reactions and skin ulcers. So, handle the bright orange dichromate crystals and solutions of dichromate carefully, with adequate protection!

However, if you should get a few drops of dichromate solution on your skin, do not panic. Immediately wash the exposed area thoroughly in cool running water. Usually it is repeated or prolonged skin contact that poses a problem.

If you should get dichromate in your eyes, flush them immediately and for several minutes with water (at school use the eyewash fountain) and seek medical attention.
Launder contaminated clothing.

The dichromate is still ‘hazardous’ when you are registering negatives so avoid touching the dry sensitized emulsion. But, once a print has been developed, thoroughly washed, and dried, whatever remains of the dichromate is locked into the emulsion and the paper and no longer poses a hazard in handling the print.

If you are planning on working in gum at home (which means bringing chromium salts into your home), don’t prepare your dichromate solution with containers or implements that may be used for cooking or eaten from in the future. Don’t mix or apply dichromate in your kitchen. Label all bottles and jars with the name of the chemical, its concentration when in solution, the date it was mixed and a warning. Don’t work with flammable ammonium dichromate near an open flame. Don’t allow crystals to build up on the mouth of a dispenser because they could crumble and then become airborne (this is true at school too). Wash and dry such surfaces. At home in my work area I dispose of contaminated rags, paper towels, stirring sticks, etc. in a closed can lined with 2 recycled plastic grocery bags, one inside the other. I carefully tie the outer bag closed on itself before it gets too full so that dust from dried dichromate won’t escape in the face of anyone who handles the trash

Be sure to keep dichromate out of reach by children especially since they may be attracted to the bright orange crystals or assume that the orange solution is a beverage.

If you must dispose of more than an ounce or two of dichromate solution, make certain it is clearly labeled and take it to a hazardous waste pick-up point. Most communities have such pick-ups scheduled at least twice a year. But try not to order and mix into solution more than you can imagine using. The dichromate crystals will keep in a sealed jar indefinitely and in solution it remains potent for a number of years.

During the 25 years that I have been teaching non-silver processes, including gum printing, I am aware of nobody who has been injured other than myself. Years ago, while helping students register negatives on their sensitized gum prints, I must have gotten a bit of dichromate lodged under the cuticle of my little fingernail. I did not wash my hand thoroughly and an abscess formed. Eventually I lost the fingernail. Although it did grow back, this incident made me more respectful of potential hazards and the need to maintain fastidious work habits.


It is possible to make a print in delicate shades of tan (that may turn slightly gray/green over time) applying liquid dichromate by itself to paper without pigmented gum. Somewhat warily, I call this a ‘chromatype’ based on this word’s definition in my 1933 edition of Webster’s New International Dictionary in English: “A picture made upon paper sensitized with potassium dichromate or some other chromium compound.” (It is possible that the expression ‘chromatype’ refers to all of the many processes — including gum, casein, carbon, photogravure, and the older generations of photo-lithographic plates and screen-printing emulsion — that rely on the light sensitivity of chromium, but I am interpreting the definition to mean dichromate used alone.)

Chromatype exposures need to be many times longer than for gum bichromate prints since you are making an image out of the secondary underprinting that comes with overexposure of a gum print. Like cyanotype, vandyke, and palladium, a chromatype image is formed around and within the organic fibers of paper by a watery chemical solution (as opposed to within an emulsion on top of sized paper) which gives a fine but limited gradation of tone from yellow to brown. Since usually dichromate solution is brushed onto un-sized paper, there is no curl from sizing or gum layers to contend with. The negative should be hinged on one side with two pieces of clear tape, as is done for the iron processes covered in this manual, so that exposure for a chromatype can be judged by inspection. The printed-out image should look over-exposed since quite a bit of yellow/tan in the highlights will wash out during development in water. If, on the other hand, the developed chromatype looks over-exposed it can be reduced by bleaching the print in sodium bisulfite or its substitutes (see ‘Clearing a Gum Print’).

Gum Arabic

Tears of gum arabic (like tears of pine resin) are harvested from the thorny acacia tree in Arabia and North Africa by Bedouin tribesmen. Burlap sacks of the rough bits of gum are delivered to processing centers where the gum is refined into various forms and degrees of purity. In the past 30 years the price of gum arabic has risen from $3 a gallon for lithographer’s gum arabic solution to nearly $30 a gallon as demand for it has increased and supply has dwindled due to drought and other factors — such as elephants scratching their backs on the acacia trees and knocking them over. One hundred years ago, a gum printer would suspend the unrefined amber pieces of gum in a cheesecloth bag in water overnight. By morning the gum dissolved in water would have formed a syrupy liquid known as a ‘colloid’. Its specific gravity should have been somewhere between 12˚ and 17˚ Baumé but I imagine that, as is true with experienced cooks, the correct viscosity of the gum was achieved intuitively. A colloid is a viscous liquid whose particles stay in suspension and do not sink to the bottom as sediment. Egg white (albumen), gelatin, and casein are also colloids that have served as emulsions for dichromates, silver salts and other light-sensitive materials. The homemade colloidal gum had to be used right away since, unlike maple syrup, it did not have much sugar to preserve it and, with its pH of about 5, was a suitable culture medium for micro-organisms. Today gum arabic is refined and hydrated to a viscosity of 14˚ Baumé for lithographic purposes. A strong preservative is necessary to keep it from going sour and although it is not listed as an ingredient on gallon containers of gum arabic, I have heard that mercuric chloride, formaldehyde or thymol is used as the preservative. As a lithographer I loved spreading the gum arabic on the stone or plate with my bare hand but now I might wear a surgical glove. The gum arabic that is used in foods must have more benign preservatives such as sugar.

One Coat versus Layers

The first time I printed on my own in complete gum bichromate emulsion, back in the late 1970s, I decided to make a strong black image using a single exposure from a high contrast lith negative. (See chapter on “Making Enlarged Negatives”.) I stirred quite a bit of powdered black pigment into 14˚ Baumé gum arabic, combined it 1:1 with a solution of ammonium dichromate, and coated BFK Rives paper with the resultant opaque black layer of emulsion. To lock the black onto the paper I exposed the contact frame, with my negative pressed against the sensitized paper, to midday sun for half an hour — this is about 30 times more exposure than I would give today. To my surprise and disappointment, the entire emulsion lifted off in the water. Eventually I understood that actinic light must be able to penetrate the pigment to effect the chemical reaction that hardens pigmented gum and attaches it to paper. I had coated with emulsion that had such a high concentration of black pigment that actinic light could not penetrate to adhere the emulsion to the paper fiber. If the white of the printing paper is not visible beneath the emulsion, as if seen through a color filter, an image usually cannot adhere and the emulsion sloughs off during development in water.

Occasionally, with the right combination of negative, pigment, and printing paper, a satisfying gum print is made with a single coat of emulsion. Demachy is known to have printed some of his most famous gum prints that way, according to Judy Seigel, editor of Post-Factory Photography, Issue #6, page 33 (see “Resources“). I encourage students learning gum to bring any paper they want to print on in the beginning and to try pigments of all sorts. In my experience, however, rich gum prints are usually built up on sized paper in increments, layer upon layer.

Two Part Emulsion: Pigmented Gum over Dichromate

When I introduce students to gum printing, we start out by making prints in chromatype (dichromate in solution by itself) so that they understand the phenomenon that occurs in a gum print when exposure goes beyond hardening and into tanning. Any kind of white paper with wet strength will do. Often the resultant print is quite appealing with delicate golden tones ranging into browns and occasionally we get a ‘keeper’.

The next exercise builds on the chromatype and was inspired by an article in an old copy of Photo Miniature that Paul Cava, Philadelphia art dealer and artist, lent to me decades ago. Unfortunately I did not record the name of the author and nowhere else have I seen or read about this approach which permits an especially free use of gesture and color in gum printing. Some of the non-silver students, especially Painting, Illustration, and Printmaking majors, choose to work in this manner even after the complete emulsion has been introduced.

With the class gathered in the non-silver darkroom under tungsten light, I coat a taped-down sheet of un-sized BFK Rives with ammonium dichromate, leaving white margins for safe handling. It is unnecessary to size the BFK Rives used for this exercise.

On the coating table is a collection of pigments mixed with gum in little jars or plastic film cannisters. One by one each student picks a pigmented gum color, stirs it with a stick of matboard (the pigment tends to settle on the bottom), pours a small puddle of it onto the sensitized BFK and brushes it out with a dry 1˝ sponge brush in a patch or arabesque on the paper. At first the paper is quite wet with deep yellow dichromate and the pigmented gum bleeds like watercolor into it, but by the time the last students (in a group of 10-12) apply their areas of color, the dichromate is nearly dry and the pigmented gum sits on top of it without spreading. Some students leave the pigmented gum puddled while others brush it on thinly and overlap with other patches. There is usually an observation made on how the color of the pigment is skewed by the deep yellow dichromate. When the paper is dry the students lay flat objects and hand-drawn light-resists, as well as negatives and printed matter, upon the paper.

The ensemble is given a prolonged exposure in our platemaker, at least three times as long as might be used for a complete emulsion in which pigmented gum and dichromate are mixed and applied together. This is because extra time is needed for light to penetrate the upper layer of pigmented gum and adhere it to the dichromate and the paper. Registration of the diverse light-resists would be lost by opening the platemaker to check underneath them for signs of printing-out, and, in any case, the materials have such a wide range of densities that no single exposure would work for all — hence the rough estimate of exposure time.

When the print has been exposed and removed from the platemaker, we note how the dichromate has darkened in the areas that were not covered with pigmented gum or light-resist, and how tanning shows through the pigmented gum, strengthening the rendition of light-resists but now affecting in a different way the hues of the watercolor pigment.

As the exposed print is slipped face up into a tray of water and gently rocked in the tray, the students have their first view of unexposed pigmented gum and dichromate releasing from the BFK. It is usually a surprise for them to see that heavy blobs of pigment are also sloughing off, even where they were given full exposure. This provides an opportunity to explain that actinic light could not make it through the heavy pigment to adhere it to the dichromated paper.

Lifting the print with gloved hands and immersing it face down and then lifting it again and slipping it back in the tray face up with unexposed pigment and dichromate streaming off the emerging image, it is usually apparent that blue and violet passages have gotten more exposure and are adhering more than red, yellow or brown passages. This demonstrates the fact that actinic light travels easily through blue and violet but has a harder time penetrating warm colors that partially mask themselves. It also becomes evident that some light-resists were suited to the exposure time and rendered well while others were so dense that only a white silhouette was recorded. Still other light-resists may have been so thin that the renderings are quite filled in.

If the light-resist was underexposed and washes off there is nothing to do but recoat the paper and give a longer exposure or find a more open light-resist. If the rendering is filled in from overexposure, subtractive manipulation often can open up highlights and mid-tones.

It is a common mistake of students to begin brushing the surface of a gum print after it has been in water for only a minute or so and then to be disappointed that detail they wanted is gone. Some of the subtlest, most beautiful gum passages are lost by this harsh approach. But since beginning students tend to overexpose, this premature brushing often becomes a way of working, for better or worse.

Manipulation of Gum Prints

Manipulation should be done in stages, beginning with the gentlest and ending with the harshest. After water development image-side-down with some agitation from rocking the tray, and after carefully flipping the print face up and face down without marring its soft surface, and after a final wash with a tray siphon, face down and face up until yellow dichromate no longer drips from it, lay the wet print face up on a piece of plexi and gently direct a small stream of cool water in a margin of the print, tilting the plexi so that water flows across the area you wish to open up or manipulate. If nothing releases, aim the stream of water directly onto the targeted area. Next, increase the impact of the stream of water by pouring it from higher up or squirting it under pressure through a hose. If there is still no release, slowly bring up the temperature of the water. If even hot water has no effect, only then should you resort to using a soft, wet 1˝ sponge brush or some other soft brush to lightly stroke the area you wish to brush away while directing cool water onto the area. Next try a dry 1˝ sponge brush, which is more abrasive than a wet one, still flooding the area with cool water. Raise the temperature of the water and strength of the spray. If still no release has occurred, take a piece of wool etching blanket or the equivalent and gently rub it on the recalcitrant area. A last resort would be to mix an alkali such as household bleach (sodium hypochlorite) or washing soda (sodium carbonate) with water in a graduated cylinder, apply some with a sponge brush, allow it to sit for a minute or two, and then hose or brush the area and see if the tough bonds of dichromate have been loosened (see upcoming section on ‘Gum Bichromate Solvents’).

Finally, after the print is completely dry, it can be cleared of as much of the tanning as possible (see next section) so that the hues of the watercolors reveal themselves.

Clearing a Gum Print

If too much tanning has taken place in either a chromatype, or in a gum bichromate print (usually after several layers), the offending golden/brown tones can be removed in a bath of approximately 5% sodium bisulfite (or sodium metabisulfite – or even sodium sulfite) which, being a weak acid, also further sets and cures the image. A more concentrated mix of the clearing agent can be applied locally and then washed off. It is important that the print be completely dry since otherwise some of the pigmented gum that hasn’t hardened may release, damaging the print and contaminating the clearing bath.

The sodium bisulfite bath should be mixed and used outdoors or under a fume hood because it smells bad and is very irritating to the respiratory system. I dissolve about 2/3 of a cup of sodium bisulfite powder or crystals in a gallon of tap water. (The exact strength of the solution is not critical since the clearing is judged by eye but it is best not to have it so strong that its action cannot be halted quickly by slipping the print into a tray of plain water — or so weak that you stand around waiting forever.) Sodium bisulfite crystals take longer to dissolve than its powdered form. The solution can be reused to clear dried gum prints until it turns a pale blue/green at which time it should be discarded.

Keep a tray of water right next to the sodium bisulfite bath to stop its action in case only partial clearing of tans is desired and also to remove some of the acidic clearing agent before putting the print in the final wash tray to remove whatever remains. Dark brown tanning does not clear entirely but turns a pale gray/green which might make a gum print cooler than intended.

Recently I learned from The Book of Alternative Photographic Processes by Christopher James (2nd Ed. Delmar, 2008), a book that I highly recommend, that sodium sulfite, which we have on hand to clear the yellow out of palladium prints, can be substituted for sodium bisulfite to clear gum prints. The concentration of sodium sulfite needs to be doubled, at least, and the action is slower, but it is effective — and odorless!


Stepping back to the beginning of the process, preshrinking your printing paper will give dimensional stability to a gum print which it needs if it is going to be built up in registered layers. If you are not working with photographic duotone or CMYK separations, close registration may not be an issue with your particular images and you can skip this discussion.

Sometimes the act of sizing with hot gelatin is equivalent to preshrinking the paper, at least for gum prints that are 5˝ x 7˝ or smaller. But for 8˝ x 10˝ prints and larger it may necessary to preshrink paper in a tray of very hot tap water for about 10 minutes as you might do with clothes that you want to make smaller. It is possible to preshrink up to 12 or 16 quarter sheets of BFK in a 14˝ by 17˝ tray that is nearly full of hot water but the sheets must be lifted and interleaved to eliminate air bubbles and to ensure that they are evenly wetted. Then hang each wet sheet by a corner from a clothespin and rotate corners as drying proceeds to avoid distortion of the soft wet paper as you would do with laundry.

Judy Seigel told me that a 36 hour soak in room-temperature water is superior to a brief hot water soak for preshrinking. I tried it and indeed paper soaked that way shrank a bit more than paper that was submerged in hot water for much less time. So if you have the foresight, time, and space in which to do the long room-temperature soak, you’ll be saving the fuel that’s consumed heating water — and getting a better result.

To Size or Not to Size

To be a successful gum printer you need to think empathetically about your materials and imagine action on a scale that you can’t see with the naked eye. Picture yourself scaled down to the size of a dust mite on the surface of an un-sized piece of BFK Rives, a soft printmaker’s paper. As a dust mite you will find yourself in a white undergrowth of rough cotton fibers that can snag jagged flakes of watercolor pigment and not let go — even in areas that received no exposure. This means that freckles of color (trapped pigment) sometimes remain where the print should have clear white highlights. These dots of color may be acceptable and even desirable; but if you definitely want clear un-freckled highlights, you need to apply a gluey substance called ‘sizing’ to coat the rough fiber enough so that pigment can release. Imagine ice coating every fiber so that unexposed flakes of pigment can slip out of the smoothed, iced underbrush and rise into the wash water. It may help in understanding the sizing issue to realize that flakes of pigment are enormous compared to molecules of dichromate or iron salts and present a physical rather than a chemical challenge.

To Size

BFK Rives is a French 100% cotton paper commonly used for etching and lithography. It is what Steichen and others printed upon in gum over 100 years ago. The paper is very familiar to me as a printmaker. It is moderately priced and commonly available at art supply stores. The fact that this paper is still being widely used gives me a sense of continuity with the past, despite two World Wars and everything else that has occurred in the past 100 years…

Preparing to print, I look for the water mark that says ‘BFK Rives, France ∞’ in the lower left corner of a horizontal full sheet of BFK (either 250 or 280 gram, one a little thinner than the other). On the side that is right reading, I put small x’s with an HB pencil in the three corners that don’t have the water mark. This is to remind myself that this is the side I wish to print on. I choose the right reading side because it is a little fuzzier and provides more ‘tooth’ for the emulsion to adhere to, but either side will work with BFK (unlike some papers which are made to be printed on a specially coated side).

I then proceed to tear a 22˝ x 30˝ sheet of BFK into 11˝ x 15˝ quarters upon which I intend to print from 8˝ x 10˝ pinhole negatives or slightly larger digital negatives. (Printmakers are taught to tear their paper rather than cut it so that the torn edges vaguely resemble the deckle at the ends of each sheet.) This is followed by preshrinking as described above.

I heard or read somewhere that the l9th century gummists used gelatin/formaldehyde sizing. (Since I am not a scholarly historian but rather a transmitter of lore, I hope that I can be forgiven for lack of references.) You will read and hear of many ways to size paper nowadays. With my own prints for many years I sized BFK with two separate sheer brushed-on coats of gelatin dissolved in hot water and then immersed the dry gelatined paper in a very diluted formaldehyde bath. The two thin layers of gelatin coat the cotton fibers of the paper without smoothing them or filling them in, but leave a good ‘tooth’ for the emulsion to grab onto. I’ve discovered that it really is necessary to coat twice initially to avoid freckles of trapped pigment in highlights.

The formaldehyde bath hardens (polymerizes) the gelatin sizing, making it tougher like hardened plastic. As I was going through the archives at the Visual Studies Workshop in Rochester, NY, I saw insect egg sacs on unprinted paper samples sized with unhardened gelatin decades ago. I saw no evidence of insects on unprinted paper samples sized with gelatin hardened in formaldehyde. However, once a layer of gum bichromate emulsion is printed on paper with unhardened sizing, the gelatin becomes less available to little critters because wherever the emulsion is printed, dichromate has hardened the gelatin in another way – with crystalline lattices rather than polymer chains.

Various starches, glues, and acrylic mediums have been recommended for sizing in recent decades. These methods seem to work better in some hands than in others. Whatever kind of sizing you use, be sure to note it on your paper since it is not always apparent to the eye or the hand. If you apply sizing on just one side of the paper as I do, note which side is which. As you grow accustomed to your paper, you will be able to feel the difference between front and back, sized and un-sized, but fine discernment can be hard to summon when you are in a hurry and you will appreciate having made notes on the paper itself.

Gelatin Sizing

To make gelatin sizing, sprinkle one level tablespoon (or a Knox packet) of unflavored gelatin as evenly as possible over the surface of 1 cup (250ml) of cool tap water in a saucepan and let it sit for at least l0 minutes as the gelatin absorbs water. I have found that any kind of gelatin works, as long as it is unflavored…. (In the past few years I have been printing on gelatined paper that I don’t harden in formaldehyde with excellent results and the added benefit that cyanotype can be brushed onto and printed on top of gum layers without beading up.) In a double-boiler or crockpot that keeps the gelatin solution away from a direct flame, raise the temperature of the gelatin-water to about 130˚F. (Many times the gelatin has gotten much hotter than 130˚F with no ill effects which makes sense since gelatin for cooking is nearly brought to a boil.) If you don’t have a thermometer, test the temperature with your fingertip. It is hot enough when you can’t keep your fingertip immersed. Just before applying the first coat of gelatin, write ‘gel 1′ in the margin of your paper.

Maintain that temperature range and dip a wide (4˝ or so), soft sponge brush into the hot gelatin and gently apply it to your taped-down paper so that the surface is evenly coated but not dripping. Leave margins un-sized to avoid gelatin build-up on the coating surface, and to avoid gluey edges that can stick to clothespins. Look at the paper at a 45 degree angle to make sure the image area is entirely coated. If you are going to hang the paper to dry, make your last brushstrokes horizontal in regard to the first hanging position. Place the clothespin in the middle of one of the ends of the sheet so that the tendency of the gelatin to flow downward is counteracted by the direction of the last brushstrokes. Make sure the wet gelatin surface does not touch or graze anything. When the paper feels dry and you are ready to apply the second coat of gelatin, write gel 2 in the margin and repeat the coating procedure but this time apply the gelatin less liberally, using a fresh soft dry brush, if necessary, to pick up excess. The paper will feel smoother as you apply the second coat. Hang the paper to dry again.

A down side of my approach is that my paper, sized, and then printed upon on one side only, curls inward. I counteract this tendency by flattening sized paper, once it has dried thoroughly, under heavy plate glass on sheets of foam. Since I also flatten the same paper between each layer of gum printing, I don’t have a problem with curling and my finished prints lie flat. Some people immerse their paper in a tray of gelatin and squeegee the excess off of the paper but it is difficult to maintain the right temperature for the gelatin in a tray and the squeegee operation can become quite a mess in a classroom situation. The benefit of total submersion is that with gelatin on both sides the paper does not curl — at least before printing commences (when the gum layers also cause the paper to curl inward). You could brush gelatin on both sides of the paper to reduce curling which would require 4 separate coating and drying operations.

Hardening with Formaldehyde

I hesitate to recommend the diluted formaldehyde bath, for fear of its being used in an improperly ventilated area. The fumes are irritating to the respiratory tract and in prolonged and high concentration can polymerize lung tissue, induce allergic asthma and do other damage. Pathologists who hunker over tissue preserved in formaldehyde are said to have the highest incidence of brain cancer of any group. If a fume hood is not available and you cannot work outdoors, skip the formaldehyde bath and make do with unhardened gelatin. The gelatin may wear off through the abrasion of brushing emulsion on and manipulating the print during development, and it will dissolve and wash off in non-printed areas (just where you want it to stay) if subjected to water temperatures over 80˚F, but a fresh layer of gelatin can be applied before each new coating of emulsion. This is how Lois Johnson worked, using fairly high contrast lith negatives. Melissa Good makes prints from inkjet separations sizing with 2 layers of unhardened gelatin and doesn’t reapply gelatin until she has printed 4 or 5 layers of gum emulsion. This approach hasn’t worked so well for me with continuous-tone pinhole negatives — but seeing Melissa’s latest prints on unhardened gelatin, encouraged me to try again, this time with digital negatives. My results were pretty good and where there was freckling in highlights I was driven to make positive digital transparencies and to print from them in titanium white to clean up the highlights. Once there was gum emulsion printed from a negative and a positive, the paper was sized by the gum itself and no more freckling occurred.

My former hardening procedure, however, is as follows. Before the formaldehyde step add an ‘f’ to the sizing notes. Then, under a fume hood (at UArts) or outdoors, wearing gloves and eye protection, tongs in hand, standing so that you are not downwind, submerge each sheet in a clean tray containing formaldehyde and water mixed 1:40 (l00ml of 37% formaldehyde to 4000ml of water — or 31/2 oz. to a gallon) for 30 seconds to a minute. Outdoors, keep a sheet of plexi covering the tray, pulling it back just enough to slip the paper in and out. Hold your breath as you drain excess formaldehyde water from the paper into a corner of the tray, cover the tray, and then hang the paper outside on a line overnight. At UArts the paper is hung over the tray in the fumehood until it stops dripping and is then hung directly in front of an exhaust fan to dry overnight. The paper will give off formaldehyde gas for at least 24 hours and should not be brought into your living space until the sweet smell of formaldehyde is gone. Meanwhile, return the remaining formaldehyde/water to a sturdy plastic container with a good screw cap and re-use. (I used the same gallon with occasional replenishment as volume diminishes for over 8 years.) Do not store 37% formaldehyde or the diluted formaldehyde in your domestic space in case a subtle out-gassing occurs.

Repeated brushing can wear down even hardened gelatin. After 3 or 4 layers of emulsion I routinely resize with one layer of gelatin that I harden in formaldehyde. After 6 or 7 layers, if the print is not completed, I apply another coat of gelatin and harden again. Sometimes the slight matte gloss of the hardened gelatin gives a satisfying depth to the image and there is no need to apply another layer of emulsion.

After reading about combining gelatin and formaldehyde in the same tray, I tried it once and found that there was an unacceptable exposure to formaldehyde in the hot gelatin and that the gelatin hardened onto the tray and brushes so that it was almost impossible to remove. I have also read of and tried glyoxal in place of formaldehyde as a hardener of gelatin. The MSDS health warnings of potential damage to the nervous system from glyoxal are even more dire than formaldelyde’s and the glyoxal solution must be discarded somehow after each use. When I found that it yellowed my paper and my image printed blurry, I definitely lost interest.

Unpigmented Gum Bichromate Sizing

A second method of sizing, which I discovered on my own, bypasses the gelatin/formaldehyde method. Apply ammonium dichromate by itself, as if making a chromatype, and, while it is still wet, generously brush pure unpigmented gum arabic on the sensitized area and buff it down with soft, dry sponge brushes. I apply the gum after the dichromate to keep the gum as much on the surface of the paper as possible to promote the sizing effect, but it can be hard to buff easily without some reduction of viscosity which is provided by the still damp dichromate solution. When the paper is dry expose it without a light-resist to any kind of actinic light until the yellow chromium salts barely begin to turn a light beige/tan. This might be 10 seconds in the sun, 40 units in the platemaker, or 1 minute under a sunlamp.

Gently wash the paper without letting the soft gum coating touch anything except cool water until it drips clear water when held in your gloved hand above the tray. Hang the sheet to dry, being very careful not to touch the gum coating which will harden eventually to provide a tough sizing for the paper. This procedure may be performed once for relatively hard papers with internal sizing but needs to be done twice for a porous soft paper such as BFK Rives. The light tan color may be removed before printing begins, with a sodium bisulfite bath, or cleared at a later stage in the gum print — or left on the paper.

Other Sizing

Students have had fairly good results sizing with diluted matte acrylic medium, PVA glue, or gesso. Each of these gluey materials should be diluted to a milky consistency and applied in at least two layers with drying between applications. Image formation varies with different sizings. I’ve seen gum printed on gesso sizing by Evan Woldow that under a loupe revealed a surface like a white dried mud flat with pigment collected in the cracks. A vivid piquant quality emerged from Evan’s work whereas my gum prints with hardened gelatin sizing are more softly luminous.

Last summer I tried using agar agar (seaweed gelatin available in Asian markets), unhardened as well as hardened in formaldehyde, as a sizing for BFK Rives but may not have diluted it sufficiently. A little of the powder goes a very long way. I also tried arrowroot starch but probably needed to cook it more and to harden it as well. More experimentation remains to be done for those who want a vegetarian alternative to gelatin sizing. You may come up with your own way of sizing. Experiment with methocellulose.

Or Not to Size

Now that I have described two rather laborious methods for sizing paper that I both teach and use myself, as well as touching on simpler ways of sizing, I must tell you that I have been shown by Martin Lennon, photographer/printmaker, that a different approach to the application of gum bichromate emulsion to un-sized paper can result in less staining and freckling in highlights than I would have thought possible. Coming from a lithographic background, I have buffed gum bichromate emulsion onto paper with soft dry sponge brushes, rather like buffing gum arabic onto a stone with clean dry surfaces of cheesecloth. Martin Lennon, instead, uses a Chinese haké brush (white animal hair sewn between wooden paddles) and slowly, gently lays out ribbons of pigmented emulsion onto un-sized BFK Rives. He demonstrated his method to a non-silver class with a layer of lamp black emulsion and Dana Leight, graduate student and teaching assistant at the time, subsequently made gum prints on un-sized BFK, using Martin’s approach, with 3 layers of pigmented emulsion. Staining and freckling in highlights was not severe enough to reduce contrast significantly and indeed the faint ghosts of brushstrokes in highlight areas gave character to those passages. I speculate that Martin’s and Dana’s success on un-sized paper came from the facts that they were using brushes made of animal hair (which I suspect releases flakes of pigment more easily than sponge), that they did not buff the emulsion into the paper fiber as I do, and that they were printing from fairly contrasty negatives. Applied Martin’s way in one generous but light stroke, the emulsion sits on top of the paper while mine gets tangled in the un-sized fibers of the paper with my many feathery buffing strokes. I encourage students to try Martin’s approach but stand ready to teach sizing strategies when they seem to be needed.
I’m not able to use his approach with my own work since I desire a smoother surface than can be achieved with one stroke of emulsion on un-sized BFK and I usually build my prints in several layers of emulsion.

Registration and Taping

The registration of multiple layers of gum emulsion can be accomplished by eye. First, make corners in pencil where the image should print on the paper. This will help you to cover the image area with emulsion yet maintain uncoated margins for safe handling. If you decide to register by eye, the initial layer of gum should be a dark one (but still sheer enough to allow light to penetrate). This dark emulsion should cover the image area and extend just beyond your marked corners so that it prints the edges of your negative. That way after printing the first layer you can see the four corners of the negative which will help you register subsequent layers by eye. Beginning with a dark layer also tests the efficacy of your sizing. It’s better to discover right away if the sizing is adequate than to print two forgiving light layers and then, when you coat with a dark layer, to have dark freckles of pigment show up in the highlights. Additional dark layers can be printed later on after resizing.

Attach minimal amounts of clear or frosted tape to opposing sides of whatever you are using as a light-resist. The hinges of tape on one side of a light-resist, recommended for easy inspection of cyanotype, vandyke, palladium, or chromatype, don’t work so well for gum bichromate. Although printing-out can show that a gum print is sufficiently exposed, sometimes a gum print is perfectly exposed with no sign of printing-out because just enough light was received to harden the emulsion without getting to the tanning stage. This is especially true with digital negatives. Pieces of tape on one side might not attach the negative properly to paper sensitized with gum emulsion which tends to curl.

Even though it is transparent or translucent, the tape may print faintly in gum. With a borderless negative, place the tape so that it overlaps dense parts on edges of the negative; that way, it will scarcely print at all. Use as little tape as possible, especially on the film. A bit more tape surface may be required to attach the negative to the border of a curling print but its tack may need to be reduced by touching it to your finger a few times. This will help prevent gum emulsion from pulling off the borders of the print when exposure is complete and the negative is removed from the print.

Use the penciled corners (which can be extended like X’s into the margins in case the emulsion hides the actual corner L’s) as a guide for placing the negative for the first exposure. After exposing, developing, and drying the first dark layer, coat the printing paper again with a second emulsion layer of any color you desire — light, dark or in-between — and place it in darkness to dry. When the second layer of emulsion is dry and you are ready to expose it, precise registration is required. Position clear tape on your negative again. Then take the sensitized print out of the dark drying area and tape its four corners with drafting tape (or some other tape that will release cleanly) to a light table that is located in an area with low ambient light.

Turn on the light table and orient the negative on top of the print. Previously printed areas should fill the open areas of the negative. If you see bright yellow edges around opaque parts of the negative, shift the negative to cover those areas or else the new color will print in highlight areas as a thin line around the previously printed layer. You can use a tube of black construction paper placed on top of the light-resist to block out extraneous light for viewing particular parts of the negative on top of the print. Check the four corners of the image to make sure the negative isn’t off-register in one direction or another. If the paper has shrunk and the negative cannot be perfectly registered, decide which segment of the image requires sharp focus and which segment(s), if any, can tolerate a looser registration. After you position the negative as well as possible, hold the negative in place with one hand and press the bits of tape attached to the negative onto the print with the other hand.

Registration by means of pin or needle holes through opposing corners of both the negative and the paper also works. Traditional pin (or button) registration is another option but tends not to work if the holes are on one side of the paper and even very slight shrinkage has occurred. Tabs can be taped to view camera negatives so that holes don’t have to be made in the film and I suppose that tabs could be attached to the edges of the print to avoid making holes in the edges of your prints.

Printed registration marks can be specified in the preparation of digital negatives, both desktop and imagesetter. These usually work well but their positions should be indicated in pencil on your printing paper so that you brush the emulsion far enough into the margins of the image for the registration marks to print.

Range of Tones

Let’s assume that you have a negative with as full a range of densities as the 21-step scale and that you are building a gum image entirely in layers of lamp black, as described earlier in ‘Approaches to Building an Image in Gum’. A refinement on the idea of giving exposures of varying durations to identical layers of lamp black gum might be to prepare 3 or 4 different dilutions of lamp black pigment in gum arabic. The first layer could use the gum with the most pigment combined 1:1 with ammonium dichromate and would get the least exposure so that the 5 steps of separation would occur in the shadow areas, the clearest parts of the 21-step scale. The next layer might contain less lamp black pigment in the gum part of the emulsion and is given enough exposure to have its 5 steps of separation fall in the mid-tone range. Black pigment in a third layer might be even more diluted and could be given much more exposure so that it did its tonal separating in the upper mid-tones and into the highlights. Sometimes an even more dilute layer with an extra long exposure is needed to print into very dense highlights. Through these multiple layers of lamp black gum exposure, and development, a very full tonal range can be extracted from a single black-and-white continuous-tone negative. This slow building of a print may seem like a limitation of the medium since it is labor intensive, but it is also a strength. Having to synthesize a full-bodied image, which entails making decisions layer by layer regarding pigment dilution and exposure time, forces an analysis of image formation and allows for creative decision-making with each exposure.

A further refinement can be brought to bear by varying the ratios of pigmented gum to dichromate solution from the normal 1:1. If fewer than 5 steps of separation are desired in a layer of emulsion, increase contrast by using 5 parts of pigmented gum to only 4 parts of dichromate solution. (Too much pigmented gum will make it hard to spread the thick emulsion evenly.) If more than 5 steps of separation are desired, decrease contrast by using 4 parts of pigmented gum to 5 parts of dichromate solution. (Too much dichromate will lower viscosity and highlights may stain or freckle.)

Placing Colors along Tonal Scale

This time, you are exposing through a single black-and-white continuous-tone negative as before, but instead of placing different dilutions of the same color along the tonal scale you are placing different colors in shadows, mid-tones, and highlights. Such an approach can produce images in which everything seems to be carved of the same material bathed in a unifying light with highlights of one color, mid-tones of another, and shadows of yet another. Some of the formal abstract qualities of black-and-white photography are preserved in this way of handling color.

For instance, brush an initial layer of Antwerp Blue gum emulsion onto your printing paper and expose it under your negative with just enough light to harden blue tones onto your paper in the most open shadow areas. After development and drying of that first skeletal layer of blue emulsion, apply a layer of Perylene Maroon emulsion and expose longer through the negative so that maroon mid-tone values adhere and the blue areas turn brown beneath the maroon gum plus tanning. After development and drying the maroon layer, apply a layer of Transparent Yellow emulsion and expose even longer, perhaps much longer, to print into highlights, turning the mid-tones a complex orange and the blue/maroon shadows even browner. A final barely visible layer of Davy’s Gray, made of iridescent ground slate, can be given a mega-exposure into the densest highlights which might separate the orb of the noonday sun from the surrounding sky or a spectral highlight from surrounding chrome (that metal again) on an old-fashioned car bumper. Since the layers are formed from transparent watercolor locked into gum arabic, shadow detail can remain visible beneath subsequent layers of more heavily exposed emulsion. If shadow detail is not visible, it may be due to heavy tan/brown printing-out of the chromium salts. A clearing bath of sodium bisulfite (or sodium metabisulfite or sodium sulfite) could help to reveal those details along with the true colors of your emulsion — aside from the gray/green left by the heaviest tanning even after clearing.

Manipulation by Deletion

Students who learn gum printing before being introduced to other non-silver processes are disappointed to find that the subtractive manipulation they do with gum is impossible with cyanotype, vandyke or palladium. With those processes, chemical solutions are embedded in paper or in some other organic support, but gum bichromate emulsion sits on top of sized paper, immediately after development in water, in a soft raised gummy layer that can be brushed away selectively, at least in highlight and in most mid-tone areas. Selective deletion can give the look of local coloration even though you may be working from a single black-and-white negative.

As the paper dries, however, the gum emulsion becomes hard and impervious. The lattice structure of the chromium crystals grips the dried gum so hard that what was raised is now recessed, and the unprinted highlight areas are in subtle matte relief. New gum layers can be added and manipulated. What is underneath will not be affected. (And as layers build up, the relief of the white areas increases to such an extent that I can recognize some of my prints from the backside. This happens even though I flatten my gum prints overnight, after each layer dries, on foam under plate glass.)


Another way to achieve an in-color effect is by masking parts of the print-in-progress with pure gum arabic. This idea came to me from reading about rubber cement-like frisket used in the selective toning of silver prints. Brush a sheer layer of unpigmented gum arabic onto the area to be masked on your print. After it dries, brush another sheer layer of pure gum onto the same area and let it dry. Avoid blobby puddles of gum. Then in the normal manner with sponge brushes, apply and buff gum bichromate emulsion over the entire image including the frisketed area and give as little exposure as is needed to adhere pigment where desired. If all goes well during development, the gum mask dissolves and the pigmented emulsion should lift from this area. But you must be warned that if the gum mask is located in a shadow area that gets a major exposure, dichromate in the emulsion may harden the gum mask and it may not release as desired.

Another way of controlling the placement of color involves cutting light-resists in the shapes to be masked. Such masks may be made of rubylith or amberlith film, goldenrod paper, black paper, or anything else that prevents actinic light from reaching the emulsion. When placed directly on top of your light-resist in contact with sensitized paper (directly on top of your negative), such a mask will give a hard-edged, cut-out look. If the mask is taped on top of the contact frame’s glass, with the negative against the sensitized paper underneath the glass, light will spread and soften the edges of the mask. If you are exposing with a point light source such as the sun, shift the angle of the contact frame during the exposure to soften harsh shadows. If you are using the platemaker which is another point light source, common sense would dictate not taping a mask on top of the platemaker’s glass since during exposure anything on the outside surface of the glass would burn, melt and/or destroy the expensive bulb. Indoor light sources that would work with the mask on top of the glass include a black light box and a sunlamp.

In addition to placing color where desired, manipulation and masking may be used to bring out some areas and suppress other areas. Instead of using opaque masks, translucent materials such as frosted mylar can serve as partial masks to dodge and burn specific areas. Emmet Gowin, the esteemed silver photographer who makes contact prints from view camera negatives, said, in a lecture at The Photography Place in the early 1980s, that he had never made a large format negative with the perfect distribution of light and dark. He showed how layers of frosted mylar can be cut into different shapes and stacked like a contour map to effect a controlled, precise, and repeatable dodging of an image. Sandwiching his view camera negative against a piece of silver gelatin paper beneath plate glass under the enlarger lamp, he placed the layers of mylar on top of the 1/4˝ glass. This complex mask allowed varying amounts of light to pass through it to the negative and photographic paper below. I was inspired by Emmet’s sharing of this technical information to try the same approach with non-silver processes, especially gum printing, and found that it worked well. But more often I found myself winging it, dodging parts of an image with pieces of cardboard and my hands as the gum layer exposed in the sun for one or two minutes.

Finally, there is now the option of a digital mask that can be printed fairly easily from a desktop printer on inkjet or laser acetate. Complex passages can be selected in Photoshop and feathered edges can be specified in ‘radius’ so that the mask can be printed in direct contact with the negative on top of the print — or even in direct contact with the print as the only light-resist for a particular layer of gum emulsion. The ‘gradient’ tool is very handy for large simple masking.

Color Separations

The methods described above permit a fairly comprehensive palette of color to emerge if watercolors that approximate the CMYK process colors (cyan, magenta, yellow and black) of commercial offset lithography are used. Antwerp Blue (Winsor & Newton name) or thalo blue can serve as cyan/process blue (as can a layer of cyanotype brushed either under or on top of gum layers). W & N Quinacridone Magenta is a true magenta and mixed with Permanent Alizarin Crimson gives a good process red equivalent for the M layer. Transparent Yellow (a relatively new W & N color that is warmer and more transparent than cadmium yellow that was used before) works well as process yellow. Lamp Black is good for process black. Oranges (from magenta and yellow), purples (from magenta and blue), and greens (from blue and yellow) plus a multitude of browns and grays are created by printing one layer on top of another.

Some gum workers print, using these conventional process color equivalents, from color separation negatives that can be generated easily in Photoshop after scanning a color image. A small color snapshot can be turned into a large gum print with great physical presence as printmaker Mira Adornetta was the first to demonstrate at UArts using the Scitex imagesetter’s maximum format that measures 18˝ by 24˝. Gum prints made this way do not have to be pale like Beatrix Potter illustrations, although a faded, nostalgic look is easily achieved with a thin layer of each process color.

Gum Bichromate Solvents

While a weak acid sets and hardens a gum print, the solvents to soften and loosen bichromate emulsions are alkalis such as sodium carbonate powder in water, diluted household ammonia, or diluted household bleach. (Never combine ammonia and bleach since together they produce tear gas.) There may be times when you wish to brush away a layer or part of a layer of heavily exposed gum bichromate (a risky but occasionally successful operation) or remove dried emulsion from a bristle brush or a graduated cylinder. A little bit of sodium carbonate, available in supermarkets as a washing aid, just a teaspoon to half a gallon, is the odorless way to loosen the tight grip of a bichromated colloid.

Because of this vulnerability to alkalinity, it is recommended that gum prints be stored over long periods of time in mats that do not contain an alkaline reserve (see ‘Light Impressions’ in “Resources“).

What follows is a step-by-step guide to producing a gum print that also serves as a summary:


l) Light-Resist: Devise some sort of negative transparency that is the size of your final image. A negative is needed to produce a positive image because gum emulsion adheres where light penetrates the negative and releases where light is held back.

2) Printing Paper: Any paper or surface (see Dana Leight’s “Gum Printing on Alternative Surfaces” at end of this chapter) can be tried, though satisfactory results are not guaranteed. The traditional paper for gum printing is sized BFK Rives, but satisfactory results have been obtained with Arches Cover, watercolor papers, Pondi paper from India and other papers too. Prepare paper, if needed, as described earlier.
3) Pigments: Select good tube watercolors, such as Winsor & Newton or Grumbacher (or others carried at Utrecht, Pearl or Daniel Smith) in hues you want to use in your print. Cheaper student grade watercolors will sometimes work out nicely but usually there is less pigment per tube and sometimes the colors are not lightfast. Students have made gum prints of great strength and beauty from inexpensive watercolors, but it is yet to be seen how some of the colors, other than black and earth tones, will hold up over time.

Finely ground dry pigments can be mixed with gum arabic, adding a drop or two of a surfactant such as Photo-Flo and a pinch of sugar to facilitate blending. Dyes, in powdered or liquid form, are not recommended. They don’t release easily, tend to stain, and later fade. They are also rather hazardous in powdered form.

According to William Crawford in his chapter on “Gum Printing” in The Keepers of Light, you should avoid the following watercolors (even from the best manufacturers) because their “permanence and durability are suspect, especially the first six: Carmine, Chrome Lemon, Vandyke Brown (the watercolor, not the process), Chrome Yellow, Rose Carthame, Mauve, Prussian Blue, Crimson Lake, Purple Lake, Gamboge, Hooker’s Green Light, Violet Carmine, Rose Dore. Chrome colors may be chemically incompatible with certain organic pigments or with the sensitizer.” Also Crawford says to stay away from Emerald Green (which I read, elsewhere, contains arsenic) and which he says should not be mixed with other chemicals.

It is interesting to note that in the years since Crawford’s book was published in 1979, many of the colors that he warned about are no longer manufactured by Winsor & Newton. This includes Chrome Lemon and Chrome Yellow that were discontinued because of ‘toxicity and lack of permanence’. Winsor Emerald, which took the place of Emerald Green, does not include arsenic in its list of ingredients but I need to test it for compatibility with gum bichromate. I have noted in classes that Winsor Green does not adhere well and that Ultramarine Blue doesn’t either and can give off a sulfurous stench when mixed with at least one kind or condition of gum arabic. Otherwise, believe it or not, there is still an ample selection of colors, each with a distinct personality and history, from which to choose.

Some Winsor & Newton colors, that I think complement the palette already available with cyanotype and the tanning effect of dichromate, are: Alizarin Crimson (the new more permanent kind), Quinacridone Magenta, Winsor Red, Bright Red, Light Red, Perylene Maroon, Winsor Violet, Caput Mortuum Violet, Antwerp Blue, Transparent Yellow, Winsor Yellow, Indian Yellow, Winsor Orange, Burnt Umber, Sepia, Neutral Tint, Blue Black, Ivory Black, Lamp Black, Payne’s Gray and Davy’s Gray. Chinese White (which is zinc white) and Titanium White can be used to lighten dark areas or for printing on dark paper. Titanium White covers better than Chinese White but has a slight yellow cast. Gum artist Melissa Good, who introduced me to Perylene Maroon when it came out, has also recommended Sap Green, Olive Green and even another green known as Oxide of Chromium which she found is compatible despite containing chromium.

You will find that blues, violets, and whites require less exposure time than reds, oranges, yellows and browns. This is because actinic light finds its way through blues, violets, and whites more easily than through colors that act as masks to it. I have not quantified the different times required for each color to adhere because there is leeway in the exposure of gum prints, but I find that I am usually successful increasing or decreasing exposures by 10-20% to compensate for warm or cool colors.

4) Gum-Pigment: Try to mix the pigmented gum part of your emulsion ahead of time so it can become thoroughly homogenized, especially if your watercolor is old and dried out. If you are short of time, mix gum with pigment, add the dichromate solution and pour the complete emulsion through a funnel that has a fine mesh screen. You need the watery dichromate solution to speed the passage of the pigmented gum through such a screen. Chunks of undissolved pigment will be held back and prevented from smearing the paper. If your pigment is fresh and the color transparent, don’t bother about straining.

To make a supply of gum-pigment that can be used for a few prints, squeeze 1/3 the contents of a small tube of watercolor (such as a W & N 5ml tube) into a little jar with a tight-fitting cap or into a 35mm plastic film canister. Add a very small amount of gum arabic that equals or is less than the volume of the pigment. Mix the pigment and gum with a stick of clean smooth matboard or something comparable and keep adding small amounts of gum until the mixture becomes a slurry. Add more gum and blend until you have about 1/2 of a film can of pigmented gum. Then dip the tip of a clean, dry 1˝ sponge brush into it and brush the mixture onto a scrap of the sized paper that is identical in kind and condition to what you’ll be printing on. Or lay down a strip of pigmented gum in one stroke with a haké brush on un-sized paper if that is how you intend to work. Then place the scrap of sized (or un-sized) paper in cool water for a few minutes or hold it under a stream of cool running water briefly. It just so happens that if the pigment or its stain remains on the test scrap, the same pigment or its stain will end up in the highlights of your print when a full emulsion, including sensitizing dichromate, is brushed on, exposed, and developed. If this amount of color is acceptable in your highlights and you like the intensity of the brushed out pigment, stop adding gum, but if you want highlights to be white, or nearly white, keep adding gum and making more tests until the pigment does release completely. Of course you will save money on pigment the more you dilute it in gum Arabic, as long as results are satisfactory.

If the color you end up with is less saturated than you wanted, it is possible to strengthen the image by recoating the print (once it has been developed and dried) with the same color and re-exposing it, or by printing over it in a related color so that the two transparent colors, like glazes, create a third color.

I have many little jars of pigmented gum that I use only occasionally. Poisonous pigments such as the cadmiums keep well but sometimes mold forms on top of the more benign colors and I have to lift it out with a stick of matboard. Over time evaporation occurs and I need to add more gum and a few drops of water. A patty of pigment in a watercolor set is pigment in dried gum that can be used for gum printing by immersing it for as long as it takes to dissolve in liquid gum and some water. A tube of dried-out watercolor pigment can be salvaged by slitting it open with a utility blade and soaking the contents in gum and water as long as necessary.

Photography student Emily Rose Chesser made a gum print using a red clay and a gray clay from Cape Cod. We used a pinch of sugar and a few drops of Photo-Flo to help the clay blend with the gum. Both clays printed beautifully in her red-clay/gray-clay gum print.

5) Safety & Mixing Dichromate Sensitizer: Before reaching for the bottle of dichromate, put on gloves. Free disposable latex gloves in medium and large sizes are stored beneath the coating table in the non-silver darkroom at UArts and should be used when handling dichromates. For dichromate solution we use sturdy plastic dispensing bottles covered with black tape that will not shatter or spill (much) if dropped or tipped over. Orange crystals tend to form on the top of the dispenser which will offset onto gloves, contaminating them immediately, so I suggest opening the dispenser with a piece of paper towel that you dispose of. If, when you pour out your dichromate solution, some of it drips down the side of the bottle, wipe it off with a damp paper towel.

For those committed gum workers outside of UArts, who are mixing their own dichromate, exercise special caution when handling the dry crystals by wearing gloves, an apron, protecting your eyes, and perhaps wearing a dust mask. Nowadays ammonium dichromate is usually sold in a granulated rather than powered form and is not dusty, thus safer to handle.

I have evolved a simple way of preparing solutions of ammonium dichromate in which I carefully spoon the crystals into a graduated cylinder up to the l ounce line and add tap water up to 15 ounce mark. (Yes, even Philadelphia tap water can be used for dichromate solutions.) You will see a variety of dilutions recommended, some of which are much more concentrated, but I have found my dilution to be as fast as the more concentrated solutions and I certainly believe in using as little of this toxic heavy metal salt as possible. In fact I mean to try using even more diluted solutions. Last summer I began using a saturated solution of potassium dichromate, an ounce by volume in close to 30 ounces of water. Although the exposures took perhaps twice as long I was very pleased with the results. There seemed to be less yellow staining and tanning.

Dichromate solutions should be stored in darkness in brown or opaque containers but can be handled in subdued room light. No safelight is needed nor should one be used since it is hard to work safely in orange or red light with orange liquid. At UArts we use 60 watt tungsten bulbs in clear glass fixtures that hang on the wall about 4 feet above the coating table. However, once the paper is coated it should be dried in darkness and placed only briefly on a dim light table for registration. Sensitivity increases as paper dries and is at its height following exposure, so get the exposed gum print into water ASAP.

After several years, a solution of dichromate will begin to lose its potency. Ideally you will have used it up before it gets old, but if you are stuck with expired solution, do not pour it down the drain. Save it for a hazardous chemical pick-up or take it to a designated place where toxic waste is disposed of as well as it can be. Such information should be obtainable through the phone book or your borough hall or may be online.

6) Necessary Supplies: Before you combine pigmented gum with dichromate, make sure the rest of your supplies are assembled. You will need:

  • printing paper, preshrunk & sized if necessary
  • a pencil for notes & a straightedge if you are designing a test strip
  • tape — white artist, drafting, or masking tape to attach the printing paper to the coating table; clear or frosted tape to attach the light-resist to the sensitized paper
  • clean, dry 3˝ sponge brushes —2 of the new finer #8505 or at least 4 of the older coarser Poly-Brush for each color (Smaller sponge brushes are OK for covering areas less than 8˝ x 10˝ and you may prefer using bristle brushes.)
  • disposable gloves for handling the bottle of dichromate, for handling the print as it develops, & for washing sponge brushes
  • graduated cylinder that measures milliliters, 45ml (11/2 oz.) or smaller
  • stirring implements for homogenizing pigmented gum in jars or film canisters & for stirring mixtures of dichromate solution and pigmented gum in graduated cylinders (We use 6˝ x ≈1/4˝ sticks of scrap rag matboard.)
  • a shallow container for the complete emulsion that is wide enough for the coating brushes (Plastic bowls or saucers work well and can be reused.)
  • tongs or gloves
  • timer
  • masking material — rubylith, goldenrod, orange mylar, cardboard — anything opaque & flat with a straight edge to block out light when making test exposures
  • tray of cool water for the initial still development
  • plexiglas or plate glass upon which to manipulate the print
  • a hose or small watering can for clearing the image with a stream of water
  • brushes, pieces of sponge, etching blanket, etc. for manipulation of the print
  • a bigger tray with a Kodak siphon or some other arrangement for final washing
  • a line with clothespins for drying the washed print
  • tofu tubs or something like them for soaking and washing sponge brushes before emulsion hardens on them

7) Working Emulsion: Prepare the working emulsion by adding one part gum-pigment mixture to one part dichromate solution.

Twelve milliliters of working emulsion should make enough emulsion to coat 8˝ x l0˝ areas on 3 sized pieces of paper. I find that if I am coating 10 or more sheets with 8˝ x 10˝ image areas, it averages out to 3ml emulsion per print but about 6ml are needed to charge the first brush initially and coat the first print. (To pre-wet the brush in water would lower the viscosity of the emulsion and could make it harder to clear highlights.)

Although the gum-pigment mixture will keep for months, or even years, in a sealed jar, once dichromate has joined it, an inexorable hardening process sets in. This happens even in the absence of light, giving the complete emulsion a working life of usually only a few hours though sometimes longer. To avoid wasting pigment and polluting any more than necessary, learn to prepare only as much emulsion as you will use during a session.

8 ) Application of Emulsion: Pour the blended complete emulsion from the graduated cylinder into a clean, dry container that is wide enough for sponge brushes (or a haké brush) to be dipped into. Make sure, if you are using sponge brushes, that you have a few lined up on the coating table that are dry and soft on the edges. This preparation is to ensure that there is no delay in grabbing a fresh brush during the critical moments just before the emulsion sets up and can’t be brushed any more.

Gently, liberally apply all the emulsion you are going to need with the first sponge brush. If that brush ceases to make progress smoothing the emulsion, grab a fresh brush and continue with quick, light strokes up and down and side to side, using one surface of the brush on the upstroke and the other surface on the downstroke. The emulsion begins to set up and will be streaky unless you work fast towards the end. You may sometimes wish for streakiness of emulsion on a print, but you should be able to brush down the emulsion smoothly when you want to. Consider in what direction to make your final strokes if they are going to be at all visible. I tend to think symbolically of horizontal strokes as denoting the passage of time and vertical strokes as gravity.

It is possible to brush complete emulsions of different colors into each other on the same layer but care must be taken to avoid buildup of emulsion where one color ends and another begins — which can result in emulsion sloughing off entirely. It helps when making a transition from one color to another on the same layer to dribble and brush out a line of pure gum arabic with a 1˝ brush between the colors which you then brush the colors into from each side.

Another approach is to coat the paper with straight ammonium dichromate and then apply pigmented gum (see ‘Two Part Emulsion: Pigmented Gum over Dichromate’ earlier in this chapter). This method produces a more contrasty image with stronger colors and makes it possible to apply different colors in patches, but it is harder to coat the viscous pigmented gum evenly without the watery sensitizer present to dilute it. It helps to work the pigmented gum into the dichromate while the latter is still wet.

9) Drying: If you have buffed down the emulsion so there are no streaks, the print is nearly dry already. Five or ten minutes of air drying in a dark space with perhaps a few light passes with a hairdryer may be all that is needed to complete the drying. A gum print does not have to be bone dry the way a vandyke brown print does, but certainly dry enough so that it does not offset on your negative and stain it irreparably with dichromate as has happened to students and myself a few times. A wise insurance would be to insert a piece of clear thin acetate between the surface of your gum print and the emulsion side of a precious irreplaceable negative. Also be sure to inspect the glass and base of vacuum tables and other contact frames before entrusting print or negative to them. In a shared space it makes sense to work defensively…

10) Test Strips and Notes: Although it is frustrating to be slowed down at this point, taking the time to make a test strip will save time in the long run and will teach you much about the nature and possibilities of the medium. One exposure time cannot be given for every light-resist (negative) for 3 reasons: 1) different film-base-plus-fog densities and other variables in light-resists; 2) differences in the speeds of different colors (i.e. red slower than blue); and 3) your idea with a continuous tone negative of where you want to place the 5 steps of separation on a 21-step scale, i.e. shadows, mid-tones or highlights.

The paper used for the test strip should be identical in kind and condition regarding sizing to that of your final print, but it need not be as large so long as the critical areas of the negative (both highlight and shadow for each exposure on the test strip) can be exposed onto it and there are margins extending beyond the negative where ruled lines and exposure times can be seen without lifting the negative. Design your test strip before applying emulsion to avoid resting your hand on the sensitized paper or possibly fogging the emulsion through prolonged exposure to light. I like to use an HB pencil and a straightedge at least 12˝ long to draw lines in the margins.

Find a passage within the negative that includes crucial densities in the parts of the image that matter to you most. With the test paper on top of the negative on a light table, design the test sheet so that each of the 5 test exposures contains the range of densities that concern you. Your test sheet may end up divided into cells on a horizontal landscape, each frame containing a segment of land and sky — the range of densities that concern you. Or the test strip may be divided into long thin strips in a vertical portrait, each strip recording forehead, eye(s), chin, and neck. Using the straightedge, draw lines in the opposing margins of the test paper to mark the outer limits of the test strip. Within those lines draw 4 more sets of lines in the margins so that there are 5 different areas for 5 exposure times. I avoid drawing lines through the actual image area because the graphite might smear and otherwise obscure slight differences between one time and another. Mark in pencil on the test strip where any corners or edges of the negative are located, for coating and registration purposes.

A good test strip goes from being underexposed to being overexposed to reveal the range of possible exposures for shadow into highlight. Sometimes an exponential progression such as 1, 2, 4, 8, 16 units of exposure runs the gamut of possibilities better than an additive progression such as 1, 2, 3, 4, 5 units, especially with a continuous-tone negative that has dense highlights. Whichever progression is used, write the numbers between the lines on both sides of the area being tested. This will help you place the masking material during the actual exposing of the test strip when the negative will be hiding your image and all you have to guide you in placing the mask are your lines with times in the margins.

When you have coated the test sheet with gum emulsion and are ready to make the test exposure, tape the test strip to the negative and start out giving the shortest exposure to the entire test strip; then cover the part that was to have that minimum exposure and give the rest of the strip the next increment of exposure, advancing the masking material to cover the strip that goes with each exposure time, and ending up with one strip at the end that receives the longest exposure.

You will thank yourself eventually for going to the trouble of making test strips, at least a few times in the beginning, to get a sense of the range of possibilities with the kind of negative you are using. You will also thank yourself for keeping notes on the watercolors used, the exposure light source and duration, the date, and any other pertinent information. I write such notes around the edges of my printing paper and later transfer the information to a notebook if I exhibit, sell or donate the print. Although it is almost impossible to produce a uniform edition of multi-layered manipulated gum prints, you can come close with comprehensive notes or can make informed changes.

11) Exposing: Ideally, gum bichromate emulsion should be exposed soon after coating. If coated paper is stored more than a few hours, especially in hot, humid weather, unexposed parts may not release or severe freckling can occur. Heat and time as well as actinic light can trigger the hardening of gum emulsion.

Tape or simply lay your negative, emulsion side down for a right-reading image, against the dry sensitized side of the printing paper (or against a piece of clear acetate that lies on top of the sensitized paper) and place the ensemble in your contact frame so that light shines through the negative onto your paper. If you are making a test strip under a 275 watt sunlamp at 18˝ with a lith or thin continuous-tone negative or an imagesetter or desktop acetate negative, try making a test strip with a sequence of 2-minute exposures up to 10 minutes. With the same negative exposing in a black light box, also try a series of 2-minute exposures. On the platemaker try a series of 20-unit exposures (at least on our machine with its current calibration). In direct sunlight, try 10-second exposures. It is OK to place the opaque test strip mask on top of plate glass, rather than directly against the sandwiched negative and print, when exposing in the sun or under a sunlamp. The light will spread somewhat but with a point light source that casts a strong shadow, differences between exposure times should be visible. Dense continuous-tone negatives, and other light-resists with a wide range of densities require much longer exposures to print highlight detail. A test strip, with exposures that double each time, will be more informative than an additive test strip. Remember that a useful test strip goes from underexposure to overexposure, covering the range of possibilities.

Sometimes a thin negative that would produce a muddy print in vandyke or palladium will print perfectly in gum, but the exposure needs to be precise so that the emulsion hardens and releases where desired. A good test strip comes in handy.

12) Developing: Traditional gum development involves immersion of the print face down in 90°F water for about one hour. Gum prints developed this way have a matte surface because most of the gum arabic dissolves out of the emulsion. To have trays tied up an hour for each layer for each gum print, not to mention maintaining 90° temperatures for those trays, is a logistical nightmare in a class situation. Out of necessity I have evolved shorter exposures and a shorter coldwater development. This handling turns out to have aesthetic advantages, in my opinion. More gum arabic is retained on the print, lending a luster as layers build up. The printed areas are in relief as the paper is drying but when completely dry, the unprinted upon, lighter areas appear raised and the printed areas, gripped by the dichromated gum, recede, subtly sculpting the paper.

Using our method of development, slip the exposed test strip (or print) face up into a tray of cool water that is at least 2˝ deep. See that the surface of the print is covered with water. Then, wearing gloves (no bare fingers!), lift the print by diagonally opposing corners, turn it over and lower it slowly into the water again, face down. This way of touching the water with the center of the print gives air bubbles an escape route from beneath the paper. Air bubbles leave underdeveloped muddy circles. Avoid having the image area touch the bottom of the tray or anything else because the emulsion is now soft and easily marred. Let the print or test strip soak for a few minutes with occasional gentle agitation and a few careful liftings and flippings to speed the release of unexposed areas.

When water in the tray has grown murky from unexposed ammonium dichromate and pigmented gum leaching into it, lift the print, drain it, and place it carefully in a large washing tray that is elevated enough to accommodate a tray siphon. The tray should have fairly deep water and, ideally, no other prints floating in it. If other prints are already in the tray, watch your print to see that it doesn’t get marred — or mar someone else’s print. One gum print in the wash tray can be face down and another face up on top of it, but not face to face or face to back. It distresses me to see a delicate gum print being ruined in the final wash through careless handling. However, beginning gum students tend to overexpose their prints so that rough handling during development doesn’t always pose a problem for them. But they are producing prints that they had to scrub to clear and which lack the subtlety and sensuous surface that gum is capable of. In any case, in the final wash tray, adjust the water flow so that the siphon begins to remove dirty water while adding clean water, but not so hard that the print is buffeted and crimped.

13) Manipulating: After development in water, you can manipulate your gum print if necessary. It is helpful to practice manipulation on a well-designed test strip. Tilt a clean piece of plexiglas or seamed (smooth-edged) plate glass against the closer edge of the sink sloping away from you. Lay the soft-surfaced test strip face up on the smooth support and, holding the print in place with a gloved hand, start gently hosing cool water around its edges with no pressure. Remember that with a continuous-tone negative the length of your exposure will determine where the 5 or so steps of tonal separation that a single layer of gum emulsion can render will land on the continuum between clear and opaque film.

The first segment of the test strip, that is grossly underexposed, will release almost entirely, handled as in the paragraph above.

The second segment of the test strip, that has had minimal exposure, will render only the clearest most open parts of the light-resist — in other words the deepest shadows and other dark parts of the image. Those areas should be able to withstand a gentle flow of cool water. There may be times when you will want a skeletal rendition of a negative. Handle a print with such a delicately adhered image with special care to avoid marring until washing is complete and the print is thoroughly dry and tough.

In the third segment of the test strip, highlights will clear in still water and mid-tones will partially release with gentle cool hosing. This exposure may give the richest rendition of the negative.

With slight overexposure in the fourth segment, highlights may be covered with a veil of pigment that can be removed with a gentle stream of cool water. Some pressure may be added to the stream if the veil doesn’t lift. If that doesn’t work, warm the temperature of the stream of water.

The fifth segment that is more grossly over-exposed may require a soak face down in warm water. If that doesn’t loosen the highlights, try hot water and then sponge brushes. Gentle rubbing with a piece of woolen etching blanket or something similarly abrasive will sometimes release emulsion.

If you have not made a test strip and you end up grossly over-exposing a layer of emulsion onto well-sized paper, try sodium carbonate as described on pages 16 and 32.
If your gum print was reasonably well exposed you can work with a stream of water, sponge brushes, and bristle brushes, selectively removing pigment in the lightly and moderately exposed areas. Fan-shaped brushes help with a seamless removal of pigment.

When Scott McMahon was a student majoring in Photography at UArts, he deleted gum in scratchy, expressive lines with the wrong end of his brush to denote rainfall. Later in his gum work he brushed away parts of layers of emulsion selectively and allowed the brushstrokes to show. Even something as apparently clumsy as a scrap of etching blanket can be used to delete color in broad lines that look like pastel on rough paper as the paper or the previously printed color beneath the layer of pigment is revealed. You will probably think of other ways to manipulate gum.

l4) Washing: When you have finished manipulating your print, lift it up by a corner and see if the water that drips off the bottom corner is clear or tinged with orange dichromate and/or the color of the pigment you exposed. If the water is not clear, wash the print some more face down in the large tray with the siphon flowing gently until the print drips clear water. Hang the print from a clothespin to dry and rotate it occasionally. (Don’t stretch it tautly between two clothespins or it might dry ever so slightly wider on that end than the other.) Clean up carefully. Graduated cylinders should be rinsed in warm water. If emulsion has begun to dry in the graduate, add some sodium carbonate (or ammonia or bleach) to dissolve it. Soak sponge brushes briefly in tubs of water (tofu containers work well) and keep changing the water until it is clear. Shake the brushes in the sink with a snap to your wrist to get rid of water in the sponges or, if that motion begins to give you carpal tunnel syndrome, press them flat between gloved hands to squeeze out excess water. Dry them flat, stacked like Lincoln logs on tofu containers with no pressure on the ends of the sponges. Never dry brushes, either bristle or sponge, resting on their working ends. They will be splayed, crimped and generally rendered useless.

The dry gum print is tough, safe to handle, and ready to receive new layers without affecting the existing layers. Wash your hands even if you’ve been wearing gloves.

15) Clearing a Gum Print: You must wait until your print is completely dry, preferably overnight, before clearing it in sodium bisulfite to get rid of subtle yellow staining of highlights as well as all but the darkest chromatype underprinting. The mild acidity of this clearing bath allegedly sets (as well as clears) the gum print but I don’t think it is absolutely necessary to use this bath if you are happy with an uncleared print or if no underprinting is visible. Be sure to use the sodium bisulfite in a fumehood or outdoors since it has a strong odor that will make you cough. The same is probably true with sodium metabisulfite though I have not used it. Sodium sulfite does not have an odor but more of it is required and it takes longer to clear a print. Whatever you clear with, have a tray of water on hand to stop the action. You may wish to retain some of the warmth of the chromatype underprinting by not clearing it entirely. You can also apply the clearing agent in different concentrations with a brush to certain areas. Be sure to wash the print for a few minutes after clearing so that the acidity of the sodium bisulfite does not remain in the paper fibers.

16) Presentation of Print: If you have flattened your print after each layer of printing you can easily mat and frame it. If your print is very curly, do not ‘break’ it by placing weights on it or putting it in a drymount press. Instead, spray a fine mist of water on the back of the print to relax the curl and leave the print face up under plate glass on top of foam or some other porous surface until it has dried.

Occasionally there will be white or light spots on your finished gum print which you will wish to bring down in tone. Spotone dyes work well on gum prints, sinking right into the emulsion. From among the 6 Spotone colors that are sold it is possible to find or mix a match with appropriate dilution to spot small areas in gum prints. If these dyes are used to darken just a tiny light spot it probably will not matter if the dye is not entirely archival and changes a little over time. If you prefer to spot with watercolor, add a drop of Photo-Flo or some other surfactant to the watercolor to reduce surface tension and allow the color to sink into the emulsion. The Pitt artist pens by Faber Castell are lightfast. Last year a range of grays, terras, and landscape colors , all with the brush tips, were introduced. I use Pitt pens to spot my gum prints. Mistakes can be sponged off with water on a bit of sponge if the print is covered with tough gum emulsion.

If you are matting your gum print, the issue of where to crop it will arise. Do you allow only the image to show, emphasizing the content of the image and bringing out its pictorial qualities, or do you permit the brushed borders to show, thus emphasizing the process and revealing the individual colors that formed the image? If showing the borders, how much should be shown? Should the print be floated on top of a piece of matboard? If you are cropping the image, do you let the edges of the film show? Can you crop into the image? There are no rules about presentation just as there are no hard and fast rules about how to work in gum. It is up to you to find your own way of working with this complex expressive medium.

17) The ‘Failed’ Print: If you are not happy with your completed gum print, do not throw it away. Your teacher, if you have one, may be able to help you understand what happened and what might help you achieve what you were after. For instance, if your highlights are freckled with pigment from the last layer of emulsion you applied, the problem is that your sizing wore away just where you wanted it to work best. This often happens when the sizing is unhardened gelatin because it is soft and abrades easily; or if you washed your print in water over 80˚, as can happen in the summer, the gelatin simply dissolved and washed away – this does not happen with hardened gelatin. You probably didn’t have freckles where previous layers of gum emulsion were already printed because those layers acted as sizing. Or perhaps over-exposure darkened your highlights. Instead of throwing a print with freckled or muddy highlights into the trash you might print the highlights from a positive transparency in a layer of titanium white emulsion. If you don’t already have a full-size positive from making an enlarged negative, a positive can be obtained in the chemical darkroom by contact printing the negative against another sheet of film or in the digital darkroom by scanning the negative and generating a positive on inkjet or laser acetate. You may be pleasantly surprised to recover your highlights and end up with a print that is more interesting than if everything had gone as planned.

Finally, there is no rule (except a self-imposed one) that says you can’t draw, paint or collage on a gum print or work the gum print into a larger piece. From apparent failure you may be led into a new way of working.

Read the next section of the book.

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3 thoughts on “A Non-Silver Manual: Gum bichromate”

  1. Hey i have printeg on a so called spooky picture that says imprinted on the back Artist Material GRUMBACHER maybe i am spelling it wrong number # thirty 30 and i am trying to figure out what date the picture looks like it is posted in London through some arch wats with tall buildings and cathedral type setting there are also five people standing around one that is in the most backround is being walked in front of and there is another picture that i have left behind that is of a big bouquet of red roses left at the so called spooky house as weel green backround looks like a spooky body of roses keep in mind

  2. @Renee. The book is no longer for sale. You can read all of it here online. Just click “Read the next section of the book” to read the next chapter. It’s all there.

  3. Thank you for posting this chapter. I am a beginning gum printer and have learned so much for reading this article. I would like to purchase this book. How can I do so?

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