Gum Bichromate: How it Works

For many years now, I have worked in various historic 19th century photographic printing techniques.  I concentrate mostly on gum bichromate; platinum/palladium; and cyanotype.  All are hand-applied contact printing processes, exposed only by UV light.  Each image is unique, one-of-a-kind, and hand-brushed on watercolor paper.

• Gum Bichromate. The creative possibilities of this process are seemingly infinite, which is what keeps me returning to it again and again.

• Layer by Layer (the short version): In a tricolor or multi-layered gum bichromate print, an image is built layer by layer. Typically, a color image is uploaded to a computer, and with proper curves applied for this particular process, CMYK or RGB separation negatives are made, via the wonders of 21st century technology. Those negatives are then printed on transparency film. Since this is a contact printing process, your image is only as big as the negative used.

  Watercolor paper (or another substrate) is brush-coated with a light-sensitive chemical (potassium or ammonium bichromate), which has been mixed in a specific concentration of gum arabic and watercolor pigment. When this coated paper is dry, the negative-- the same size as the final print-- is placed on top, and exposed to ultraviolet light. The light activates the dichromate, which hardens the gum in proportion to the amount of light it receives. After UV exposure, the print is then 'developed' in trays of tap water. The dichromate and any unhardened gum will be leached from the print in the wash water. Specific watercolors, used with the corresponding negative (ie, cyan to cyan; yellow to yellow, etc), will ideally remain. Any number of layers can be added- or even just one layer can be used to make a print. However, to build a full, rich image, multiple layers (with perfect re-registration of negatives) are required. What ultimately remains is watercolor pigment encased in hardened gum arabic. This is one of the most archival photographic printing processes available.

Below is the evolution of a tricolor gum print. In this example, three separation negatives were made via the wonders of 21st century technology.  The original color image was scanned, uploaded, and separated onscreen into cyan, yellow, and magenta negatives.  Each of these separation negatives was printed digitally on Pictorico transparency film.  

Each layer was then brushed on (in this case, using BFK Rives watercolor paper) with the corresponding pigment color (cyan to cyan negative; yellow to yellow negative, etc), in specific ratios/emulsion of gum arabic, pigment, and bichromate.  Each negative was re-registered for each layer, exposed again, and  water-developed.  

The example below illustrates the first layer (cyan), then the second (yellow) layer, and magenta-- the final layer.

Sometimes I start with the magenta color and end with cyan; most often, I start with the cyan layer, since the darkness of the blue makes the subsequent layers a little easier to see for registering the negatives.  Exposure times can vary for each layer.  Although this is an example of only 3 layers to achieve a final image, more layers can be added, with varying ratios (gum to pigment to bichromate) and exposure times to achieve your desired look.

Below is a tri-color gum print, using 3 separation negatives (cyan, yellow, and magenta), and only one pass for each on this particular image.  I use Pictorico transparency film for the digital negatives and BFK Rives heavyweight watercolor paper.

The light-sensitive bichromate, exposed to ultraviolet light, hardens the gum arabic in direct proportion to the amount of light that has passed through it.  When the print is placed in the development water, soluble gum arabic, carrying pigment with it, washes away from the sections of the image that did not receive enough light to harden it.  What remains is an image of pigment encased in hardened gum arabic.

The image below was originally made with a 'Diana' toy camera.  The first cyan layer is shown left; final image is to the right.

Initially, this process appears relatively simple and straight-forward, yet it is rife with variables, frustration, mistakes, unpredictability, and frequent heartache.  It is labor intensive; often, each print can take days to make.  This unpredictably and lack of control is, oddly enough, part of its charm and why the creative possibilities are truly endless.

Another example of the evolution of a print, starting with a cyanotype base, so you can see the difference color can make.  The first print is cyanotype; the second with black pigment added to that cyanotype base, and then yellow and magenta layers were added to that in the 3rd image here.

I always believe that the printing process I choose for my images should mesh with and prove meaningful to the subject matter itself.  I often find myself printing an image in a couple of different processes to determine what technique best fits with what I want to say with a particular image.  

I printed this image, titled Ribboned Water, in two different ways.I first printed in a cross-process of platinum/palladium and gum.  

The first layer was platinum/palladium; after that layer was exposed, developed, cleared, and dried, I added 3 layers of gum. 

This first layer of platinum/palladium resulted in an overall sharper image and a somewhat cooler and softer color palette.  Once dry, the three CMY gum layers were added.

Platinum and palladium are sister metals that work well together and provide beautiful long tonal range prints that offer a mix of warm brown and cool gray.

This emulsion is a mix of pt/pd and ferric oxalate (the light sensitizer).  Exposed under UV light, the pt/pd print is then developed in a bath of potassium oxalate and distilled water.  I then clear the image in a bath of EDTA and water, and sometimes an additional bath of citric acid and water, and multiple clear water rinses.

The second print was done in tricolor gum bichromate alone, using 11 layers in various calibrations and ratios (gum to pigment to dichromate).  With this tricolor gum print, I followed Tony Gonzalez's (not the NFL great, but the amazing gum printer) 11-layer gum recipe that he generously provides in Christopher James' The Book of Alternative Photographic Processes.  I had my doubts, after about layer 7-- when I thought the print looked "done," but I pressed on.  Those 11 layers took many days to complete, but I love the soft, yet vivid and rich colors, so a big thanks to Tony for helping to positively transform the way I now choose to gum-print my images.  Now, my mantra- in all things pertaining to gum- is more along the lines of "This seems finished, but maybe I should add just one more layer . . "  

This 11-layer gum print was made on BFK Rives Heavyweight paper, which I did not pre-shrink, nor add any additional sizing.  

So how you choose to print is a personal preference, and the differences can be subtle- as shown in these examples- but one will probably be closer to your goal and your meaning.

To print the same image in various processes offers a different mood, feeling, and look.  Veiled Woman, on the left is printed in cyanotype, on the new/revised Arches Platine paper.  In the middle, is the same image cross-printed in cyanotype over platinum/palladium. On the right, the same image printed in tricolor gum bichromate (7 layers).  All were made on the new Arches Platine-- a beautiful paper for alternative printing processes.  No pre-shrinking or additional sizing for the multi-layered prints was required.