Detecting Art Forgeries: A Problem-Based Raman Spectroscopy Lab

Laboratory Experiment pubs.acs.org/jchemeduc

Detecting Art Forgeries: A Problem-Based Raman Spectroscopy Lab Sara E. Nielsen, Jonathan P. Scaffidi, and Ellen J. Yezierski* Department of Chemistry and Biochemistry, Miami University, Oxford, Ohio 45056, United States S Supporting Information *

ABSTRACT: A real-world context can provide an interesting and engaging framework for an upper-division instrumental analysis laboratory experiment. A lab based on the authentication of art with Raman and fluorescence spectroscopy was developed. In this adapted problem-based learning (PBL) approach, students make real-time decisions about experimental conditions and write their final report in the form of a journal article. Laboratory implementation demonstrated that the majority of students met the desired learning outcomes.

KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Problem Solving/Decision Making, Applications of Chemistry, Fluorescence Spectroscopy, Lasers, Raman Spectroscopy, Student-Centered Learning variety of optical spectroscopic techniques find use in the chemical analysis of artwork.1−12 Optical spectroscopic techniques such as infrared (IR) absorption,1,4 UV−visible reflectance (UV−vis),5 fluorescence,6,7 and Raman spectroscopy8 allow molecular-level analysis of artwork and are particularly appealing because they are at most microdestructive, as compared to more destructive techniques such as laser-induced breakdown spectroscopy (LIBS) or inductively coupled plasma−atomic emission spectroscopy (ICP−AES). In addition, some of these spectroscopic molecular analysis techniques require only optical access to the sample, making them especially well suited to noncontact examination of artwork (e.g., paintings behind glass). As a result, optical spectroscopy is gaining utility during art conservation and when identifying potential forgeries. The specificity of Raman spectroscopy in particular has proven useful when determining the composition of both inorganic and organic pigments.9−11 Most pigments are easily distinguishable by their Raman spectra when compared to a reference library.12 In addition, because the location of vibrational peaks in a Raman spectrum is independent of the excitation wavelength, lasers with wavelengths in the nearinfrared region can be used to limit fluorescence while preventing damage to potentially priceless artwork. Despite the scientifically and experimentally straightforward nature of Raman spectroscopy and the engaging context of art, there are very few student-centered, undergraduate laboratory experiments exploring the way Raman spectroscopy can be used in the examination of artwork.13,14 Techniques such as XRF, UV−vis, and FT-IR have been used in undergraduate laboratory experiments to study pigments and paintings.15

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© 2014 American Chemical Society and Division of Chemical Education, Inc.

Research has shown that student-centered laboratory experiences help students strengthen their problem-solving and reasoning skills.16 Problem-based learning (PBL) is an active learning pedagogy that engages students in solving real-world problems.17,18 Some essential features of PBL include a strong connection between the required content and the problem at hand, framing the students as individuals with a vested interest in the problem, and a student-centered learning environment where teachers guide student exploration and problem solving. In addition, it is crucial that assessment be authentic to the scenario. Throughout the PBL laboratory experience described in this paper, students encounter problems that generate a “need to know”. Through their use of resource materials both during and after the lab, students synthesize the content that would otherwise be presented in lecture. Traditional PBL activities are designed to be more long-term and overarching, allowing students to iterate through multiple cycles of discovery and problem solving. In this case, the curriculum requires that the laboratory experiment fit within a 4 h laboratory period. The PBL format was modified to work within this time constraint. Most of the essential features remain the same, but the students are given added guidance to allow the learning experience to fit into a single laboratory period.

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EXPERIMENTAL OVERVIEW

The following experiment is designed to be performed by groups of 2−3 students in a 3−4 h undergraduate instrumental analysis laboratory. The simplified scenario presented to the Published: February 6, 2014 446

dx.doi.org/10.1021/ed400319k | J. Chem. Educ. 2014, 91, 446−450

Journal of Chemical Education

Laboratory Experiment

spectrum. The frequency-doubled Nd:YAG laser is used to ensure that the students have enough time to take spectra of the samples and the paintings, as the exposure times are often much shorter. Using this excitation wavelength also highlights the potential interference from or usefulness of fluorescence for the examination of artwork. Although the use of the frequencydoubled Nd:YAG laser may cause photobleaching, we feel that it is an important factor the students need to consider when making decisions about analyzing the paintings. The design of this laboratory experiment requires that students make a variety of procedural decisions and be able to justify them when reporting their results. These decisions are primarily associated with sample preparation constraints, spectral exposure time, reducing or using fluorescence and photobleaching when identifying individual pigments, and optimizing signal-to-noise ratios. Because the paint samples are mounted on glass slides, the students need to decide if the laser should pass through the glass before hitting the paint or if the slide should be oriented such that only the paint is excited by the laser. The students quickly discover that the suggested exposure time is not optimal for every sample, and they must identify conditions that provide a characteristic spectrum for each pigment. Strong fluorescence can overpower the Raman signal from several of the pigments and saturate the detector; therefore, students must adjust exposure times as needed when examining different samples. Furthermore, as reliability of the data is always a concern for researchers, students are tasked with deciding whether their spectra are both reproducible and informative. In contrast to providing students with a particular experimental protocol, the instructor helps to guide the students’ experimental design by asking leading questions or highlighting particular spectral features. Several of the pigments’ spectra are quite similar, adding another layer of complexity and authenticity to the context of the lab. The final product of this experiment is a report in the form of a journal article. This format is standard for all the reports in this particular course, and it was chosen to help students develop the scientific writing skills they will need later in their careers as researchers. The students are given the report guide19 before they come to the lab to help them focus on the important and required criteria when performing the experiment. The report guide, which can be found in the Supporting Information, requires students to interpret and analyze their data and to apply their knowledge to the problem presented in the lab.

students is not completely reflective of the role chemists play in art conservation and authentication but does provide an interesting and engaging context to the experiment. The students play the role of analytical chemists contacted by a museum to examine two seemingly identical paintings. The museum director needs to determine which painting is likely authentic and which is likely a forgery. The students acquire fluorescence and Raman spectra and use them to successfully identify the time frame in which the paintings were likely created. Students explain the origins of fluorescence and photobleaching and how to either avoid these effects or use them as tools when identifying the pigments in the paintings. They also consider a scenario in which the paintings are displayed under glass on the wall of a museum and use their experience in the lab to construct a solution. Prior to the laboratory, the students are supplied with the “Student Laboratory Guide” included in the Supporting Information.

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LEARNING OUTCOMES The lab aims to help students build content and procedural knowledge of fluorescence and Raman spectroscopy. In this experiment, students (1) acquire fluorescence and Raman spectra of solid samples, (2) use evidence to make and justify decisions about experimental conditions in real time, (3) apply the concepts of fluorescence and Raman spectroscopy to a realworld problem, (4) find relevant and analytically useful information in the scientific and conservation literature, and (5) write a final report in the form of a journal article. Problem solving and writing are embedded in these procedures, and the learning outcomes were observed in the majority of the students.

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EXPERIMENTAL COMPONENTS

Prelab Exercise

Students are required to complete a prelab exercise that assesses their preparedness to perform the experiment (available in the Supporting Information). They need to be able to describe specific safety procedures and equipment required, what experimental parameters are expected to be important, what data they will be collecting, and what those data are expected to look like. Procedure

A key feature of this lab is that students are not given a formal procedure. Rather, they are presented with a goal and through experimentation, data analysis, and use of source materialdevelop an appropriate experimental procedure to achieve their goal. However, as the students have little or no experience with Raman spectroscopy, especially with respect to the application of Raman spectroscopy to art, they are given a student guide and a brief hands-on tutorial demonstrating the safe operation of the Raman microspectrometer. The students are supplied with 15 glass slides with different paint samples and two visually identical paintings. The students take spectra of each slide and each different color on the paintings. They are given general experimental settings, but care was taken by the authors to ensure that the default settings cannot be used to analyze every sample. The students use a frequency-doubled Nd:YAG laser with a wavelength of 532 nm. When analyzing art, red or NIR lasers are most commonly used as they are less likely to excite fluorescence or damage the artwork. The drawback of such IR excitation wavelengths is that longer exposure times are required to acquire a high-quality

Equipment

A wide variety of Raman spectrometers could be used for this experiment. A custom Raman microspectrometer built with commercial off-the-shelf components was used when developing this laboratory (see the Supporting Information), but a wide variety of instrumentation can fulfill the requirements to perform this experiment. Glass microscope slides with paint samples and two seemingly identical paintings are also required. A list of all of the paints used for the slides and in the paintings is available in the Supporting Information. Pictures of the paint samples and paintings used can be found in the Supporting Information. Authors have created kits containing all of the necessary paint samples and two identical paintings for instructors who wish to implement the lab but are unsure of their ability to prepare suitable paint samples and paintings. Contact the authors for more information.20 447




Figure 1. Student spectra (left) and author spectrum (right) for King’s Yellow. Students were not instructed to report their results using Raman shift (cm−1).

Figure 2. Student spectrum (left) and author spectrum (right) for Dioxazine Purple. Students were not instructed to report their results using Raman shift (cm−1).

Post-Lab Exercise

their decisions about the time frame in which each painting was likely created. During the course of the lab, students acquired multiple spectra for each sample to ensure the reproducibility of their data. As seen in Figures 1 and 2, the spectra obtained by students are qualitatively quite similar to spectra obtained by the authors, highlighting the students’ ability to collect representative data. Intensity differences are due to differences in focusing, laser power, and acquisition time. When collecting their spectra, students found that intense fluorescence or Raman scattering by several pigments saturated the detector. Saturating the detector leads to nonrepresentative spectra, so the students had to make decisions regarding exposure time to avoid saturation while simultaneously maintaining a suitable signal-to-noise ratio. The students also discovered that some pigments undergo photobleaching, so they had to decide how to either use this phenomenon or work around it when identifying their pigments. The report guide (excerpted in Table 1) and rubric designed to evaluate the student reports functioned very well. Two of the authors graded the reports separately then compared and negotiated scores. Before any negotiation took place, there was 95% agreement between the assigned grades, which demonstrates the reliability and functionality of the report guide and rubric. While grading the reports, a few questions from the report guide were determined to be worded somewhat ambiguously, and some questions were repetitive. After

The instructor’s guide, which can be found in the Supporting Information, contains a list of questions for a postlab discussion that addresses important conservation principles (e.g., ethical considerations) for analyzing artwork.

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HAZARDS As designed and tested, this experiment uses a high-powered 532 nm laser that can easily damage students’ eyes if proper precautions are not taken. Suitable laser goggles designed to block 532 nm light (OEM Laser Systems, Salt Lake City, UT) must be worn for protection any time the laser is on and the shutter is open. Proper selection of such laser goggles can additionally block laser light while allowing student to observe the intense fluorescence produced by several of the paint samples. In addition, several of the paints contain heavy metals such as lead, cobalt, and cadmium. The paints should not be ingested. Students should wash their hands after handling the samples.

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RESULTS AND DISCUSSION This experiment was performed by groups of 2 or 3 undergraduate students during a 3−4 h instrumental analysis lab. In total, 14 students (6 groups) performed the experiment. The students were able to correctly identify the pigments used in the paintings based on their fluorescence and Raman spectra. Using this spectral information, they were also able to justify 448




Table 1. Excerpts from Experimental Methods and Results and Discussion Sections of Report Guide Report Guide Section and Learning Outcome Conditions used Conditions used Explain decisions Explain decisions Explain decisions Effects of conditions Effects of conditions Theory of Raman Optimization Optimization Optimization Analysis Analysis Analysis Analysis

Question What acquisition time(s) was(were) used? How many exposures/frame were used? Why did you use the acquisition time you used? Why did you use exposures/frame you used? Why did you use the room conditions you used? What effect does the laser excitation wavelength have on the spectra? What effect does the laser power have on the spectra? Why is the fluorescence so much stronger than the Raman signal? What techniques were used, or could have been used, to limit fluorescence? Why are the fluorescence-limiting techniques effective? What effect does photobleaching have on the spectra? What is the oldest painting A could be? Give evidence for your assessment of painting A′s age. What is the oldest painting B could be? Give evidence for your assessment of painting B′s age. Which painting is the original and which is the forgery? If you had to choose 1 or 2 paints to make your determination, which would you choose? Why would you choose those paints?

Table 2. Number of Students Meeting the Learning Outcomes at a 70% Level

Point Value 1 1 3 3 3 3 3

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Learning Outcomes

Number of Students

Introduction Conditions used Optimization Effects of conditions Explain decisions Theory of Raman Analysis Application Conclusion Spectra

10/10 10/10 7/10 6/10 8/10 8/10 8/10 10/10 9/10 9/10

SUMMARY This experiment was designed for and successfully implemented in an undergraduate instrumental analysis laboratory course. Raman and fluorescence spectroscopy were used to analyze paint samples to determine which of two paintings is likely authentic. The undergraduate students were able to successfully use the instrumentation and make decisions based on the data they collected. The students also found the context to be interesting and engaging.

3 3 3 3

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ASSOCIATED CONTENT

S Supporting Information *

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Student laboratory guide including the prelab exercise, experiment guide, and report guide; a facilitation guide for the instructor, including a prelab lecture and a list of questions for a postlab discussion; spectra collected by students and the authors; report grading rubric; the initial pool of materials and those that were selected for the final experiment; photo of the custom-built Raman microspectrometer; pictures of the paint samples and paintings. This material is available via the Internet at http://pubs.acs.org.

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discussion between the authors, the repetitive questions were removed and the wording was modified to minimize ambiguity. Successful student reports document procedures while explaining their rationale and document results while describing their significance. The complete report guide and rubric are in the Supporting Information. Most students correctly answered the factual questions about experimental settings and could explain the settings’ effects on the obtained spectra. By comparing their reference spectra to spectra collected from the paintings, students were able to identify several pigments used in the paintings. They were then able to research the timelines of the paints used and make a determination of the approximate ages of the paintings. Most of the students made correct determinations, though not all students could provide satisfactory reasoning justifying their decisions. To add authenticity to the scenario, as well as a layer of complexity to the analysis, the colors seen in the paintings are a mixture of different paints. This reflects typical artistic practice, as artists seldom use pure paints in their work.

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AUTHOR INFORMATION

Corresponding Author

*E. J. Yezierski. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the students of the CHM 455 Instrumental Analysis course for their participation as well as the three graduate students and three undergraduate students who participated in the pilot studies of the lab. We appreciate the helpful suggestions from reviewers, particularly their conservation perspectives on the experiment. We are grateful to the Yezierski, Scaffidi, and Bretz Research Groups for their assistance with the structure and the content of the lab. We also thank Miami University for funding this research.

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STUDENT LEARNING OUTCOMES One of the strongest features of this PBL laboratory experience is its strong focus on analysis and explanation, as evidenced by 57% of the total points on the report guide requiring analysis, synthesis, or evaluation, the highest levels of Bloom’s Taxonomy.21 The students are expected to demonstrate their reasoning skills in their reports, and many of the students were able to do this successfully. The majority of the students met the learning outcomes for the lab, as shown in Table 2 for the ten students who gave consent.

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REFERENCES

(1) Adriaens, A. Non-destructive analysis and testing of museum objects: An overview of 5 years of research. Spectrochim. Acta, Part B 2005, 60, 1503−1516. (2) Moioli, P.; Seccaroni, C. Analysis of art objects using a portable X-ray fluorescence spectrometer. X-Ray Spectrom. 2000, 29, 48−52. (3) Anglos, D. Laser-Induced Breakdown spectroscopy in art and archaeology. Appl. Spectrosc. 2001, 55 (6), 186A−205A.

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(4) Casadio, F.; Toniolo, L. The analysis of polychrome works of art: 40 years of infrared spectroscopic investigations. J. Cult. Herit. 2001, 2, 71−78. (5) Leona, M.; Casadio, F.; Bacci, M.; Picollo, M. Identification of the Pre-Columbian pigment Maya Blue on works of art by noninvasive UV-Vis and Raman spectroscopic techniques. J. Amer. Inst. Conserv. 2004, 43 (1), 39−54. (6) Romani, A.; Clementi, C.; Miliani, C.; Favaro, G. Fluorescence spectroscopy: A powerful technique for the noninvasive characterization of artwork. Acc. Chem. Res. 2009, 43 (6), 837−846. (7) Melo, M. J.; Claro, A. Bright light: Microspectrofluorimetry for the characterization of lake pigments and dyes in works of art. Acc. Chem. Res. 2009, 43 (6), 857−866. (8) Vandenabeele, P. Raman spectroscopy in art and archaeology. J. Raman Spectrosc. 2004, 35, 607−609. (9) Hayez, V.; Denoel, S.; Genadry, Z.; Gilbert, B. Identification of pigments on a 16th century Persian manuscript by micro-Raman spectroscopy. J. Raman Spectrosc. 2004, 35, 781−785. (10) Kendix, E.; Nielsen, O. F.; Christensen, M. C. The use of microRaman spectroscopy in architectural paint analysis. J. Raman Spectrosc. 2004, 35, 796−799. (11) Pages-Camagna, S.; Calligaro, T. Micro-PIXE and micro-Raman spectrometry applied to a polychrome wooden altarpiece from the 16th century. J. Raman Spectrosc. 2004, 35, 633−639. (12) Burgio, L.; Clark, R. J. H.; Hark, R. R. Raman microscopy and X-ray fluorescence analysis of pigments on medieval and Renaissance Italian manuscript cuttings. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 5726. (13) Advances in Teaching Physical Chemistry; Ellison, M. D., Schoolcraft, T. A., Eds.; American Chemical Society: Washington, DC, 2008. (14) Active Learning: Models from the Analytical Sciences; Mabrouk, P. A., Ed.; American Chemical Society: Washington DC, 2007. (15) Nivens, D. A.; Padgett, C. W.; Chase, J. M.; Verges, K. J.; Jamieson, D. S. Art, meet chemistry; chemistry, meet art: Case studies, current literature, and instrumental methods combined to create a hands-on experience for nonmajors and instrumental analysis students. J. Chem. Educ. 2010, 87 (10), 1089−1093. (16) Science as Inquiry in the Secondary Setting; Luft, J., Bell, R. L., Gess-Newsome, J., Eds.; NSTA Press: Arlington, VA, 2008. (17) Barrows, H. S. Problem-Based Learning in medicine and beyond: A brief overview. New Dir. Teach. Learn. 1996, 69, 3−12. (18) Torp, L.; Sage, S. Problems as Possibilities: Problem-Based Learning for K-16 Education. Association for Supervision and Curriculum Development: Alexandria, VA, 2002. (19) Initially, the report guide had no points allotted for references or quality of writing. Some of the students’ reports did not include references, and the writing was not always as strong as desired. To address these issues, points for references and writing were added to the report guide. http://chemistry.miamioh.edu/yezierski/ ramanlabkit.html (accessed Jan 2014). (20) All material is available on the Internet at http://chemistry. miamioh.edu/yezierski/ramanlabkit.html (accessed Jan 2014). (21) Taxonomy of Educational Objectives: The Classification of Educational Goals. Handbook 1: Cognitive Domain; Bloom, B. S., Ed.; David McKay Company, Inc.: New York, 1956.

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Detecting Art Forgeries: A Problem-Based Raman Spectroscopy Lab

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