Understanding Photography as Applied Chemistry - ACS Publications

Laboratory Experiment pubs.acs.org/jchemeduc

Understanding Photography as Applied Chemistry: Using Talbot’s Calotype Process To Introduce Chemistry to Design Students Esther S. Rösch*,† and Silke Helmerdig‡ †

Institute for Materials and Material Technologies (IMMT), Pforzheim University of Applied Sciences, Tiefenbronner Strasse 65, 75175 Pforzheim, Germany ‡ Faculty of Design, Pforzheim University of Applied Sciences, Holzgartenstrasse 36, 75175 Pforzheim, Germany S Supporting Information *

ABSTRACT: Early photography processes were predestined to combine chemistry and art. William Henry Fox Talbot is one of the early photography pioneers. In 2−3 day workshops, design students without a major background in chemistry are able to define a reproducible protocol for Talbot’s gallic acid containing calotype process. With the experimental concept presented herein, students can be taught to approach an issue in a systematic way, to practice their problem solving skills, and to experience chemistry in a hands-on learning environment. Students are coached individually in accordance with their progress. The students can understand the chemical process, manipulate it, and translate it into artwork. However, the molecular interpretation of a photograph is the means to an end. Photography is a well-known, ubiquitous process, and even today, young students are fascinated by the moment when the picture becomes visible in the dark room. Labor intensive photographs are appreciated in a different way than images taken with digital cameras or smartphones. Students succeeded in formulating a reproducible protocol for the calotype process and are able to pass on their knowledge to fellow students. KEYWORDS: General Public, Upper-Division Undergraduate, Inorganic Chemistry, Collaborative/Cooperative Learning, Hands-On Learning/Manipulatives, Problem Solving/Decision Making, Applications of Chemistry, Oxidation/Reduction, Photochemistry

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students held by a photography professor and a chemistry professor.

enerations of teachers and lecturers of chemistry have aimed at stimulating the curiosity of their students. Kindling an interest in chemistry for nonchemistry majors such as design students is even more of a challenge. Traditional photographic processes are a link between the artistic and scientific disciplines. Many excellent articles have been published using the interdisciplinary approach to teach chemistry and art.1−3 However, few articles make photography in particular a subject of discussion.4 To the best of the authors’ knowledge the articles mentioning the calotype process describe the early Talbot salted paper method, which is sometimes referred to as a photogenic drawing and which was mainly used to make prints.5,6 In this paper, the calotype process of sensitizing and developing the paper with gallic acid is described. The calotype process can not only be used to make prints, but also to take pictures in camera obscuras and lens cameras.7,8 Furthermore, many articles describe how to use scientific methods to analyze artworks2,5 or describe the chemical process of photography.6,9 In contrast, our approach is to integrate the understanding of the chemical process into the formation process of artwork. Herein we present a 2−3 day experimental photography workshop for upper-level design © XXXX American Chemical Society and Division of Chemical Education, Inc.

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ANCESTORS OF MODERN PHOTOGRAPHY During the beginning of the 19th century, the fascination of the idea to imprint a picture of physical reality durably set the scene for many experiments with photographic processes. William Henry Fox Talbot was one of those experimenters. His aim was to take a picture with a camera obscura and fix the image on paper.10−12 Talbot was born in 1800 in England and became a mathematician with a passion for photography.13 His early attempts of imprinting an image on paper started with silver salts. He took advantage of the difference in light sensitivity. Silver halides, in particular, silver iodide, formed with an excess of halide, i.e., potassium iodide, are less light-sensitive than those formed with excess silver.10,14 Silver nitrate and potassium iodide form insoluble silver iodide which precipitates on the paper surface. An excess of soluble potassium iodide is Received: November 30, 2016 Revised: May 16, 2017

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DOI: 10.1021/acs.jchemed.6b00932 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Scheme 1. Anticipated Formation of the Photographic Image24,40,42,43

detailed negatives.11 Due to the fear of not being recognized for the invention, Talbot not only registered patents but also took legal actions against anyone who would use his process without license.20,21 Out of an interest in analogue photography furthered with a strong interest in the scientific beginnings of the medium, we, a professor of art photography and a professor of chemistry, intended to hold workshops that aimed at introducing design students to applied chemistry and students of chemistry to creative processes. Due to the less hazardous chemicals Talbot used for his calotype process (compared with daguerreotypes and other processes) and the availability of instructions, we decided to reproduce Talbot’s negative process, and due to the many setbacks that we experienced, we obviously tried to improve the chemical process. After the fourth workshop, we coincidentally realized that we ended up with almost the same process as Louis-Désiré Blanquart-Evrard. Blanquart-Evrard was a French experimenter, who invented albumen paper and founded a photographic printing business.22 He heard about the Talbot process in 1844, and introduced an improved process in 1847. The main difference from Talbot’s process was that Blanquart-Evrard did not brush the solutions on paper, but floated the paper on or immersed it in the respective solutions. We also used this processing method because it yielded more evenly coated papers. Further advantages are described such as a better tonal scale in the negative and the longer storage of the paper in the dark until it is needed. The difference between our and Blanquart-Evrard’s process is that we did not put the paper between two sheets of glass during exposure. Blanquart-Evrard presented his improved process in 1847 and did not give Talbot any credit. After Talbot found this out, he called this an “act of scientific piracy”. In this case, he did not take legal action against Blanquart-Evrard.13

adsorbed by the silver iodide surface which primarily causes the apparent light insensitivity.15,16 Talbot experimented with different silver halides such as chloride, bromide, and iodide.10,17 The outcome was a negative on paper that could be turned into a positive image when copied again to sensitized paper. Talbot termed this process photogenic drawing. About the same time, Louis-Jacques-Mandé Daguerre also succeeded in producing what we nowadays call photographic pictures. Talbot was afraid of losing the merit of his efforts after discovering Daguerre’s process, and the following quote referring to this is handed down to us (ref 10, page 34): I was placed, in a very unusual dilemma (scarcely paralleled in the annals of science,) for I was threatened with the loss of all my labors, in case M. Daguerre’s process proved to be identical with mine. Talbot registered several patents to protect his rights as inventor. In 1841, he patented his improved photographic process, the calotype process, where he described that the paper was sensitized by brushing the sensitizing solution containing silver nitrate, acetic acid, and gallic acid (also referred to as gallo-nitrate of silver) to produce negatives using a pinhole camera (British Patent 8,842). The image was developed by again brushing the paper with the same gallo-nitrate of silver solution. The image was fixed with potassium iodide, sodium chloride, or potassium bromide.13,18 In 1843, Talbot described the use of sodium thiosulfate to fix the image in a second patent (British Patent 9,753), which he also registered for the United States in 1847 (US Patent 5,171).7,13 Talbot became aware of this superior fixing process during a visit to Sir John Herschel in 1839, who had described in 1819 that sodium thiosulfate dissolved a number of insoluble silver salts.19 Nevertheless, Talbot did not mention sodium thiosulfate in his first patent, probably because potassium bromide yielded better transparency.13 As a next step he used lens cameras to obtain more B




The inherent difficulties of the process23 together with the inaccurate or misleading descriptions in protocols allowed us even about 170 years later to empathize with the tedious task of obtaining a picture and to deplore the personal vanities of early photographers.

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chemicals were mainly purchased from Carl Roth, Germany, but can be obtained from most chemical suppliers. During the first two workshops a common calotype protocol was tested.8 Due to heavy process issues, the procedure found in The Book of Alternative Photographic Processes was successfully used instead.22 On the basis of this procedure, the students came up with a revised protocol, which can be found in the Supporting Information. Briefly, the paper is first immersed in a silver nitrate solution and after a short drying period covered by a potassium iodide solution. Then, the paper is washed in softly running water and dried diagonally overnight. By this means the paper can be handled under artificial lighting (i.e., room light such as LED or fluorescent lights) and stored for several months. AgI coated paper can be handled in room light because silver iodide is the least light-sensitive of the three silver halides. The paper is cut to size and sensitized with a solution of silver nitrate and acetic acid. The camera or cassette is loaded with damp paper. After exposure, the photograph is developed with a mixture of the acetic acid containing silver nitrate solution and a saturated gallic acid solution. The picture is fixed with thiosulfate solution and washed. If desired, the photograph can be immersed in a commercial gold toner to obtain a better tonal scale. In many experiments with different paper qualities, such as ordinary printing papers in different weights or tracing paper, we discovered that the back of a baryta paper for digital printing (Adox Art Baryta 190 g, 18 × 24 cm2) gave the best results. The back of the paper is uncoated, but has a smooth surface. The preparation of the paper and the loading of the cameras took place in a darkroom. A safelight was switched on when necessary and described in the protocol. Photographs were taken with self-made camera obscuras and lens cameras such as Bilora Boy, Exa 1a, Sinar P, and Linhof Master Technika 5 in. × 4 in. The cameras were loaded with a sheet of damp sensitized paper and exposed for 30 s to 5 min depending on the season, time of day, and light intensity.

FUNDAMENTAL CHEMICAL PROCESS

Silver-based photography has been known for over a century and a half now. The chemistry behind it appears on first sight to be undemanding, especially for advanced chemistry students and chemists. However, the supposedly easy questions often reveal an unexpected complexity. Many scientists tried to investigate and explain the chemistry of the latent image formation.24,25 Gurney and Mott were the first to formulate an explanation for the nucleation process in 1938.26 In 1957, Mitchell published his thoughts and refined his idea with the concentration theory of latent image formation.27−30 Bayer and Hamilton established a different theory in 1965, the nucleation and growth model, which is a quantitative approach to describing the latent image formation.31−34 Malinowski, Moisar, and Granzer described the thermodynamic process of light induced silver phase deposition.35−38 The detailed mechanistic processes described in these articles shall not be part of the discussion herein. Our aim is to formulate a scientifically accepted chemical reaction for the latent image formation. All theories have in common that light induces the transfer of an electron from a halide ion to an interstitial silver ion. The lattice energy of silver halides is low enough to allow silver ions to easily move through the crystal lattice and form interstitial silver ions Agi+. Silver reduction occurs preferentially at clusters with a size of Ag2 and larger to form the latent image. The temporary association of an electron and Ag+ to elementary silver is reversible. Silver cluster growth starts at a size of Ag4+ to reach Agn at the end of the exposure.24,39 Talbot used gallic acid as developing agent. The developing is a redox reaction,40 which is catalyzed by the Agn silver clusters. Thus, the image develops faster at the latent image where the silver clusters are larger due to the exposure, than at places with smaller silver clusters.41 The oxidizer gallic acid was also supposed to be added as sensitizer in Talbot’s patent,7 which can lead to undesired side effects such as fogging and a barely reproducible process. This step was mainly the reason why the calotype process was called kind of fuzzy.23 Leaving out gallic acid in the sensitizing solution, the process becomes reproducible and yields better photographs. Gallic acid was supposed to increase the sensitivity of the paper during exposure because the catalytic silver cluster growth was accelerated by this reducing agent. However, the latent image formation itself does not necessarily require the presence of a reducing agent at this point. The fixing agent is sodium thiosulfate, which was described early on as the superior fixer and is still used today. The chemistry of the process is shown in Scheme 1.

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WORKSHOP SETUP The setup was a nonassessed extracurricular workshop open for undergraduates from the School of Design whose last formal exposure to chemistry was in secondary school. The workshops were planned as a series with one workshop per semester lasting 2−3 days. Both professors were present at all times and supported the students, who were free to work individually or in groups. Although students preferred to work individually, they shared their cameras and helped each other during the preparation and development of the paper. To date, four workshops have been run. The professors had conducted preliminary experiments to reproduce the Talbot process knowing the difficulties and pitfalls but without being able to reproduce the process properly. The students were informed during the registration process that the outcome of the workshop is uncertain. Nevertheless, 20 students participated in the first workshop. In all workshops, student participation was between 5 and 20 students depending on the workshop time slot and work load of the students. As the workshops took place at the end of a semester, preferably after the exam weeks, interested students were recruited through an open call for an extracurricular workshop. The first part of the workshops consisted of two lectures, one about the history of photography and one about the chemistry of the photographic process. The second part was the safety instruction. The third part was the preparation of the iodized paper, which takes about 1 day

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EXPERIMENTAL DETAILS Due to its ubiquitous availability, photography as an image making process has become devalued. To raise the students’ awareness, the intention of this course was not to handle readyfor-use photography solutions but to start from the bulk chemicals and prepare the required solutions as given in the protocols. The only exception was the gold toner, which was used in the fourth workshop as a ready-for-use solution. The C




including the drying of the paper. Since Talbot took the first pictures with a camera obscura, the students of the first workshops were also supposed to build camera obscuras. Thus, this was the fourth part. The fifth and sixth parts were finally to take pictures and to develop them (Table 1). Table 1. Course Structure Session Tasks for Students by Workshop Type Day

Session

Duration, Hours

1

1

1.5

2 3

0.5 6

4

4

5

4

6

4

2

3

Using Pinhole Cameras (Camera Obscura)

Using Cameras with Lenses

Lectures in history of photography and chemistry of photographic processes Safety instructions Preparing the iodized paper, colloquia Building a pinhole camera, colloquia Taking pictures, developing colloquia Taking pictures, developing wrap-up

Lectures in history of photography and chemistry of photographic processes Safety instructions Preparing the iodized paper, colloquia Taking pictures, developing colloquia Taking pictures, developing wrap-up a

a

The workshop for students using lens cameras had one fewer session, ending with session 5.

Figure 1. First successfully developed calotype photograph taken by a student, showing the Pforzheim University School of Design inner courtyard. Reproduced with permission.

The workshop can be held over 2 days when the making of pinhole cameras is omitted and lens cameras are used. During the sessions, the professors discussed the progress and the chemical theory in small group colloquia with the students to prove their understanding of the chemical process and coach them individually. For the end-of-semester show, which takes place in the School of Design twice a year at the end of each semester, the participating students produced a presentation. Results from the workshops were presented on posters with information about the process and the progress throughout the workshops. Our first successfully developed calotype photograph which was taken by a student was a necessary element of the presented progress (Figure 1). An experimental protocol was developed which was refined workshop by workshop (see Supporting Information). Many aspects of obtaining a successful outcome were discussed with the students, namely, the difficulty of reproducing early chemistry protocols due to different units, the chemical names and availability of chemicals, the necessity of accurate documentation, the consequences of inaccurate protocols, and personal perseverance and dedication. Failure modes were analyzed during each workshop, which helped to set goals for the next workshop (Table 2).

Table 2. Course Aims of Workshops Workshop No. 1 2

3 4

Objective Feasibility experiments without students Reproducing the calotype process without any changes Finding the right camera obscura dimensions Understanding the chemistry of the process to obtain a reproducible process Finding the right paper First trials with small format lens cameras such as Bilora Boy and Exa 1a Developing a reproducible protocol for the Talbot process Trying large format lens camera such as Sinar P Making artwork using a Linhof Master Technika 5 in. × 4 in.

collected and declared as hazardous waste. Waste was disposed by the students under the presence and instruction of the chemistry professor. For additional information about the chemicals, please refer to the Supporting Information.

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DISCUSSION Since these workshops were extracurricular and not assessed, the emphasis was not only on teaching the basic chemistry of photography but also on many other objectives. At the beginning, design students were not familiar with chemical protocols. Thus, reading, interpreting, and applying the procedures was a very important lesson. Through this, students learned that protocols are sometimes difficult to reproduce. Reasons can vary, but in our case the published protocols were flawed. Since some protocols were more than 100 years old and had their origin in Great Britain, the units had to be transformed from the imperial into the metric system. Due to the many setbacks, students realized that documenting the lab work is essential and a systematic approach helps to improve the process (i.e., to change only one parameter at a time). At

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HAZARDS All students were instructed before the experimental part started and had to sign an understanding of the safety instruction. Through this, the students became familiar with the correct use of chemicals. Students were also instructed how to dispose of chemicals. No waste was disposed in the sink. Solid waste was collected in plastic bags and removed. Silver nitrate and silver containing waste was collected in special canisters, which were intended for silver recycling. Oxidizing agents (e.g., gallic acid containing solutions) were collected in canisters dedicated to photographic developer agents. Thiosulfate solution was collected in canisters dedicated to photographic fixing agents. The iodide solution was separately D



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the beginning the discipline to work accurately was very low. But over time, students quickly improved their skills. Fingerprints for example were visible on the final image. As they found this disturbing, they started to use gloves when cutting the paper to avoid it. By communicating and documenting the failures and successes, they established a reliable process over four workshops and were able to reproduce the photographic procedure. Students showed great perseverance and dedication to improving pictures and valued the successfully developed photographs. During the series of workshops, students developed a natural curiosity for the history of early photography, for the lives of the photographers, and for chemistry. Even though the workshops started with a lecture on the chemical fundamentals, students were eager to know more about the process and asked many questions during the lab experiments. In several colloquia of small student groups, which took place during the sessions, the professors were convinced that the students easily got used to the chemical vocabulary; acquired a deeper understanding of redox reactions, complex chemistry, latent image formation; and taught each other on their level what they had just learned. The participants understood that failure is part of the success and errors may also help with future work. In the third workshop the course participants were not able to find a suitable paper until one student accidently coated the back of a baryta paper. Suddenly, the process worked, and the back of this type of paper became the standard. The aim of the professors was not to teach the history and chemistry of photography on slides but to offer students the possibility to discover the chemical process on their own and to translate the procedure into creative artwork.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Esther S. Rösch: 0000-0002-9996-8390 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Study Commission for Didactics at Universities of Applied Sciences in Baden-Württemberg (GHD) for the initial funding of our interdisciplinary project between art and chemistry, which allowed us to foster this series of workshops. We also thank Hannah Hekel for permission to use the first successfully developed calotype photograph showing the Pforzheim University School of Design inner courtyard.

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REFERENCES

(1) Kafetzopoulos, C.; Spyrellis, N.; Lymperopoulou-Karaliota, A. The Chemistry of Art and the Art of Chemistry. J. Chem. Educ. 2006, 83 (10), 1484−1488. (2) Greenberg, B. Art in chemistry: An interdisciplinary approach to teaching art and chemistry. J. Chem. Educ. 1988, 65 (2), 148−150. (3) Jacobsen, E. K. JCE Resources for Chemistry and Art. J. Chem. Educ. 2001, 78 (10), 1316−1321. (4) Rigos, A. A.; Salemme, K. Photochemistry and Pinhole Photography: An Interdisciplinary Experiment. J. Chem. Educ. 1999, 76 (6), 736A−736B. (5) Rogge, C. E.; Bezur, A. An Investigation into the Creation, Stability, and X-ray Fluorescence Analysis of Early Photographic Processes: An Upper-Level Undergraduate Laboratory. J. Chem. Educ. 2012, 89 (3), 397−400. (6) Sattar, S. The Chemistry of Photography: Still a Terrific Laboratory Course for Nonscience Majors. J. Chem. Educ. 2017, 94 (2), 183−189. (7) Talbot, W. H. F. Improvement in photographic pictures. US Patent No. 5,171, [Online] June 26, 1847. https://www.google.com/ patents/US5171 (accessed May 10, 2017). (8) Barnier, J. Coming into Focus; Chronicle Books: San Francisco, 2000; p 296. (9) Kahn, B. E. The Chemistry of Photographic Color Dye Formation. J. Chem. Educ. 2004, 81 (5), 694−697. (10) Newhall, B. The History of Photography from 1839 to the Present Day; The Museum of Modern Art: New York, 1964; p 256. (11) Talbot, W. H. F. The Pencil of Nature; Longman, Brown, Green and Longmans: London, [Online] 1844; p 82. http://www.gutenberg. org/files/33447/33447-pdf.pdf?session_id= dd9075c1b1c4fb8c7289189045cdd39f2ed5f1d5 (accessed May 10, 2017). (12) Sheppard, S. E. The chemistry of photography. I. Historical considerations. J. Chem. Educ. 1927, 4 (3), 298−312. (13) Crawford, W. The Keepers of Light: A History & Working Guide to Early Photographic Processes; Morgan & Morgan, Inc.: New York, 1979; p 318. (14) Germann, F. E. E.; Hylan, M. C. The Photographic Sensitiveness of Silver Iodide. J. Am. Chem. Soc. 1923, 45 (11), 2486−2493. (15) Lottermoser, A.; Rothe, A. Adsorption of silver nitrate and potassium iodide by amorphous silver iodide. Zeitschr. phys. Chemie 1908, 62, 359−383. (16) Lüppo-Cramer, H. Photochemistry of Iodine Silver. Z. Wiss. Photogr. Photo. 1913, 1, 11−18. (17) Talbot, W. H. F. Photographic Pictures. Improvements in Obtaining Pictures, or Representations of Objects. British Patent 8,842,

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CONCLUSION In this series of workshops students learned how to prepare chemical solutions and approach a project without a known outcome. The aim of the workshops was to reproduce the Talbot calotype process. The workshops were not assessed in the classical sense because the objective of the professors was not to teach pure information about photography and chemistry but instead to coach the design students individually during their creative discovery of the process. We showed the students with this real problem of reproducibility how to approach an issue in a systematic way. They learned in a collaborative learning environment that perseverance and dedication to a given task are needed to finally come up with a successful result. These are measures which are difficult to assess. Photography is an appropriate topic because it is a wellknown, ubiquitous process, and even today, young students are fascinated by the moment when the picture becomes visible in the dark room. Labor intensive photographs are appreciated in a different way than images taken with digital cameras or smartphones. Students succeeded in formulating a reproducible protocol for the calotype process and are able to pass on their knowledge to fellow students.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00932. Instructor notes; lists of equipment, chemicals, and hazards; and images (PDF, DOCX) Student instructions (PDF, DOCX) E




(43) Hamilton, J. F. The silver halide photographic process. Adv. Phys. 1988, 37 (4), 359−441.

[Online] February 08, 1841. http://www.nonesuchsilverprints.com/ art-of-silver/PDFs/Fox%20Talbot%20Patent%208842%20-%201841. pdf (accessed May 10, 2017). (18) Ware, M., Mechanisms of Image Deterioration in Early Photographs: The Sensitivity to Light of WHF Talbot’s Halide-Fixed Images, 1834−1844; Science Museum: London, 1994; p 95. (19) Herschel, J. On the Hyposulphurous Acid and its Compounds. Edinburgh Philos. J. 1819, 1 (19), 8−29. (20) Wood, R. D. J. B. Reade, F.R. S., and the early history of photography: Part II. Gallic acid and Talbot’s calotype patent. Ann. Sci. 1971, 27 (1), 47−83. (21) Wood, R. D. The involvement of Sir John Herschel in the photographic patent case, Talbot v. Henderson, 1854. Ann. Sci. 1971, 27 (3), 239−264. (22) James, C. The Book of Alternative Photographic Processes, 3rd ed.; Cengage Learning: Boston, 2015; p 847. (23) Forman, S. A. The dynamic interplay between photochemistry and photography. J. Chem. Educ. 1975, 52 (10), 629−631. (24) Nietgen, M. Bildung und Zerfall von Silberclustern bei Mikrokristallen fotografischer AgCl-Emulsionen nach der Belichtung in Abhängigkeit von Kristalleigenschaften und äußeren Faktoren. Dissertation, University of Wuppertal, Wuppertal, Germany, 2001. (25) Sahyun, M. R. V. Mechanisms in photographic chemistry. J. Chem. Educ. 1974, 51 (2), 72−77. (26) Gurney, R.; Mott, N. The theory of the photolysis of silver bromide and the photographic latent image. Proc. R. Soc. London, Ser. A 1938, 164, 151−167. (27) Mitchell, J. W. The Eleventh Renwick Memorial Lecture: The Nature Of Photographic Sensitivity. J. Photogr. Sci. 1957, 5 (3), 49−70. (28) Mitchell, J. W. Stable Latent Image. Photogr. Sci. Eng. 1978, 22 (1), 1−6. (29) Mitchell, J. W. Chemical sensitization and latent image formation: a historical perspective. J. Imaging Sci. Technol. 1989, 33 (4), 103−114. (30) Bilke, W.; Pietsch, H. A Tribute to Professor Mitchell, John, Wesely On the 80th Anniversary of His Birthday. J. Inf. Rec. Mater. 1993, 21 (3), 215−226. (31) Bayer, B.; Hamilton, J. Computer investigation of a latent-image Model. J. Opt. Soc. Am. 1965, 55 (4), 439−452. (32) Hamilton, J. F. Investigation of a Latent Image Model by an Analytical Approximation. Photogr. Sci. Eng. 1970, 14 (2), 102−111. (33) Hamilton, J. Mathematical-Modeling of Latent-Image Formation. Photogr. Sci. Eng. 1974, 18 (4), 371−378. (34) Hamilton, J. Toward a quantitative latent-image theory. Photogr. Sci. Eng. 1982, 26 (6), 263−269. (35) Malinowski, J. Latent image formation in silver halides. Photogr. Sci. Eng. 1970, 14 (2), 112−121. (36) Malinowski, J. Photographic Process as Photostimulated Phase Formation. Photogr. Sci. Eng. 1979, 23 (2), 99−106. (37) Moisar, E.; Granzer, F.; Dautrich, D.; Palm, E. Formation and Properties of Sub-Image and Latent-Image Silver Specks. Part I: Thermodynamics of Silver Speck Formation. J. Photogr. Sci. 1976, 25 (1), 12−17. (38) Moisar, E.; Granzer, F.; Dautrich, D.; Palm, E. Formation and Properties of Sub-Image and Latent Image Silver Specks. Part IV. The Formation of Silver Specks - A Process of Nucleation and Phase Formation. J. Photogr. Sci. 1980, 28 (2), 71−82. (39) Fayet, P.; Granzer, F.; Hegenbart, G.; Moisar, E.; Pischel, B.; Wöste, L. The Role of Small Silver Clusters in Photography. Z. Phys. D: At., Mol. Clusters 1986, 3, 299−302. (40) Shen, H.; Duan, C.; Guo, J.; Zhao, N.; Xu, J. Facile in situ synthesis of silver nanoparticles on boron nitride nanosheets with enhanced catalytic performance. J. Mater. Chem. A 2015, 3 (32), 16663−16669. (41) Fotografie. Fonds der Chemischen Industrie: Frankfurt/Main, 1999; Vol. 26, p 76. (42) Eslami, A. C.; Pasanphan, W.; Wagner, B. A.; Buettner, G. R. Free radicals produced by the oxidation of gallic acid: An electron paramagnetic resonance study. Chem. Cent. J. 2010, 4 (1), 14. F


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