Multidisciplinary Learning: Redox Chemistry and Pigment History

Laboratory Experiment pubs.acs.org/jchemeduc

Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

Multidisciplinary Learning: Redox Chemistry and Pigment History Marcie B. Wiggins,† Emma Heath,†,‡ and Jocelyn Alcántara-García*,‡ †

Department of Chemistry & Biochemistry, University of Delaware, Newark, Delaware 19716, United States Department of Art Conservation, University of Delaware, Newark, Delaware 19716, United States

‡

J. Chem. Educ. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 11/29/18. For personal use only.

S Supporting Information *

ABSTRACT: The interface of art and science provides a broad range of educational and collaborative projects at various learning stages. Therefore, the use of historic artists’ materials for teaching chemistry is receiving more attention. We prepared and used copper acetate (verdigris pigment) for a series of interconnected, lab-based activities, which can be applied to highschool-level chemistry, to undergraduate general chemistry, and further to heritage conservation science research for emerging art conservators. The synthesis and degradation processes of artists’ materials like this pigment allow instructors to illustrate scientific concepts like redox chemistry, while extending the vision of science to arenas beyond the classroom.

KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Graduate Education/Research, Interdisciplinary/Multidisciplinary, Laboratory Instruction, Hands-On Learning/Manipulatives, Applications of Chemistry, Dyes/Pigments, Oxidation/Reduction, Undergraduate Research

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INTRODUCTION Interdisciplinary classes are becoming increasingly popular, as numerous workshops,1 projects,2−4 and courses5−9 illustrate. These efforts merge the arts and humanities with science, which has proven successful at illustrating challenging topics in science curricula.10−12 Emerging art conservators, as students, are preparing themselves for the challenge of keeping our material cultural heritage safe. This requires incorporating more science into art conservation curricula. Art conservation students learn hands-on and preventive conservation techniques, connoisseurship, (art) history, documentation, and applied conservation science, to mention just a few areas of the vast curriculum. Conservation professionals must learn fundamental principles of chemistry and physics, as well as some characterization techniques. Because of students’ interest in applied material science, in addition to time constraints, accelerated degradation, often referred to as “accelerated aging” or “artificial aging”, allows us to replicate chemical phenomena that students can easily relate to historic, “naturally degraded” objects. Although the interest of this audience is very specific, similar experiments can be successfully adapted for college and high school students, who are not necessarily interested in cultural heritage. The experiments detailed in the following lines show successful and collaborative strategies to teaching science to both nonscientific and scientific audiences. Our set of activities can be easily adapted to audiences spanning from college-level general chemistry to art conservation students. The latter will use similarly tailored projects to address unique conservation-related questions © XXXX American Chemical Society and Division of Chemical Education, Inc.

throughout their careers. As such questions are often related to the original state of the material, synthesis and evaluation of those materials can and will likely affect treatment, housing, and display decisions. Additionally, the role of these materials on “problem-substrates”, or substrates such as paper and paint that react and degrade with the material, contributes to these decisions. The materials in question can also be used to illustrate simpler subject matter to younger chemistry students. Integrating artists’ materials as a part of a general chemistry lab connects lessons to real world situations. Therefore, this project presents a format that can grow and expand from fundamental high school chemistry, to undergraduate science courses, to conservation-related research projects. Copper-Containing Artists’ Materials

Copper reactivity offers seemingly endless opportunities to teach chemistry. Besides their high reactivity, copper compounds are ideal candidates for teaching several science topics due to both their relatively low cost and toxicity. Copper compounds provide a wide range of colors, from various shades of blue to red, depending on their chemical state (oxidation states, crystal structures, ligands, etc.).13−18 Numerous historic pigments are copper-based, and artists’ treatises contain guidelines for their preparation and use (supported by modern analytical investigation), e.g., blue Received: June 5, 2018 Revised: October 18, 2018

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DOI: 10.1021/acs.jchemed.8b00358 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. Objects showing signs of decay and discoloration due to verdigris’ degradation. Leaves show some green intact verdigris as well as brown discolored verdigris. (Left) Courtesy, Winterthur Museum, Fraktur: Birth and baptismal certificate of Julyanna Biehl by George Peter Deisert, 1792, Adams County, PA, Watercolor, Ink, Laid paper, Museum purchase with funds provided by the Henry Francis du Pont Collectors Circle, 2013.31.2.1. (Right) Courtesy, Winterthur Museum, Fraktur: Birth and baptismal certificate of Anna Maria Biehl by George Peter Deisert, 1792, Adams County, PA, Watercolor, Ink, Laid paper, Museum purchase with funds provided by the Henry Francis du Pont Collectors Circle, 2013.31.2.2.

...made by grinding up in a mortar of true Cyprian copper with a pestle of the same metal equal weights of alum and salt or soda with the very strongest white vinegar. This preparation is only made on the very hottest day of the year, about the rising of the Dogstar. The mixture is ground up until it becomes of a green color... To remedy any that is blemished, the urine of a young boy to twice the quantity of vinegar that was used is added to the mixture. Verdigris has been extensively used in many works of art, including, but not limited to, 17th century illuminated manuscripts, where the pigment was identified using Raman spectroscopy.25 More commonly, however, verdigris is not unequivocally identified, but discolored areas corresponding to otherwise blue or green (sea in a map, leaves of a plant) are assumed to contain it (Figure 1). On paper, these remarkably brittle areas represent oxidized cellulose.26−30 Verdigris’ low stability was well-documented even in the 15th century. In “Il libro dell’arte”, which is pivotal to understand historic artistic practices, Cennino Cennini states that verdigris “makes a green for grass most perfect and beautiful to the eye, but not durable”.31 Objects with verdigris are of interest in cultural heritage and serve as an engaging case study for both fundamental chemistry and art conservation alike.

azurite (Cu3(CO3)2(OH)2) for Mary in Christian depictions.19−22 Because of the varying interpretation of these historical synthesis descriptions, these pigments provide interesting and challenging real world, artistic, and historic examples for students. Verdigris (copper(II) acetate) is possibly one of the most widely used pigments in western art since antiquity. This material can range in color from pale blue to a dark emerald based on its crystal structure, which is related to its synthesis. The most common form is neutral verdigris (Cu(CH3COO)2· H2O). Historic recipes for verdigris are as simple as described by Pliny:19,23 [I]t is scraped off the stone from which copper is smelted or by drilling holes in white copper and hanging it up in casks of strong vinegar which is stopped with a lid... Metallic copper reacts with acetate ions to form the oxidized blue-green corrosion crust, which was harvested for painting and/or writing material. Although suspending copper materials over acetic acid is the simplest of methods, more complicated syntheses are also described in Pliny’s and other treatises:19,24 B




This set of activities is adaptable for different students and subject matter. Herein, we describe how we utilized copperbased pigment synthesis to teach redox reactions in a 200level undergraduate general chemistry course. Building on this concept, we highlighted its historic use and relevance in material cultural heritage, i.e., watercolors on paper. We kept students engaged in the activities through the use of their own pigments to create their own artwork. We used the pigments and artwork they prepared to work with emerging art conservators. The secondary part of the laboratory activity used extreme conditions of relative humidity and temperature to induce the artworks’ decay (artificial/accelerated degradation). Degradation of paper, a combination of acid hydrolysis and oxidation, is a very visual effect and can be monitored by instrumental methods, as we later explain. Learning Objectives

Two methods of synthesis are described below. Each can be applied to specific chemical lessons. The simple method or direct synthesis19,23 of verdigris illustrates redox chemistry, limiting reagent, stoichiometry, and the role of d-electrons in color, to name a few concepts. The multistep method,8 on the other hand, can be used for explaining ligand field theory and coordination chemistry, acidity, and crystallization methods, in addition to the concepts mentioned before. The follow-up activity (degradation of cellulose) is a prime example of organic redox chemistry, which is seldom covered. Last, the role of these pigments in the degradation processes shows the applicability of chemistry in day-to-day life. For an upper-level undergraduate chemistry or a graduate art conservation course, characterization techniques can be easily added to the experimental design. Namely, X-ray diffraction (XRD), Raman and Fourier transform infrared (FTIR) spectroscopies, and X-ray photoelectron spectroscopy (XPS) were used to to track chemical changes.25,29,32,33

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Figure 2. Copper sheet suspended on plastic cups over 50% (v/v) acetic acid. It was sealed in a glass container for 3 weeks.

Students prepared duplicates of all their artwork. Most students took some pride in their work, thus becoming personally invested in the outcomes of the aging experiment, including preparation “accidents” that we will discuss in detail later (Figure 3). Alongside these samples, several standardized samples were prepared with the same dyes and watercolors to age as well for comparison.

EXPERIMENTAL SUMMARY

See Supporting Information for full lab experiments. Both handouts describe the synthesis and can be used for both upper-undergraduate and graduate classes on chemistry and heritage conservation science. Only one handout contains postlab questions; the second handout includes photographs of the procedure, as well as conditions for the aging study. The synthesis of verdigris can be performed in one of two ways: the simple or the multistep synthesis. For the simple synthesis, pure copper objects are suspended over acetic acid inside sealed containers at ambient conditions (Figure 2). Depending on the acid’s concentration, this synthesis can take anywhere between 2 days and 6 months, but even cooking white vinegar will color green within 60 min when put in touch with copper pipes that can be purchased from a hardware store. Figure 2 illustrates a green-blue “crust,” formed after 3 weeks with 50% (v/v) acetic acid. For the multistep synthesis, copper sulfate is converted into basic copper(II) sulfate with ammonia. Then, sodium hydroxide is used to form copper(II) hydroxide. Acetic acid is added to convert to copper(II) acetate, which will then crystallize over a week to form usable pigments.8 Students created their own artwork using their own synthesized pigments, either as dyes (using concentrated aqueous solutions of the pigment) or watercolors (using gum Arabic as a binder) on Whatman filter paper No. 1.

Figure 3. Copper(I) oxide forms by Fehling’s reaction upon addition of excess heat to the verdigris and gum Arabic mixtures.

Accelerated aging was carried out with an ESPEC BTL 433 test chamber located at the Winterthur Museum.34 Both the pigmented Whatman paper and control unpigmented Whatman paper were aged under the same conditions (60 °C, 85% RH).35,36 Visible changes were clearly observed as discoloration, or “browning”, of the pigmented Whatman filter paper compared to the aged controlled paper. Figure 4 compares the aging of the pigmented papers with controls. At the high school and undergraduate level, these visual changes exemplify the interactions taking place between the copper-based verdigris and the organic substrate, cellulose and in some cases gum Arabic, as a result of accelerated degradation. This is related to why many historical documents, maps, etc. appear brown and C




Figure 5. FTIR spectra of aged Whatman paper dyed with simple synthesized verdigris, with noted bands at 1316 cm−1 (solid arrow), 1425 cm−1 (small dashed arrow), and 1560 cm−1 (big dashed arrow).

Figure 4. Whatman no. 1 filter papers aged over 7 days (60 °C, 85% RH) matted on white board to observe various verdigris pigments applied as a dye or watercolor: (A) no pigment, (B) multistep synthesized verdigris in water (dyed by students), (C) commercially available verdigris in water, (D) simple synthesized verdigris in water, (E) multistep synthesized verdigris in water (dyed by instructors), (F) gum Arabic, (G) commercially available verdigris in gum Arabic, (H) simple synthesized verdigris in gum Arabic, and (I) multistep synthesized verdigris in gum Arabic.

discolored today, and these students can see this in a series of mock-ups, as well as in their own artworks. For art conservation graduate students, this sample served as a case study to observe the chemical changes taking place in paper painted with copper pigments. This case study was used as a part of the introduction to FTIR spectroscopy. The behavior indicated in Figure 5 is a direct consequence of the degradation promoted by the pigments via mostly acid hydrolysis and cellulose’s oxidation.37

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HAZARDS AND SAFETY Most starting compounds used are inexpensive and nonhazardous for students of all ages. The greatest concern arises from the dilution of concentrated acetic acid and sodium and ammonium hydroxides to the desired concentrations. Undergraduate students performed this without any problems, but these solutions can be diluted by the instructor in future experiments.

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OUTCOMES AND CRITICAL ASSESSMENTS Although the synthesis, application, and aging were the main elements of this activity, there were numerous spontaneous conversations because of unplanned student observations. 1. Yields were lower than expected. Although students followed a given procedure based on Solomon et al.,8 not all teams obtained the same quantity (please see details in Supporting Information). This framed discussions on limiting reagent and molarity, although they had not been covered at that point.

2. Students crystallized ammonium copper complexes by accident likely due to excess ammonia in the first step of the multistep synthesis. Students reported it was hard to know when to stop, as the “deep blue” mentioned in ref 8 occurred for some students at the beginning of the addition, whereas others added almost twice as much of the reactant. Excess ammonia in the first step resulted in ammonium copper crystals in the final product (appearing dark blue compared to verdigris crystals). This framed discussions on Le Chatelier’s principle, “complex formation, acid−base chemistry and solubility”.13 3. Addition of the gum Arabic initiated a color change. Gum Arabic consists of natural gums (monosaccharide sugars), and it was historically a common used as a paint binder for watercolors. Depending on the purity of the students’ final product, the verdigris solution appeared either lighter or deeper green-blue. In all cases, the addition of gum Arabic caused the solutions to become a uniform deep blue-green (Figure 6). Some groups used more of the gum Arabic resulting in thicker watercolors. Excess gum Arabic or heating the solutions for too long or at a higher temperature caused the mixtures to change colors and within a few minutes to form a red precipitate (Figure 3). These “accidents” framed discussions on the formation of copper(I) oxide, the Fehling’s reaction, and redox reactions on organic compounds. The test of the same name identifies reducing sugars,38,39 whose unique role in verdigrispigmented artwork decay we are currently investigating.40

ASSESSMENT Altogether, synthesis of pigments, preparation of watercolors, applications, and aging studies made an engaging activity in which students learned about artists’ materials and how chemistry and art can be studied in tandem. These laboratorybased activities proved to be characteristically attractive to a broad audience, because they allowed for a visual assessment of a real-life situation. Additionally, our activities illustrated various aspects of the chemistry of historical objects and fundamental chemistry topics through synthesis and applicaD




tion for this activity, such as a commercial accelerated aging unit and an FTIR spectrometer, was available. However, alternative instrumentation can be used for similar results, such as custom accelerated degradation systems and fiberoptic and UV−vis spectrometers.41−43 These pigments are so reactive that alternative and simplified degradation chambers are effective. For instance, any sealed container with saturated ionic salts solutions will hold said relative humidity.44 If these boxes are placed in a warm environment (like in direct sunlight), the temperature will rise, resulting in similar changes. Another alternative is placing samples inside a dark container with UV lamps set with timers. The research aim is to speed up degradation in a laboratory setting “in order to elucidate the chemical reactions involved and the physical consequences thereof”.45

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Figure 6. Appearance of the dissolved verdigris changed upon the addition of gum Arabic from a light, semi-insoluble blue crystal (left) to a deep-green solution (right).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00358. Lab handout for our undergraduate class, including postlab questions (PDF, DOCX) Synthesis using both methods (with illustrations), the application, and the aging protocol for use with an upper-undergraduate as well as a graduate class on heritage conservation science (PDF, DOCX)

tion. We encourage a visit to a museum or archive after finalizing these activities, so students can compare the rapid chemistry they did in a laboratory to naturally aged cultural heritage. Students were personally invested in the activities, because they synthesized and prepared their own artist’s materials. This translated into first-hand interest in the degradation processes: from generic objects to “my” objects. Students already interested in degradation processes (art conservation and related) took special interest in aging, as historic materials rarely exhibit the colors they present when freshly applied. Upper-level students benefited from the collaborative activities that provided an interesting and multidisciplinary context for learning characterization techniques. The samples’ wide span of visual results sparked other more in-depth discussions, such as the role various chemical substrates (fibers, fillers, sizing agents) and media (oil, gum, water) play into either promoting or slowing down degradation. These activities are versatile both for audiences and materials. The lab, as written, can be easily adapted to teach redox chemistry of inorganic and organic compounds, coordination chemistry, and/or degradation mechanisms. Different aspects of the activity can be highlighted on the basis of the age group being instructed, from high school students to master-level art conservation students. The chemistry of numerous other artists’ materials is well-known for their effects on material culture degradation. Hence, with the interchangeability of various artists’ materials like Prussian blue (Fe4(FeCN6)3) or historic inks (copper- or iron-based), instructors can adjust the subject matter to better suit their lessons and materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jocelyn Alcántara-García: 0000-0003-1780-6687 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank University of Delaware’s ARTC/CHEM 210 Science of Color Phenomena classes (Spring and Fall 2017). They would like to thank the Andrew W. Mellon Foundation and Winterthur Museum, Garden, and Library for the use of the ESPEC BTL 433 test chamber, and they would also like to thank Dr. Karl S. Booksh for use of the ATR-FTIR for measurements.

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LIMITATIONS Drawbacks regarding this activity include its time-consuming nature: from the verdigris synthesis to the accelerated aging. However, this can be easily fixed by performing some of the activities beforehand. For instance, the simple synthesis can be set up one semester/year in advance, by students from a semester/year ahead. Students would then use the products slowly synthesized for over a semester/year, in addition to setting up the experiment for the next class. Courses that meet once every week over the semester fit this activity well if several lab periods are available for this activity. InstrumentaE




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Multidisciplinary Learning: Redox Chemistry and Pigment History

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