Some years ago, I stumbled upon a book called The Photographic Instruction Book, by Townsend T Stith © 1903. The book was filled with all sorts of advice on how to take successful photographs using the tools and technologies of the day. Surprisingly, many of the Mr. Stith’s suggestions, including advising against taking pictures from moving bicycles, hold true today.
I especially enjoyed reading the instructions for what we nowadays call “alternative processes,” including cyanotypes and gum bichromate prints, aka gum prints.
Gum prints, which date back to the earliest days of photography, are essentially photographic watercolor prints with color palettes not unlike the muted tones of Polaroid transfer prints, a process popular among fine art photographers in the 1970s and ’80s.
To make traditional gum prints, you first size the paper with clear gelatin or egg albumin, let it dry, and follow up with an emulsion consisting of gum arabic, potassium, (sodium, or ammonium) bichromate, and watercolor pigment. Once dry, you place the paper emulsion-to-emulsion against a film (or paper) negative in a glass contact frame, expose it to bright light, and process it in a tray of cool water.
Gum printing requires commitment. Once you start the process, which start-to-finish can take the better part of a day, you can’t pause and put it off for a more convenient time. Having learned the art of gum printing some thirty-plus years ago, I decided to revisit the process and update it using tools and materials that didn’t exist my first time out with this process.
Creating Large Negatives
I started the process using 35mm color slides that were enlarged to final size using an enlarger, onto Kodak Kodalith Type III film, which is no longer available. In its place you can use Ilford Ortho Plus B&W Negative film or Rollei Ortho 25 B&W Negative film, both of which can be handled and processed under red safe lights.
Alternatively, you can easily produce digital negatives on your inkjet printer using transparency films for inkjet printers, which are available in sheets and rolls. You don’t need a darkroom, and digital negatives are quicker to output.
Choosing a Paper
Any paper can be used for gum printing, but thicker, heavier 100% rag watercolor papers are recommended for reasons of aesthetics, physical vulnerability, and longevity. Watercolor paper stocks are available with smooth (hot-press) or textured (cold-press) surfaces—the choice is yours.
You can also use many of the fine art and watercolor papers made for inkjet printing. Available in weights up to 500 gsm and sheet sizes up to 44 x 60", many pre-sized fine art inkjet papers are available at B&H Photo.
Sizing
Back in the day, paper had to be sized with clear gelatin or egg albumen to prevent the pigments from bleeding into the paper fibers. Today, you can save your eggs for omelettes because there are numerous pre-sized watercolor papers available including most fine art inkjet media, which are perfectly suitable for gum printing.
The Photosensitive Emulsion
The photosensitive emulsion is a mixture of gum arabic (the base ingredient of chewing gum!) and potassium, sodium, ammonium, or potassium dichromate (the photosensitive component), and pigment—powdered or in tubes.
Gum Arabic can be mixed from powder or purchased pre-mixed from B&H, which is preferable to powdered gum for reasons of consistency. The ratio of pigment to gum solution is approximately 7 to 8 grams of pigment to 100ml of gum solution, though after a bit of testing your ratios might differ.
Liquid gum Arabic and potassium bichromate solutions have long individual shelf lives when refrigerated. Once mixed, the solution has a shelf life of about 24 hours, so plan accordingly.
Pigments
Lamp black was once the go-to pigment because it was readily available. Color pigments were also used and often blended for deeper colors and tonality. Today, powdered and liquid pigments in tubes are readily available in art supply stores and online. Once dry, pigment is as permanent as the paper it’s printed on.
Applying the Emulsion to the Paper
Apply at least two coats of sensitized emulsion onto the paper surface omnidirectionally using a camel hair or polyfoam brush or a sponge. When finished, pin all four corners of the paper to a flat surface in a cool, dark location until dry.
Once dry, the emulsion only remains light-sensitive for about 8 to 12 hours, which is why if you plan on using sunlight as your light source, you should start the process immediately after your morning coffee.
The terms “dichromate” and “bichromate” can be used interchangeably. Regardless of spelling, handle with care. Always wear gloves, keep the wet emulsion away from your skin and eye, and work in a well-ventilated space.
Exposure Times
Exposure times vary based on the light source and the chemical composition of your emulsion. The best results (and shortest exposure times) can be achieved by using strong UV light sources such as sunlight, black lights, HMIs, fluorescent, or mercury vapor lamps. Expect exposure times ranging from 20 minutes to several hours.
Establish your exposure times by making test strips in 10-minute intervals by placing your print-size negative and coated paper emulsion-to-emulsion, negative facing the glass of the contact frame with an opaque covering over the surface. Every 10 minutes, slide the covering a half and repeat until you expose the entire strip. Process the print and choose your best exposure.
Processing Gum Prints
Take your exposed print and gently float it on top of a tray of (approx.) 80° water, emulsion side down, keeping movement to a minimum. In time, unexposed pigment will begin falling away from the print surface. Lift the print out of the water for inspection every 10 minutes to check its progress and return the print emulsion side down to a fresh tray of water. If the emulsion begins falling away quickly (or entirely) your exposure is too long. If little or no emulsion falls away during processing, your exposure is too short. To better clear highlight areas you can take a soft, wet camel-hair brush or moistened cotton swab and gently brush away unwanted pigment.
The emulsion is extremely delicate when wet, so handle prints gingerly until they’ve thoroughly dried.
3-Color Gum Printing
Having figured out monochromatic gum printing, I decided to try to reproduce a few 35mm slides as three-color gum prints by enlarging each slide through red, green, and blue filters onto negative film. I then coated a piece of paper with a layer of yellow-sensitized gum emulsion and exposed it through the blue-filtered negative. After processing and drying the print I coated it with magenta-sensitized emulsion and exposed it through the green separation negative, processed and dried the print, and repeated the process again using cyan-sensitized emulsion exposed through the red separation negative. It took several days to produce each print, but the experience was worth the time and effort.
Additional gum printing tips, recipes and workflows can be found online. Have you tried gum or other alternative print process? Drop us a line and tell us about it.
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