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The advent of photography has had enormous impact on our civilization. Our record of the late 19th century is much richer and more complete than that of the early 19th centrury, in part because of the use of photographs to document historic events, places, and people. The story of photography really begins with the camera obscura (c. 1580 AD). This was a dark room with a lens mounted in one wall which projected a scene onto the opposite wall. An artist could sit in the room and trace the image to produce a very accurate record of the scene. In 1725, Johann Schulze discovered that silver nitrate was light sensitive. A sheet of paper soaked in silver nitrate would turn black wherever light hit it as silver ions were reduced to silver metal. The image was not fixed, however, in the sense that when the photograph was brought into daylight, the whole image would turn black. It was almost 100 years later (1819) that sodium hyposulfite was found to remove the unexposed silver ions, leaving a permanent or "fixed" image behind. The first widely used photgraphic process was developed by Daguerre and Niepce in 1839. They used copper plates coated with silver which had been exposed to iodine vapors (producing silver iodide). Wherever light hit the silver iodide coating, the silver ions were reduced to silver metal. The unexposed areas were removed with sodium hyposulfite, leaving exposed copper. Thus the image was bright (silver) where light had exposed the plate, and dark (copper) where it had been unexposed.

Silver processes were improved continuously throughout the 19th and early 20th centuries. Silver bromide replaced the earlier silver chloride and iodide processes beginning in 1885. The first celluloid films were introduced in 1895. This film was composed of nitrocellulose, which is extremely flammable. Celluloid was phased out in the 1930's with the advent of safety film. Color photography was commercialized in 1935 with the introduction of Kodachrome. Polaroid instant prints were introduced in 1948. Today almost all photographic film consists of silver bromide on acetate film. Almost all photographic paper is silver bromide on various papers.

Most photography is based on silver as the light-sensitive agent. But there are several non-silver processes. Cyanotypes (blueprints) are based on photosensitive iron compounds. Platinotypes are based on platinum chloride. And the gum bichromate process is based on light-sensitive chromium compounds. The gum bichromate process was popular from about 1880 to 1920, particularly among artists. It has the advantage of relatively simple chemistry and color photography is as easy as black and white with this process. It has the disadvantage that it is less sensitive than silver bromide and so exposure times are much longer. Consequently the bichromate process has been used only for prints, not for negatives. After the invention of color silver bromide photography, the bichromate process fell into obscurity. Beginning in the late 1970's the bichromate process regained some popularity in artistic circles.

In devising this project, I was torn between two competing desires. First, I have always promised to "teach you as little as possible." We have already discussed silver and we are fully prepared to discuss the redox chemistry involved in the exposure and processing of silver-based photography. But I have also tried to have you make things from scratch to the extent possible. With gum bichromate photography, we will be able to make up our own color photographic emulsion and use it to coat even potentially homemade paper, but we will have to discuss a new metal, chromium. This is the route I have chosen to take.

Chromite Ore

Chromium compounds are manufactured from chromite, Cr2O3. Chromium minerals were first discovered in Siberia in 1763. These brightly colored minerals, chiefly lead chromate, were used as artist's pigments. By 1797, chromium was recognized as an element. In 1811, chromite ore began to be mined in Maryland. These days, South Africa and Russia supply about 60% of the world's chromite ore with lesser contributions from Albania, Turkey, Zimbabwe, Finland, and the Phillipines.

The ore is composed of chromite (44-53%), iron compounds, silica, alumina, and a host of minor components. To separate the chromium from the other constituents, the ore is crushed, mixed with soda ash, and heated under oxidizing conditions in a kiln at 1100-1200 °C:
2 Cr2O3(s) + 4 Na2CO3(s) + 3 O2(g) -----> 4 Na2CrO4(s) + 4 CO2(g)
The sodium chromate is soluble in water while the other components are not. So the calcined ore is leached with water, removing the chromium and leaving the gangue behind. The leachate is concentrated and acidified with sulfuric acid:
2 Na2CrO4(aq) + H2SO4(aq) -----> Na2Cr2O7(aq) + Na2SO4(aq) + H2O(l)
Sodium dichromate is about 10 times as soluble in water as sodium sulfate. The acidified solution is evaporated until the sodium sulfate precipitates and this precipitate is filtered off. The remaining solution is evaporated to produce the raw sodium dichromate product which is the starting point for most chromium compounds.

We will use ammonium dichromate for our photographic process. This chemical can be produced by a simple metathesis reaction:
Na2Cr2O7(aq) + (NH4)2SO4(aq) -----> (NH4)2Cr2O7(aq) + Na2SO4(aq)
Again, while sodium sulfate is soluble in water, it is much less so than ammonium dichromate and so can be removed by evaporating the solution and filtering off the sodium sulfate.

Chromium compounds are used chiefly for pigments because of their bright colors. The second most important use for chrome plating of other metals. The third most important use is for mordanting textiles and tanning leather. The dichromate ion is a powerful oxidizing agent and it is for this reason that dichromates are used as mordants and tanning agents. Its oxidizing power is also the reason we will be able to use it for photography. .

Gum Bichromate Photography

The second compound we need is gum arabic, also known as acacia. This is a gummy compound exuded by the acacia plant. It is collected, purified, and dried into a powder. Acacia is soluble in water but loses its solubility when it is oxidized. It is this change in solubility which we will exploit in our photographic process

Gum arabic has been used for centuries as a component of paints. Initially soluble in water, it is oxidized over time by the oxygen in the air to an insoluble form. Bum bichromate photography simply uses dichromate as an oxidizing agent to speed up this "hardening" process. Since the dichromate ion is brightly colored, we know it absorbs light. In fact, its oxidizing power is increased by exposure to light. Let us then imagine a mixture of gum arabic and ammonium dichromate painted onto watercolor paper and left to dry in the dark. Initially, we could simply wash the paper in running water and all of the gum arabic would wash off. If left for a day or more, the dichromate will oxidize the gum arabic to its insoluble form. But if we expose the freshly dried paper to bright light, the light will increase the rate of oxidation in the exposed portions and the exposed gum will harden to its insoluble form. The unexposed gum will remain water soluble. After exposure, if we wash the paper, the exposed gum will remain on the paper while the unexposed gum will wash off. This is the principle at work in gum bichromate photography.

Such a photograph will be quite dull and boring, since gum arabic is naturally light tan in color. But remember the use of gum arabic for mixing paints. If we add watercolor pigments to the gum, any hardened gum will retain its pigments when the print is washed. Thus it is every bit as easy to make colored prints as to make black and white prints. Granted there will be only one color, but the artistic possibilities are much enhanced by this capability.

Because bichromate photography is less sensitive than silver-based photography, a bright light and long exposure times are needed. In practice this means that bichromate photography is done by contact printing. In this process, a negative is placed on the emulsified paper and exposed to light for several minutes. The paper is then washed in water, leaving a reverse image of the original negative.

Other Bichromate Pages

Books on Reserve

The Photography Quiz

The photography quiz consists of three questions on the following topics:

Safety and Common Sense

Gum Arabic is relatively safe. Don't drink it. If you do, drink lots of water.

Ammonium dichromate is the strongest oxidizer we will use this semester. It should be treated with respect. It will stain the skin and, like lye or sulfuric acid, could cause blindness if splashed in the eyes. Glasses and gloves should be worn by anyone handling this substance.

In addition, chromium compounds have been shown to produce cancer in laboratory animals. This doesn't mean that one drop of a dilute solution is certain to give you cancer. But ammonium dichromate exposure over long time periods could increase your risk of cancer. So it exhibits both an acute (immediate) and a chronic (long term) hazard. This is why we wear gloves and glasses when handling it: to minimize our exposure.

On the other hand, ammonium dichromate does not warrant paranoia. It is a wonderful little chemical and can be used safely with simple precautions. Skin should be washed if you get a little careless. If you get it in your eyes, you should wash them immediately with cold water and call a hospital. A taste will leave a very unpleasant taste in your mouth. If you are foolhardy enough to eat it by the spoonful, you should call a poison control center and head immediately for the emergency room. I hope you make it.

Information on chemical hazards is summarized in a Material Safety Data Sheet for each compound. These sheets often tell you more than you want to know, but they are worth looking at.


Mix 1 tbsp each of the gum arabic solution and the ammonium bichromate solution with enough watercolor to give a deeply colored emulsion. Paint this emulsion onto a piece of paper and let it dry in a dark place. When dry, place the paper under the light box, place a negative or object on the paper and turn on the lights for a minute or two. You may have to experiment with exposure times. Wash the exposed paper in water to remove the unexposed pigments and allow to dry.

Criteria for Success

When you present your photograph to me, I will look at it. Well, either it is a photograph or it isn't. If it is, you pass, if not, you fail.

Pinhole photograph of Gilmer Hall by Paul Mueller
Pinhole camera with 4x6 inch Ilford paper negative
Gum bichromate contact print using black watercolor on Arches paper
Self Portrait by Paul Mueller
SGI Indy camera laser printed in negative to transparency film
Gum bichromate contact print using green watercolor on Canson paper
Self Portrait by Kevin Dunn
SGI Indy camera laser printed in negative to transparency film
Gum bichromate contact print using blue watercolor on Canson paper
Jon Vordermark's Keys by Kevin Dunn
Direct contact keys on paper
Gum bichromate contact print using blue watercolor on Canson paper