Introduction to Gum Bichromate

Dunes. Tricolor gum bichromate. Copyright Katharine Thayer 1995 or 1996. Private Collection.

What is gum bichromate?

Gum bichromate is a 19th-century, hand-coated, contact-speed photographic process in which paper is coated with a light-sensitive chemical mixed with gum arabic and pigment, then exposed to ultraviolet light through a negative the same size as the final print. The print is developed in tap water. Monochromes, duochromes, trichromes (realistic color or interpretive color) or any number of chromes you like can be made by this process.

A great range of expression is possible, including prints with normal photographic tonality and near-photographic detail, high-contrast graphic effects similar to silkscreen prints, and moody impressionistic "photo-paintings" reminiscent of pictorialists' work. The images represented in the image gallery on this site show a few of the many ways I have personally used gum bichromate. This gallery is not intended to represent the entirety of my work, but simply to show a few examples of the printing effects that gum is capable of.

How the process works in brief:

The light-sensitive chemical (ammonium or potassium dichromate), when exposed to ultraviolet radiation, hardens the gum arabic in direct proportion to the amount of light that has passed through it. When the print is placed in water, soluble gum arabic, carrying its pigment with it, gradually floats away from the sections of the image that didn’t receive enough light to harden it. What is left is an image made of pigment encased in hardened gum arabic.


Gum has the advantage of permanent color (as long as permanent pigments are chosen), as opposed to most modern color photographic processes. In addition, gum offers the printer flexibility in creative expression, unlimited color choice, and the satisfaction of making a handcrafted art object with a unique alternative appearance. Compared to other photographic processes, gum is forgiving in many respects: overexposure, underexposure or color imbalance can usually be corrected without having to throw the print out. In addition, a darkroom is not absolutely necessary, since the process can be carried out in ordinary light and negatives can be made digitally. The materials necessary for gum printing are quite inexpensive in comparison with the materials for some other processes, and the process itself is quite easy to do.


Gum is a short-range, rather labor-intensive, slow emulsion (that's why a contact negative is required, because the emulsion isn't sensitive enough to allow projection printing) often requiring multiple printings in order to realize the range of tones a printer may wish to express in an image. While the process is one of the easiest to do, it's one of the hardest to control in the sense of understanding all the variables well enough to reliably get the results you want. Learning gum printing involves some trial and error and there's no short cut to mastery; a person successful in mastering the process will have some staying power and possess a sense of humor and some tolerance for failure.

Can Gum Printing be Taught?

Only to a point. From teachers or texts one can learn general principles, but since each combination of materials and equipment requires different methods to produce similar results, and each gum printer may have different results in mind, each gum printer must work out the specifics that relate to his own particular situation by trial and error. There's simply no way to get around this. Well, sure, if you're learning at school, then you're using the same lab and materials as your teacher is using, in which case you should be able to get the same results with the same specific instructions. But doing that, you won't be learning how to print gum, only to copy specific instructions, and when you leave there and go somewhere else, or set up your own lab, you'll have to start over from scratch as far as developing a more general understanding of the method. At any rate, the audience I envision myself being useful to are people learning the process on their own, as I did, not academic students.

One of the great early masters of the process had this to say:

"There are, therefore, many questions upon which the worker who wishes to make himself acquainted with all the capabilities of his medium will do well to satisfy himself. Amongst them the effect upon the image of different relative proportions of gum, colour, and chromic salt, and the thickness of the coating, are worth his careful study. For all these matters it is impossible to formulate direct instruction. In the preparation of the coating mixture and its application to the paper, a certain amount of practice will teach a good deal. For instance, the right consistency for coating will be instinctively recognized without the necessity of weighing and measuring. One finishes by feeling what is the right proportion. It cannot be explained. It is like the knack of an engraver wiping the plate with the palm of his hand -- he feels that it is right--while another man doing the same thing would do it wrong." -- Robert Demachy, from "Photo-Aquatint or the Gum-Bichromate Process" by Demachy and Maskell, 1898.

This kind of talk is often dismissed, in our era of instant results or forget it, as deliberate evasion designed to keep others from learning the particular printer's secrets. I can't know the motives of 19th century workers, but in the 21st century I still believe this to be true: one learns to print gum only by printing gum.

Why I am a gum printer:

I had a mental picture of the kind of photograph I wanted to make. I had never seen any photographs like them, but I was determined to find a way to make them, these pictures I saw in my head. Their colors were soft and relatively unsaturated, but with a kind of glow about them. I had never seen autochromes, but years later when I saw some reproductions of autochromes, I realized that the look of autochromes is probably closer to what I was imagining than anything that was ever done in gum historically. It's probably a good thing I didn't see the autochromes before I discovered gum, or I may have spent the last dozen years trying to make autochromes.

At any rate I tried and discarded several other possibilities, such as Polaroid transfer, solvent transfer, lith printing, toning, before I found gum. I dabbled in digital art before most photographers had ever heard of Photoshop, using channel operations and calculations in a very early version of Photoshop to create primitive jury-rigged filters that gave my photographs an impressionistic sort of watercolory look. Each of these methods produced interesting results, but none of them was what I was looking for. Finally I came across Suda House's book "Artistic Photo Processes" in a library and saw Todd Walker's dreamy images printed in gum on silk; though his images weren't quite like the ones I was thinking of, I was sure I'd found my medium. I set out first to teach myself to print in gum, then to adapt the method to produce the kinds of pictures I wanted to make, and have been making them ever since.

Copyright Katharine Thayer. All rights reserved.

Introductory notes page