The Photographic Development Process

and Its Potential Applications in Interdisciplinary Instruction


Andrew DeVita
CHEM 367
6 December 2011

Abstract

This paper will discuss the photographic development process and potential applications of the chemistry behind it for use in an educational setting, particularly in laboratory courses for non-science majors. First, a short history of photography will be presented along with a brief description of the chemistry involved in early photographs (daguerreotypes), as well as in modern color and instant photographs. Finally, a survey of experiments designed by researchers in science education will be discussed and an argument in favor of such interdisciplinary lessons will be presented.


Introduction

With the increased emphasis on STEM (Science, Technology, Engineering, and Math) achievement in the United States comes the challenge of getting young adults who have been raised in a society that is often merely tolerant, if not openly derisive of people in scientific fields interested in science. One method for getting young people interested in their science courses is to up the “cool” factor and give them a visible change that they can see and know a reaction has taken place. Developing photographs is one such experiment that has a visible chemical reaction, a solid background in several branches of chemistry, as well as providing students with the opportunity to take home a photograph and share what they learned in lab with friends and family, thus having a built-in mechanism for reinforcing the subject matter outside of class.


The History and Science Behind Photography

One of the earliest photographic processes was introduced by Louis-Jacques-Mandé Daguerre in the mid-19th century. His processes, called the daguerreotype, started with a copper sheet plated with silver, polished, and exposed to sensitizing iodine vapors, before being put in a camera and exposed to light.[4] After the picture was taken, the plate would be developed with mercury vapors that would condense where the light had fallen, creating an amalgam with the silver iodide.[3] Then, the image would be fixed by removing the undeveloped silver with a concentrated solution of table salt (NaCl) or sodium thiosulfate.[5] The plate would sometimes be treated with a layer of gold chloride solution before being washed and dried.[4]

The silver-mercury amalgam would appear white where the silver iodide had been exposed to light, whereas the other parts of the plate would be dark, thus giving a very good black-and-white image.[3] The “photograph,” then was the same copper plate that had been used in the camera—this means that each plate became the picture itself and no copies could be made.[4] In 21st century America, where people have cameras on their computers, phones, and even MP3 players, it could be difficult to imagine only getting one or two chances to get a good family picture, let alone not being able to share it easily with friends and family!

Below in Figure 1 is a daguerreotype taken in 1840 by photographer Robert Cornelius, just one year after the daguerreotype process was disclosed to the public [4]. It shows the intersection of 8th and Market Streets in Center City Philadelphia as it looked over 150 years ago!

daguerreotype-philadelphia_8th_and_mkt.jpg
Figure 1: Daguerreotype of downtown Philadelphia, part of the Daguerreotype collection of the Library of Congress


Still, Daguerre's images required only a 30 minute exposure time, a tiny fraction of the 8 hours' exposure time needed for older images, such as those made by Joseph Nicéphore Niépce's heliograph procedure.[7] Where photography had once been just a passing curiousity, the much shorter exposure time brought photography indoors and into the age of portraiture.

As groundbreaking as these images are, they were not without problems. When the silver on the plate was exposed to chloride-containing contaminants, silver chloride would form, which is a light-sensitive compound that would appear as white spots.[4] Additionally, daguerreotypes were prone to tarnishing, due to the reaction of the metals with sulfide present in the air.[3] These chemically-based complaints should be taken in addition to the practical problems—the inability to make copies of a daguerreotype, for example, as well as the chemistry knowledge required for photography in contrast to the popularity of photographs.

The first color image produced by photography was made by Clerk Maxwell in 1861, who used three filters—one each transmitting blue, green, and red light—and three projectors to create a colored image on a screen.[20] This setup used an additive color mix[20], in which red, blue, and green can be combined to create all colors of the spectrum by blocking the other wavelengths. In an old-fashioned television set, an additive color process changes black-and-white images to color by using a mosaic of red, green, and blue filters.[18]

As much as Maxwell's experiment paved the way for color photography, it was simply not practical for someone to carry around three projectors to make one image.[20] In addition, because the additive process transmits a particular color while blocking out other wavelengths, layering blue on top of red, or vice-versa, would result only in a black part of the image,[18] as shown in Figure 2 below, therefore making it impossible to use an additive color process when taking photographs.[20]

additive.png
Figure 2: Colors superimposed using additive process [20]


In contrast, modern color photography uses a subtractive process, in which the colors used are the complements of those used in the additive process. The colors used in the subtractive process are yellow, magenta, and cyan.[18] Because each color absorbs only one third of the visible light spectrum, the colors can be superimposed. By proper mixing of yellow, magenta, and cyan, a very good range of colors can be created, as demonstrated in Figure 3.[20]


subtractive.png
Figure 3: Colors superimposed using subtractive process [20]



Now that the basic theory behind photography has been discussed, it is necessary to take a closer look at the chemistry that takes place when a photograph is taken, the negative produced from the latent image, and the negative printed into a picture. An in-depth examination of the relevant reactions and mechanisms have been studied[8], but goes beyond the scope of this paper. However, the basic chemical principles behind the image developing process will be briefly discussed below.

On its most basic level, photography uses the transformation of silver halide crystals embedded in gelatin when film is exposed to light. The gelatin of the film replaces the silver plating on the copper sheets used in daguerreotypes, although they both serve the same function by keeping the silver in place to be exposed to light.[1] A flowchart showing steps in image formation is presented in Figure 4 below.[13]


photography_flowchart.png
Figure 4: Steps in image formation in the conventional silver halide photographic process[13]

Figure 4 shows how the silver halide of the photographic material is transformed into a latent image by exposure to light. The latent image is then developed through a redox reaction and “fixed” through a complexation reaction with the silver.[13]

Color film consists of many layers of silver halides treated with special dyes that are sensitive only to specific wavelengths of light. The addition of the dye also has the desirable effect of reducing the exposure time necessary to produce a good image.[10] These layers are stacked onto a support in a specific order. This arrangement has been determined to result in the best color. The blue-sensitive layer is on the top, then a green-sensitive layer, and finally a red-sensitive layer. Because silver-halide salts are especially sensitive to blue and ultraviolet light, a layer of yellow dye is put in between the blue and green-sensitive layers to prevent blue light from reaching the bottom two layers and obscuring the proper colorization of the image. [20]

Development of both black-and-white and color photographs is done through a two-step procedure. In the first step, a latent image is created on the film by exposure to light through a camera. The latent image is developed to a “negative” silver image in the first processing step,[20] and then the negative is then printed on positive paper to form the color photograph.[18] There are four main solutions used in the processing of photographic film—developer, bleach, fixer, and stabilizer.[11] Additionally, the reactions that occur with these solutions and the dyes on the film are detailed in Table 1 below.[11]


table_--_rxns_in_color_photo_processing.png
Table 1. Five Reactions Used in Color Photographic Processing with their Photographic and Chemical Names[11]

Note: D is the developing agent (reducing agent), B is the bleach (oxidizing agent), C is the color coupler (containing the leaving group Y), and L is the fixer (complexing agent).


As shown in the table, photographic processing involves several types of reactions covered in general and organic chemistry classes, from the reduction of silver with developer, to the complexation of silver with fixer towards the end of the development process.[11] The wide range of reactions that occur in such an everyday process will allow students to better understand the chemistry that occurs on a microscopic level through what happens visually on the macroscopic level as their prints are made. This will be further discussed in a later section.

The color photographic development process was further streamlined through the development of instant photography in the early 1970's.[9] Instant photography—the recently-discontinued Polaroids, for example—is developed through a one-step procedure.[18] An instant color print film combines exposure, development, and the resulting image into one cohesive unit.[19] A diagram showing the different layers that make up a developed instant photograph is below in Figure 5.

diagram_of_instant_film.png
Figure 5: Diagram of a processed Kodak instant print.[19]


This film has the same basic blue-green-red sensitive layers going from top to bottom as conventional color photography film, as discussed above. It even contains the layer of yellow dye that prevents blue light from reaching the green and red-sensitive layers of the film.[19] Instant photograph film generally used two means of accomplishing the exposure and development of a photograph neatly in one package. In the first system, called the dye developer route, was the one used in the original Polarcolor film. The dye developer route uses dyes that are mobile when wet and are fixed by the development of silver halide. The second route is known as the dye-release route, uses dyes that start out fixed and are released when the photograph is developed.[15]

Despite how cool it is to see a lengthy process that took sheets of metal and a bunch of toxic chemicals in the 1840's become a relatively simple operation that probably facilitated the blackmail of many a public figure, instant “Polaroid” pictures do have some limitations. One well known limitation is the fact that the color images that result from an instant photograph have a tendency to fade when exposed to visible and ultraviolet light.[19]


Photographic Development as an Interdisciplinary Laboratory Exercise

Chemistry and art are often taught as entirely separate disciplines with their own definite boundaries and scope; [17] however, what is art but chemistry happening on a macroscopic level? Chemistry education is a major focus for organizations like the American Chemical Society, and one way they focus on spreading chemical knowledge is through developing programs to help teachers improve their skills.[12] The need for interdisciplinary coursework relating chemistry and art has been recognized by many scientists, and the development of courses designed to interest non-science majors in chemistry have been supported by the National Science Foundation through funding and workshops.[2] These courses encourage students of all academic backgrounds to explore the role of chemistry in painting, making dyes, metal-casting—and yes, even photography.[14]

From the development and marketing of chemistry sets for children back in the 1950's and 1960's, when chemistry enjoyed a place of prominence in society due to the Cold War, kids and young adults respond well to experiments with visual effects[6]. What will engage a reluctant student more—the ability to watch the development of a photograph and experiment with development conditions or one involving salinity of water on its melting point, involving time spent staring at a thermometer for tiny changes in temperature? While the latter type of experiment certainly has relevancy, it's not going to win any points with an already science-hostile student who thinks of science as 'boring' and a 'waste of time.'

Of course, interdisciplinary and art-focused approaches to chemistry would not result in frivolous courses with little real scientific content. Fortunately, the photographic process provides a basis for introducing concepts from a wide variety of chemical disciplines—from electro-chemistry to organic chemistry, kinetics to catalysis.[13]

An experiment involving black-and-white photography could allow students to discover how factors such as the amount of absorbed light (exposure time), the development temperature, and concentration of the reducing agent (developer) affect how quickly the photograph develops.[17] Furthermore, an investigation into the concept of contrast can be done by keeping exposure and development times constant, merely adjusting the concentration of the developer and the temperature as variables.[16] A high-contrast image will have a very quick rate of development whereas a low-contrast image will be developed slowly.[16] Both high and low contrast images have worth in art, depending on the effect that the photographer was trying to create. Examples of high and low contrast images are shown below in Figure 6

hi_lo_contrast.png
Figure 6: high and low contrast images of ruins[17]
The low-contrast image is on the left, and the high-contrast images is on the right.


An alternative experiment in the chemistry of black-and-white photography uses “bleached-out” photographic prints in place of ordinary photographic paper that can be developed in broad daylight.[9] This experiment has the advantage of being able to be done in a normal laboratory setting instead of a dark room. That way, supervision is simple for the instructor to provide as well as increasing the safety of the experiment. Although the chemicals used in photo processing aren't particularly hazardous[17], there is the risk of spills and the risk of injury could be increased with the addition of a dark room. Also, light-tolerating development of “bleached-out” prints will enable students whose schools lack the proper chemistry lab facilities to participate in these experiments.

Another experiment takes a conventional black-and-white image and allows students to examine the chemistry of dye formation. First, the image is bleached and washed before being placed into a developer solution that is mixed with color couplers. The images resulting from this procedure are similar to “toned” black-and-white images—that is, the image is made from both dye and silver.[11] A flow chart showing the procedure for this experiment is shown in Figure 7 below and demonstrates how this process can be used to make full-color images from a black-and-white image as shown in Figure 8 below.

dyeing_exper_flowchart.png
Figure 7: Flow chart of dyeing experiment, where y is yellow, m is magenta, c is cyan, and X is a halide ion.[11]


dye_exper_image.png
Figure 8: full-color image produced by an undergraduate student following the procedure in Figure 7. [11]


All of these experiments have both relevance in a chemistry course as well as providing students with the ability to have a tangible result of their experiment. The “bleached out” black-and-white developing experiment done in full light might be more appropriate for younger students who would be getting their first taste of how chemistry and art are related. A class of older students would be more interested in using dye to color their image, such as in the last experimental procedure described in the paragraphs above.

The first two experiments described above on exposure time and contrast would probably be most appropriate for older students, since they require time in a dark room and have the potential for students to craft their own experimental parameters, as well as providing for more in-depth mathematical and chemical analysis.


Conclusion

Although strengthening the commitment to quality STEM education in the United States is commendable, focusing merely on one field while failing to relate it to few other branches of human knowledge will almost certainly result in the further narrowing of American education. While this viewpoint may provoke the ire of some scientists, what point is there to an education that limits itself to isolated memorization of formulas and equations without encouraging students to develop the interdisciplinary links and critical thinking skills that will help them become full, engaged citizens in a global society? A developed nation needs to be populated by citizens who can understand the interplay of science and society and art, which will allow them to become better participants in the world.

Photography is one such subject that can tie science, history, politics, and art together, ultimately enabling students to become better scientists who are more prepared to fight for the relevancy of their work, especially in this age where budgets are being slashed right and left, as well as allow them to better understand how their work fits into the wealth of human knowledge available today. Success in life depends on more than just the ability to remember that if Suzie has two apples and Johnny has eighteen, they have twenty apples total. When that problem takes in to account information from other disciplines, it could expand to “if Suzie has two apples and Johnny has eighteen, what are the potential ramifications of income inequity?” Similar examples can be found across physics (“how does the distance between guitar frets change the sound quality of a guitar?”).

With the use of an interdisciplinary unit on the photographic development process, students will not only be able to answer questions about the redox reaction that occurs when an image is developed, but they could talk about how moving from expensive painted portraits requiring special care and storage to relatively inexpensive and easy-to-procure photographs changed historians' ability to research the daily lives of people across income levels—use the Great Depression as an example. Nearly a hundred years after Daguerre's images, photographs allowed the capturing of iconic images of that era. Chemistry facilitated the freezing of an instant in time, allowing us to look back and further interpret pattens and chains of events in history.

Ultimately, the example of the photographic process is a good model for how interdisciplinary lessons can be used to ease the very tangible gap between science and the arts and humanities. Non-science majors will learn an appreciation for science, potentially resulting in a better public understanding of science and the roles of scientists. Science majors would be encouraged to see beyond the immediate impact of their work and how it might play a part in society at large. For once, everybody wins.


References

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