The Chemistry of Photography: Still a Terrific Laboratory Course for

Oct 6, 2016 - The course is enriched by the intertwined histories of chemistry and photography and the work of contemporary photographers. Judging fro...
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The Chemistry of Photography: Still a Terrific Laboratory Course for Nonscience Majors Simeen Sattar* Bard College, Annandale-on-Hudson, New York 12504, United States S Supporting Information *

ABSTRACT: Examination of photographic processes from the 19th century to the present digital age is an effective means to intellectually engage nonscience majors with science. A laboratory course for nonscience majors exploring these processes is described in this article. Ionic and covalent compounds, oxidation−reduction reactions, precipitation reactions, and complex ion formation are major themes of the course. Other important themes are light and color and the generation of color in photographic images. Of the 11 experiments that comprise the laboratory component, nine involve making photographic prints, and nearly all of them involve hand-coated paper. Nearly all required supplies are found in an ordinary general chemistry laboratory. The experiments keep the chemistry of the processes in the foreground while letting students exercise their creativity. Figures in the article exemplify the quality of images that students can achieve. The course is enriched by the intertwined histories of chemistry and photography and the work of contemporary photographers. Judging from the level of student participation, engagement remains high from the beginning to the end of the course. KEYWORDS: First-Year Undergraduate/General, Nonmajor Courses, Curriculum, Interdisciplinary/Multidisciplinary, Laboratory Instruction, Problem Solving/Decision Making, Oxidation/Reduction, Precipitation/Solubility

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INTRODUCTION Amid the profusion of topics for courses for nonscience majors, one of the earliest such topics, the chemistry of photography, is too often overlooked. The emergence of digital photography has not weakened the case for “photography as a basic science” made 70 years ago in this Journal: “It gives basic training in and logical use of many facts of chemistry, physics and mathematics as a part of one naturally integrated field. It stimulates interest in other widely divergent fields of knowledge. It gives rigorous training and cumulative experience in many phases of the scientific method.”1 The goal of this article is to persuade readers to reconsider this topic for its delightful experiments, the coherence of the chemistry it encompasses, and its seamless integration of the histories of science and photography. Students’ fascination with photography has the potential to draw them into chemistry. Inspired by this realization, authors have contributed articles about the chemistry underlying photography to this Journal from its earliest years. These articles have ranged from tutorials2,3 to classroom demonstrations4 to descriptions of experiments for college students at all levels, including both silver5,6 and nonsilver7 photographic processes, chiefly the cyanotype process.8−11 Two textbooks from the 1970s and 1980s attest to the popularity of photography among undergraduates: William Jolly’s innovative general chemistry laboratory manual, which included a sequence of experiments investigating the chemistry of silver and photography,12 and Roger Bunting’s book for nonscience majors devoted to the chemistry of photography.13 Although articles involving photographic processes continue to appear in this Journal, the advent of digital photography has virtually displaced conventional photography not only from the marketplace but also from photography departments in colleges and universities. Courses about the chemistry of photography © XXXX American Chemical Society and Division of Chemical Education, Inc.

have begun to disappear as well. This is a pity, because students are still captivated by the subject, learning a great deal about chemistry and the history of science along the way. The author has offered a course called Photographic Processes in 10 independent sections across five semesters since 2010 to a total of 188 students, of whom 54% were art history, film, photography, or studio art majors, with the rest drawn from virtually all other majors besides the sciences. The enrollment is overwhelmingly composed of juniors and seniors, who were given precedence at registration.

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CLASSROOM TOPICS To capitalize on the main attraction for students, namely, making photographic prints by diverse methods, the course is structured around different photographic processes; see Table 1. Chemical and physical principles are introduced in the context of these processes. Light and color, ionic and covalent compounds, and oxidation−reduction reactions are the foundational topics whose essentials are covered in the first 4 weeks of the semester. These topics are then revisited throughout the course, accruing detail. Students therefore have many opportunities to develop mastery over basic skills such as writing chemical reactions, drawing molecules, interpreting line bond structures, identifying oxidation−reduction reactions, and working with additive and subtractive color. Mass−mole relationships are introduced organically, with minimum fuss, as a means to understand the relative concentrations of solutions used in the laboratory. The semester closes with Received: June 1, 2016 Revised: September 8, 2016

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Table 1. Topics Covered and Associated Experimentsa Week

Topics

Experiment

1

Requirements for photography. First photograph. Wave and particle models of light. Electromagnetic spectrum. Reflection, absorption, transmittance. Additive color (lights) and subtractive color (filters). Interpreting spectra of dyes. Attributes of color. Ionic and covalent compounds. Writing formulas of and naming ionic compounds. Dissolution of ionic compounds in water. Determining oxidation states of metal ions. Oxidation and reduction. History and chemistry of the gum dichromate process.

1. Light and color

2 3

6

History and chemistry of the cyanotype process. Chemistry of toning cyanotypes. Shell structure of atoms. Why certain atoms form characteristic ions. Valence electrons. Drawing Lewis structures. More Lewis structures. Line bond structures. Electronegativity and bond polarity. Chemistry of the diazo process. Applications of bond polarity: solubility; why certain reactions happen; oxidation states of atoms in covalent compounds. Chemistry of silver: precipitation reactions and net ionic equations; complex ion formation; comparing the solubility of insoluble salts; reduction. Molarity. Talbot and the invention of the salted paper and calotype processes. Chemistry of Bayard’s direct positive process.

7

Daguerreotypes. Collodion process. Ambrotypes and tintypes. Flexible supports: cellulose nitrate, cellulose acetate, polyester.

8 9

Review of printing processes. Test 1 Proteins and denaturation. Proteins in photography: gelatin and albumin. Julia Margaret Cameron: collodion negatives, albumen prints. Vandyke prints: iron−silver process. Platinum prints: iron−platinum process. Overview of black and white film processing: exposure through development. Preparation of the emulsion. Factors affecting sensitivity of film: crystal shape and size, iodide, sulfur, sensitizing dyes. Gurney−Mott model of latent image formation. A closer look at the composition of processing chemicals: developer. Acids, bases, pH. Closer look continued: stop, fix and clear. Push processing. IR and instant black and white film. Producing color: additive color; Autochrome; colorizing black and white prints. Chemistry of image reversal. Structure of layered color films. From exposure to processing: negatives, positive prints and slides. Cross processing. What made Kodachrome special.

4 5

10 11 12 13 14 15 16

2. Gum dichromate process 3. Cyanotype process 4. Diazo process 5. Chemistry of silver 6. Salted paper process 7. Bayard’s direct positive process 8. Albumen process 9. Vandyke process 10. Chemistry of photography 11. Colorizing a black and white photograph

Color masking. Color balancing: RGB and CMY filters. False-color infrared film. Instant color film. Conductors and semiconductors; p- and n-doping. CCDs: photons to electrons to voltages to brightness levels. CCD versus CMOS. Producing color in digital cameras: mosaic filters. Producing color continued: three sensors, three layers. Digital IR photography. Digital photography in astronomy. Review. Test 2

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The course meets twice a week for one 2 h class and one 3 h class, which is divided between the classroom and laboratory, for a total of 30 meetings during the semester, of which two are tests.

For instructors wishing to educate themselves, curators’ notes at the J. Paul Getty Museum, George Eastman House, Metropolitan Museum of Art, National Media Museum (U.K.), and Smithsonian Institution are particularly informative about materials and methods and history. In addition to the books cited above, one aimed at conservators summarizes the enormous variety of materials and methods that have been used to produce photographs.17 A captivating way to illustrate specific techniques is to highlight the work of individual photographers and artists: Anna Atkins (cyanotypes),18 Julia Margaret Cameron (collodion process, albumen prints),19 Francesco Capponi (tintypes),20 Cy DeCosse and Keith Taylor (three-color gum dichromate),21 Ed Drew (tintypes),22 Robert Mapplethorpe (instant film),23 Richard Mosse (false-color infrared),24 Sergei Mikhailovich Prokudin-Gorskii (three-color additive process),25 Lauren Redniss (cyanotypes),26 and Carrie Mae Weems (colorized black and white photographs).27 Photographic images made by the inventors themselves (Sir John Herschel, William Henry Fox Talbot, Louis-Jacques Daguerre, Hippolyte Bayard, and the Lumière brothers) further enliven the course.

digital photography and the working principles of charge coupled devices. Nonsilver printing processes dominate the first half of the course, while silver-based processes dominate the second half. Only 2 of the 11 experiments do not involve making prints. In the first of these, students collect and interpret absorption and transmission spectra of cyanine dyes and reflectance spectra of paint color samples. (Physical chemistry instructors will recognize cyanine dyes as a familiar test case for the freeelectron model.) As students discover later, cyanine dyes are used to sensitize silver halide emulsions to green, red, and infrared light. The second conventional chemistry experiment is derived from Jolly’s manual and explores the chemistry of silver halides: their precipitation, sensitivity to light, relative solubility, dissolution by complex ion formation, and reduction to silver.12 In the summaries of the experiments below, readers will notice many references to articles in this Journal. Not to be overlooked are books aimed at artists that are filled with practical advice about materials and procedures, historical background, and examples of work by past and contemporary photographers.14−16 Indeed, the history of science and the photographic images are among the great pleasures of teaching this course. The historical development of photography is rich with personalities; some of them are noted in the Supporting Information along with their discoveries and inventions. That many scientific discoveries arose from bold inferences grounded in careful observations (e.g., the discovery of infrared and ultraviolet radiation) is eye-opening for students. Sadly, chemistry majors seldom see their own field in this light owing to the pressing need to cover scientific content.

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LABORATORY EXPERIMENTS Specialized needs are few and are fortunately common to nearly all the experiments: watercolor paper, foam and bristle brushes, blow dryers, glass plates, and plastic trays. While sunlight can serve as the light source for exposure (and some students prefer the natural, unpredictable element), it cannot be relied upon. A photographer’s light box is worth the investment as the light intensity from the fluorescent tubes is constant and evenly B

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distributed. One 26 × 21 × 8 in.3 light box suffices for a class of 18 students. For all but one experiment, turning off the room lights and avoiding direct sunlight are adequate measures as the lightsensitized papers are not all that light-sensitive, requiring several minutes of exposure to bright light to form an image. The one experiment which uses commercial photographic paper requires a room whose windows can be covered, but complete darkness is not necessary. As to what image to print, the choice is vast. Photography majors often prefer to use their own 4 × 5 in.2 black and white negatives. Other students enjoy using digital photographs they have taken with a phone or digital camera; these positives are easily inverted using software (e.g., Microsoft Paint and Adobe Photoshop) and printed or photocopied onto transparency film. Found images can be treated in the same manner. Thin botanical samples produce pleasing images with tonal gradations. The figures shown here and in the Supporting Information exemplify the variety of images that have been used with great success. For good results, it is essential to press the transparency or negative flat against the sensitized paper. A simple assembly consists of a sandwich of a 4 × 6 in.2 rectangle of 1/8 in. plywood, sensitized watercolor paper, a transparency, and a glass plate clamped together with metal binder clips. Since the experiments involving printing processes share the same basic tools and procedures, students quickly gain confidence in the laboratory. Reagent concentrations are fixed, leaving exposure time as the principal experimental variable. A good exposure time depends on the density of the transparency selected. Students start from an estimated exposure time and, after making their first print, decide whether to increase or decrease the time and by how much. Most processes yield an inverted (negative) image, but some yield a duplicate (positive); thus, the direction in which to vary the exposure time can present students with a puzzle. Precise times for exposure and subsequent processing steps are absent by design. This is especially disconcerting for photography majors at first because they are accustomed to detailed instructions; knowing when precision is required and when it is not is a valuable laboratory skill for all students to learn. Summaries of the print-making experiments with notes on the underlying chemistry are given below; chemical reactions are given in the Supporting Information. A primary reference containing details about reagent preparation and procedures is cited at the end of the first sentence of each section; any modifications made are mentioned. Information about hazards is given in the Supporting Information and ref 16.

exposed to light, simultaneously washing away excess dichromate and watercolor. Thus, tones in the image are reversed from those in the original negative; see Figure 1.

Figure 1. Gum dichromate print tinted with alizarin red made from a 4 × 5 in.2 black and white negative. Photo and print by Ying Jing Zheng. Used with permission.

Cyanotypes

Gum Dichromate Process

In this process, invented by Herschel in 1842, watercolor paper is sensitized with a solution of ammonium ferric citrate and potassium ferricyanide.14 In areas exposed to light, citrate reduces Fe3+ to Fe2+ and is itself oxidized to acetone and carbon dioxide.29 A second oxidation−reduction reaction between Fe2+ and ferricyanide generates Prussian blue, FeIII4[FeII(CN)6]3, whose color arises from intervalence charge transfer from Fe(II) to Fe(III).8 Salts containing colorless FeII[FeII(CN)6]2− precipitate as well, diluting the blue color. If exposure is carried out in sunlight, the progress of the reaction can be monitored, making this a printing-out process. The paper is washed to remove unexposed reactants. The image reverses the tones in the transparency or negative. Numerous methods exist for altering the color of cyanotypes.8,30 Two of them illustrate some interesting chemistry. The blue color can be deepened by immersing the print in dilute H2O2 which oxidizes FeII[FeII(CN)6]2− to FeIII[FeII(CN)6]−. Alternatively, the color can be changed altogether by first immersing the print in dilute ammonia, which after a few minutes fades the image due to formation of light brown Fe(OH)3. Immersion of the faded print in a gallic acid or tannic acid solution results in a gray or rich brown tone, respectively, due to the formation of insoluble iron salts. A lovely violet-gray cyanotype toned in gallic acid is shown in Figure 2.

This process was developed over a period of years during the mid-19th century.14 A solution of gum arabic, tube watercolor, and potassium dichromate (below 0.2 M in the mixture) is brushed onto a sheet of watercolor paper. A transparency or negative and a sheet of glass are laid over the dry, sensitized paper, which is then exposed to light for a few minutes. During exposure, Cr(VI) in the dichromate ion is reduced to free Cr3+, while alcohol groups in the gum (a mixture of polysaccharides and glycoproteins28) are oxidized to carbonyl groups. The Cr3+ ions cross-link the gum, causing the gum to harden, trap pigment particles, and become insoluble in water. Pigment particles are thus retained in areas exposed to light. The paper is then floated in water. Gum dissolves from areas that were not

Figure 2. Cyanotype toned with gallic acid. Image: selmandesign. Print by Justin Tobey. Used with permission. C

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Figure 3. Two diazo prints on the left were made on hand-coated paper with the difference that the left-most print was exposed for a shorter time. The third print was made on commercial diazo paper. The prints are made from a transparency of a photograph of Virginia Woolf. Copyright National Portrait Gallery, London. Prints by Jordon Soper. Used with permission.

solution (0.8 M) is brushed over the dried paper, precipitating silver chloride over the entire surface of the paper. Exposure to light reduces Ag+ to Ag, which gives the exposed areas a deep brown tone. The print is next washed in tap water; a telltale milkiness indicates removal of excess silver nitrate. Lastly, the print is immersed in a sodium thiosulfate solution (recommended by Herschel in 183931) to dissolve unreacted silver chloride via formation of [Ag(S2O3)2]3− and thus stabilize (or fix) the image. See Figure 4 for an example of a salted paper

Gallic and tannic acids show up in a different context later in the course. In the 1790s, Thomas Wedgwood found that white leather treated with silver nitrate was more light-sensitive than similarly coated paper.31 Talbot was the first to use gallic acid (3,4,5-trihydroxybenzoic acid) to develop a latent image, i.e., reduce silver halides to silver.31 Hydroquinone (1,4-dihydroxybenzene), the key component of modern black and white developers, is very similar. Diazo Prints

This experiment is based on a classroom demonstration that involves coating acetate transparency film with sensitizing solution.7 The original procedure requires fashioning a tool for coating and some dexterity. However, good results can be obtained by simply brushing the solution on paper (see the leftmost image in Figure 3). Paper is coated with a solution of 4-diazo-N,N-diethylaniline fluoroborate and a color coupler (3-hydroxy-2-naphthoic acid anilide for blue, 1,3-benzenediol for brown) in acetone−ethanol thickened with cellulose acetate, tinting it yellow. The solution also contains a small amount of sulfosalicylic acid to suppress acid dissociation of the color coupler whose conjugate base would otherwise react prematurely with the diazonium salt. Exposure to light decomposes the diazonium salt, fading the yellow tint. The paper is then suspended over concentrated ammonia in a lidded jar. The ammonia vapor deprotonates the color coupler, thus shifting its acid dissociation equilibrium toward the conjugate base, which instantly and dramatically reacts with any diazonium salt retained in unexposed areas, producing a dye. Thus, the diazo process produces a direct positive. This outcome startles students: This is the first time the tonal relationships in the original are not reversed in the print. Lively debates ensue as to how to change the exposure time to lighten or darken the final image. Figure 3 shows a pair of diazo prints made from a positive transparency; the darker image was exposed for less time. Diazo paper is commercially available, and students enjoy using it, but working with hand-sensitized paper has its own satisfaction and reinforces the chemical processes involved in a way that commercial paper cannot.

Figure 4. Salted paper print made from a 4 × 5 in.2 black and white negative. Photo and print by Andrew Warner. Used with permission.

print. Prior to Herschel’s suggestion, chloride, bromide, and iodide were used by Talbot, Daguerre, and Bayard to fix the photographic images. The effect of these treatments on the color of the print is illustrated in an article in this Journal.6 See ref 32 for a thorough discussion of the chemistry of Talbot’s salted paper process, including quantitative aspects. Bayard’s Direct Positives

Invented in 1839, Bayard’s process has been known for its difficulty. However, its interesting chemistry makes it worth the effort.33 Paper is first coated with potassium iodide solution and then with silver nitrate to precipitate silver iodide. Next, the entire paper is exposed to light until a uniform deep brown coating of silver particles forms. The brown paper is then washed in tap water to remove excess silver nitrate and soaked in a sodium thiosulfate solution to dissolve any remaining silver iodide. It is then soaked in a potassium iodide solution, covered with a transparency or negative and glass, and exposed to light a second time. In the exposed areas, iodide undergoes air

Salted Paper Prints

This procedure is little altered from Talbot’s original process dating to 1834. A paper is first coated with a chloride solution (0.4 M).14 Optional additions to this solution are sodium citrate and a gelatin or starch thickener. Next, a silver nitrate D

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oxidation to iodine, which in turn reacts with silver to form silver iodide. Exposed areas, therefore, become light yellow, whereas unexposed areas remain brown, yielding a duplicate of the transparency. The print is again washed in water and fixed in aqueous sodium thiosulfate. As with the diazo process, students must think through the chemistry to decide how to change the exposure time to lighten or darken their prints. Some staining, arising from uneven dispersal of silver particles in the preparation of the paper, is evident in the Bayard print shown in Figure 5, but it is a lovely example nonetheless. Staining is difficult to eliminate, but it can be reduced by multiple coatings of AgI and exposures. Figure 6. Albumen print on cotton cloth made from a photograph taken with a phone. Photo and print by Andre Burger. Used with permission.

image is fixed with sodium thiosulfate. Although Vandyke prints are named for their deep brown tones, delicate sepia tones can be achieved as well, as can be seen in Figure 7.

Figure 5. Bayard print made from a photograph taken with a digital camera. Photo and print by Andre Burger. Used with permission.

Juxtaposing Bayard’s and Talbot’s processes highlights the differences between them and also Bayard’s creative thinking about silver halide chemistry. Albumen Prints

Many students in the arts have heard about or seen albumen prints, and this experiment demonstrates what the term means.16 It motivates inclusion of the classroom topic of protein structure and denaturation, which is also relevant to gelatin, used to make conventional photographic film and paper. The albumen process is very similar to the salted paper process, with the modification that egg whites are used to thicken the coating in order to lift it above the paper fibers. After many years of enduring the odors associated with separating yolks from whites of a dozen fresh eggs, we switched to powdered albumen. Albumen powder is dissolved in water, and the proteins are denatured by addition of acetic acid and ammonium chloride. The mixture is briefly whisked to denature those albumen proteins susceptible to foaming. After excess liquid drains, the foam is skimmed off, and the remaining liquid is used to coat the paper. Once the coating has dried, it is brushed with silver nitrate solution to precipitate AgCl, exposed, washed in tap water to remove excess silver nitrate, soaked in sodium thiosulfate solution, and washed again. A film of dried albumen is quite apparent to the eye and touch on the finished print on paper. Figure 6 shows an albumen print on cotton; the rich brown coloration is distinctive.

Figure 7. Vandyke print made from a 4 × 5 in.2 black and white negative. Photo and print credit by Ying Jing Zheng. Used with permission.

Palladium and platinum prints, which are more familiar to art students, share the same chemistry as Vandyke prints, with palladium(II) and platinum(II) salts taking the place of silver nitrate. Photogenerated Fe(II) remains the reducing agent. Printing and Image Reversal

This experiment is the first to employ commercial silver chloride contact printing paper (cut into 2 in. × 2 in. squares) and developer.12 This is the only experiment requiring a room whose windows can be covered. In the first part of the experiment, students make two prints by the conventional process. Photographic paper overlaid with a simple cutout or leaf is exposed for 1−2 s under a small desk lamp, developed in a commercial developer, washed in dilute acetic acid, and fixed in sodium thiosulfate solution. One print is set aside for colorizing the following week. In the second part, students replicate the first steps of the previous process (expose, develop, and stop), but this time, instead of fixing the image (i.e., dissolving unexposed AgCl), the print is placed in an oxidizer to dissolve silver from the exposed areas: the image fades away. The only modification to Jolly’s procedure is replacing 0.1 M Na2Cr2O7 with 0.1 M K3[Fe(CN)6]. After washing, the bleached print is developed under light to expose the previously unexposed areas and

Vandyke Prints

Paper is coated with a solution of ammonium ferric citrate (also a component of the cyanotype process), silver nitrate, and tartaric acid.14 To the students’ surprise, silver halides are not involved as in the previous three processes. Silver nitrate is reduced by the Fe(II) formed by the photoreduction of Fe(III).13 Areas exposed to light turn a deep brown-black color indicating formation of silver particles. Tartaric acid prevents silver citrate from precipitating.34 After the paper is washed, the E

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further expansion, as connecting to modern technology provides a welcome change of pace for students and the instructor at the end of the semester. Enthusiasm for the course is evident from start to finish. That many students elect to make more than the minimum number of required prints and a couple will linger on past the end of the laboratory period to get better results is extraordinary. Students regularly report reading topical articles (e.g., the demise of Kodachrome, the revival of instant film) and reviews of shows at museums and galleries and bring questions arising from their own experiences (e.g., what makes film fast or slow, how does push processing work, what is the rationale for color balancing, what does ISO mean in digital cameras). Students who visit museums during the semester are surprised to discover how meaningful object labels such as “albumen silver prints”, “salted paper print from calotype negative”, etc., have become. Many students are impatient to discover whether the historic processes being discussed are still in use by contemporary photographers and artists. It is a positive sign when they access the Internet during class to check. Their finding such photographers and artists (as is nearly always the case) affirm that nondigital photographic processes, in their great variety, are alive, relevant, and inspiring, thus making the effort of understanding the chemistry worthwhile.

washed. An inverse image magically materializes; see Figure 8. To heighten their surprise, students are not told what to expect.

Figure 8. Positive print of leaves placed on photographic paper made using the image reversal process (left) and a colorized negative print (right). Prints by Rebecca Ganellen. Used with permission.

The chemistry of image reversal is teased out in the following class by recollecting that, in the cyanotype process, ferricyanide is an oxidizer. Colorizing Black and White Prints

Colorizing entails replacing the silver in the black and white print made the previous week (or any other black and white photograph) by a cyan, magenta, or yellow dye.35 The source article gives directions for preparing the required solutions from scratch; it also mentions a commercial kit that is economical and less labor-intensive. In the first step, the silver on the print is oxidized and precipitated as AgCl by immersion in a solution of copper(II) chloride and sodium chloride: Hence, it is called a rehalogenating bleach; the complex ion [CuCl2]− is formed. Simultaneous exposure to bright light activates the silver chloride, forming the latent image. The print is then placed in a tray containing a developer (a phenylenediamine) and color coupler with the net result being that silver chloride is reduced back to silver and the oxidized developer reacts with the color coupler to produce a dye. To demonstrate that dye has not simply stained the gelatin, as some students will aver, the silver is removed by treating it again with the oxidizer solution, revealing gradations of tone. Lastly, the print is fixed with sodium thiosulfate. By this point in the course, students readily catch on to the logic underlying the sequence of the steps. See Figure 8 for a colorized leaf print. The significance of this experiment is that it illustrates the working principle of developing color film and paper with the difference that yellow, cyan, and magenta color couplers are already present in separate layers of the silver halide emulsion rather than diffused in from the developer solution.

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COURSE EXPECTATIONS Students are evaluated on the basis of 10 homework assignments, 11 laboratory reports, and 2 tests. The homework and reports require using the basic chemistry skills listed in Table 1. Two-fifths of the first test consists of questions that would not be out of place on a general chemistry test: writing formulas for ionic compounds, drawing Lewis structures for polyatomic ions and small molecules, labeling bonds as polar and nonpolar, determining oxidation states in ionic and covalent compounds, identifying oxidation−reduction reactions, and solving problems involving properties of light. Apart from a question about history, questions about photographic reactions and processes and general scientific reasoning make up the remainder of the tests. Examples of questions about photographic reactions and processes include placing reactions in a photographic context, arranging reactions in a logical sequence, and describing the chemical composition of prints at various stages of processing. Examples involving general scientific reasoning include relating a dye’s color to its spectrum and predicting the outcome of scrambling the layers in color film. An explanation is a required component of a complete answer.

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STUDENT RESPONSE Every topic stimulates some questions from students, and the resulting discussion makes the course enjoyable for all. Topics that rise to the top as student favorites are the composition of commercial developers, historical strategies for making color images, color film processing, and digital photography. That the intricacies of the composition of developers12 with numerous reagents working in concert could be a topic that rivets students never ceases to be a source of amazement. Understanding and implementing historic processes is particularly illuminating to art history and photography majors; what was mere vocabulary suddenly becomes clear. The logic of the color negative and reversal processes36 is deeply satisfying to many students. The time devoted to the workings of charge coupled devices has steadily expanded to the current 5 h of class time. It could bear

CONCLUSION If the goals of a beginning science course are to introduce students to “important chemical principles”,12 train them in “the logical use of many facts”,1 convey “the enjoyment and excitement of experimental chemistry”,12 and “spark curiosity, and occasionally··· to entertain”,12 then the chemistry of photography is an ideal vehicle for reacquainting nonscience majors with science. It is an equally revitalizing experience for instructors to exercise their chemical knowledge by applying it to a possibly unfamiliar subject and to exercise their creativity in designing a course for which there is no template by developing content and assembling a suite of experiments. F

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(15) Farber, R. Historic Photographic Processes; Allworth Press: New York, 1998. (16) James, C. The Book of Alternative Photographic Processes; Delmar Cengage Learning: Clifton Park, NY, 2009. (17) Lavédrine, B. A Guide to the Preventive Conservation of Photograph Collections; Getty Publications: Los Angeles, 2003. (18) The J. Paul Getty Museum, Anna Atkins. http://www.getty. edu/art/collection/artists/1507/anna-atkins-british-1799-1871/ (accessed Aug 2016). (19) Lane, A. The portraits of Julia Margaret Cameron. New Yorker, September 2, 2013. ̂ http://francescocapponi.tumblr. (20) Capponi, F. Chair-en-boite, com/chair-en-boite (accessed Aug 2016). (21) DeCosse, C. http://www.cydecosse.com/processes/ (accessed Aug 2016). (22) Curtis, E. The war in Afghanistan in tintypes. New Yorker, June 26, 2013. (23) Wolf, S. Polaroids: Mapplethorpe; Prestel: Munich, 2007. (24) Mosse, R. Infra. http://www.jackshainman.com/exhibitions/ past/2011/richard-mosse/ (accessed Aug 2016). (25) The Prokudin-Gorskii photographic record recreated: the empire that was Russia. http://www.loc.gov/exhibits/empire/ making.html (accessed Aug 2016). (26) Redniss, L. Radioactive: Marie & Pierre Curie, A Tale of Love and Fallout; HarperCollins: New York, 2011. (27) Cotter, H. Testimony of a cleareyed witness: Carrie Mae Weems charts the black experience in photographs. New York Times, January 23, 2013. http://www.nytimes.com/2014/01/24/arts/design/ carrie-mae-weems-charts-the-black-experience-in-photographs.html?_ r=0 (accessed Aug 2016). (28) Stuart, B. Analytical Techniques in Materials Conservation; Wiley: New York, 2008. (29) Abrahamson, H. B.; Rezvani, A. B.; Brushmiller, J. G. Photochemical and spectroscopic studies of complexes of iron(III) with citric acid and other carboxylic acids. Inorg. Chim. Acta 1994, 226, 117−127. (30) Berkowitz, S. Cyanotype toning. http://www.berk-edu.com/ HYB_subsite/PDF_hyb/CyanotypeToners.pdf (accessed Aug 2016). (31) Schaaf, L. J. Out of the Shadows: Herschel, Talbot and the Invention of Photography; Yale: New Haven, CT, 1992. (32) Ware, M. Mechanisms of Image Deterioration in Early Photographs; Science Museum and National Museum of Photography, Film & Television: London, 1994. (33) Osterman, M. Understanding Bayard’s direct positive process. George Eastman House, International Museum of Photography: Rochester, NY, 2007. (34) Ware, M. Positives: Minor Processes. In Encyclopedia of Nineteenth-Century Photography; Hannavy, J., Ed.; Routledge: New York, 2008; Vol. 2; pp 1154−1162. (35) Kahn, B. E. The Chemistry of Photographic Color Dye Formation. J. Chem. Educ. 2004, 81, 694−697. (36) Guida, W. C.; Raber, D. J. The Chemistry of Color Photography. J. Chem. Educ. 1975, 52, 622−628.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00400. Information about supplies, scientists, and inventors who made significant contributions to photography; a glossary of basic terms; reactions for the photographic processes carried out in the laboratory; chemical hazards; and additional student prints (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS This course was inspired by the NSF-sponsored “Chemistry and Art” workshop conducted by Patricia Hill and Michael Henchman in 2006. The author is grateful to Maureen O’Callaghan-Scholl, science laboratory manager, for cheerfully and creatively rising to the challenges presented by these experiments; to Ying Jing Zheng for meticulously scanning the prints shown here; to Johnny Selman of selmandesign for permission to include the print shown in Figure 2 made from a transparency of his artwork published in The New York Times (31 October 2015); and to Robert Olsen for critiquing this manuscript.

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REFERENCES

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DOI: 10.1021/acs.jchemed.6b00400 J. Chem. Educ. XXXX, XXX, XXX−XXX