Characterizing Color with Reflectance - Journal of Chemical

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

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Characterizing Color with Reflectance Simeen Sattar* Bard College, Annandale-on-Hudson, New York 12504, United States

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S Supporting Information *

ABSTRACT: Pigments, dyes, and transition-metal compounds are made in courses across the undergraduate chemistry curriculum, but student characterization of these compounds’ most striking features, their colors, seldom goes beyond verbal descriptions. Affordable, hand-held, fiber-optic reflectance spectrophotometers make it possible to advance students’ understanding of color. Reflectance spectra provide graphical information about color, whereas CIE L*a*b* color coordinates describe color quantitatively. This article describes how collection and interpretation of reflectance data are used to round out pigment syntheses in a course for nonscience majors. This approach is transferrable to any experiment in which students obtain colored solids. KEYWORDS: First-Year Undergraduate/General, Nonmajor Courses, Interdisciplinary/Multidisciplinary, Laboratory Instruction, Hands-On Learning/Manipulatives, Applications of Chemistry, Dyes/Pigments, UV−Vis Spectroscopy

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INTRODUCTION Artists’ pigments, organic dyes, and transition-metal compounds are made in laboratories across the undergraduate chemistry curriculum, encompassing courses for science and nonscience majors. Although these syntheses illustrate key concepts (e.g., types of reactions and spectrochemical series), the colors of the products are undeniably part of the appeal of these experiments. However, students’ vocabulary for describing color is limited and qualitative. With the ready availability of affordable, hand-held, fiber-optic reflectance spectrophotometers, it is now possible to correct this deficit and expand students’ understanding of color by making collection of reflectance curves and CIE L*a*b* color coordinates (discussed below in Quantifying Color) a routine part of any experiment in which students prepare colored compounds. Because reflectance measurements are simple and rapid, they add little time to already busy laboratory periods. Besides the correspondence between color and wavelength, the only essential background knowledge for students is additive color mixing of light (red + blue = magenta, blue + green = cyan, red + green = yellow, and red + blue + green = white), which can be easily and vividly demonstrated as needed. Incorporating visible-reflectance spectroscopy into the undergraduate curriculum is an important goal for a few reasons. For one, our visual experience of the world comes mostly from reflected light, yet reflectance is neglected compared with absorbance and transmittance. As evidence, a 2012 survey of UV−vis spectrophotometers in the undergraduate chemistry curriculum1 cited just a single application of reflectance spectroscopy.2 Second, absorbance measurements in the UV−vis region are restricted to transparent samples and cannot be made on opaque solids, the end result of many laboratory syntheses, unless they are first dissolved in a solvent. Although serviceable, absorbance (and transmittance) spectra of a solution are one step removed from the observed color of the solid and, given the availability of reflectance spectrophotometers, an unnecessary compromise. © XXXX American Chemical Society and Division of Chemical Education, Inc.

A third reason for incorporating reflectance spectroscopy into the curriculum is its widespread use in cultural-heritage sciences, a family of fields into which students with a background in chemistry might plausibly enter. In one study, art-conservation scientists compared the colors of a natural Japanese malachite and synthetic basic copper(II) carbonates, Cu2CO3(OH)2, on the basis of their reflectance curves and color coordinates.3 Another study conducted by an interdisciplinary team cataloged the reflectances and L*a*b* coordinates of fabrics colored with a variety of traditional dyestuffs with the aim of identifying dyes in historical textiles.4 These are just two of many examples. Thus, introducing students to quantitative measures of color opens up the conservation-science literature to science and nonscience majors alike with nascent interest in the field. A few undergraduate experiments involving the measurement of color using a portable, fiber-optic reflectance spectrophotometer have been published. In an experiment cited in the 2012 survey,1 students in a forensic-science course match a “paint smear” to one of a set of white house paints tinged with blue by comparing their CIE L*a*b* color coordinates.2 An article inadvertently overlooked by the survey describes a trio of experiments, suitable for a nonmajor artconservation course and analytical and analytical−instrumental courses, involving analysis of reflectance curves and L*a*b* coordinates of samples collected from paintings.5 A recent contribution to this Journal uses transmittance spectra of indicator solutions and wines as the basis for computing color coordinates.6 Differing from the stand-alone experiments described above, this article advocates adding collection and interpretation of reflectance curves and L*a*b* coordinates to existing experiments in which colored compounds are made. Received: October 15, 2018 Revised: February 21, 2019

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DOI: 10.1021/acs.jchemed.8b00845 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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Table 1. Artists’ Pigments, Chemical Formulas, Preparation Sources, and CIE L*a*b* Coordinates CIE Coordinatesa Pigment Name

Formula

Ref

L*

a*

b*

Cadmium yellow Carmine lake Chromium oxide green Cobalt blue Cobalt violet Indigo Lead orange Lead yellow Malachite Prussian blue

CdS C22H20O13·nAl(OH)3 Cr2O3 CoAl2O4 Co3(PO4)2 C16H10N2O2 PbCrO4 PbCrO4 Cu2CO3(OH)2 K3Fe[Fe(CN)6]

12 13 14 15 16 17 18 18 19 20

58.14 43.34 35.48 46.91 64.14 21.46 57.49 74.75 80.44 22.65

31.90 34.22 −13.15 −7.48 29.32 7.48 32.16 19.63 −23.92 3.42

49.96 −16.38 14.52 −21.66 −11.30 −5.59 36.36 75.75 0.03 −11.68

Variations in the L*a*b* values of the same sample are about ±0.1.

a

a* and b* are complicated; in practice, a range of ±100 is ample. Colors on opposite sides of the a* and b* axes are called color opponents because reddish-green and yellowishblue do not exist, whereas reddish-blue (magenta), reddishyellow (orange), greenish-yellow, and greenish-blue (cyan) do exist. Looking ahead to the color coordinates of pigments prepared by students (Table 1), indigo is very dark (L* is small) and nearly achromatic (a* and b* are both small); malachite is a light (L* is large) and almost pure green (a* < 0, b* ≈ 0). Examples drawn from student-laboratory reports are given in the Results.

This practice has been incorporated into a nonscience-major course about pigments, paints, and paintings in which students synthesize artists’ pigments. More than two-fifths of the 63 students enrolled in the three offerings of the course that included this component were studio-art or art-history majors. Reflectance measurements are also included in a course about photographic processes for nonscience majors.7 Students in this course may choose any set of samples they wish. The diversity of their selections (paint swatches, fruits and vegetables, blue jeans, and skin tones, to name a few) highlights the students’ curiosity about color. It would be a pity not to cultivate this.

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Quantifying Color

In 1855, James Clerk Maxwell postulated that color possesses three attributes, which he identified as hue, intensity, and tint (or purity).8 Although studio-art majors nod at these terms, they are more accustomed to parallel sets: hue, value or lightness, and chroma or saturation. Just as these examples suggest, the array of triads is bewildering; the apparently interchangeable vocabulary only worsens the confusion. As a result, a numerical system for expressing the attributes of color comes as somewhat of a relief. Of specific interest here are the 1976 Commission Internationale d’É clairage (CIE) color coordinates L*, a*, and b*. They are calculated by numerical integration of the product of the spectral distribution of the light illuminating a sample; the sample’s reflectance curve; and the CIE standard-response curves of the nominally red, green, and blue receptors in the human eye. This calculation yields the tristimulus values XYZ, from which L*a*b* and other coordinate sets (e.g., xyY) can be derived. A multitude of algorithms are available online to transform one set of coordinates to another (see, for example, ref 9). RGB values, familiar to students with experience manipulating digital images, can also be calculated from the XYZ coordinates. Formulas and stepwise calculations of different sets of color coordinates from transmittance or reflectance curves may be found in the documentation and workbooks accompanying ref 6. The L*a*b* coordinates lie along three mutually perpendicular axes defining a color space. L* measures lightness, with L* = 0 corresponding to black and L* = 100 corresponding to white; a* < 0 corresponds to green, and a* > 0 corresponds to red, whereas b* < 0 corresponds to blue, and b* > 0 corresponds to yellow. Colors near the origin in the a*−b* plane are dull, nearly devoid of hue, or achromatic; colors become more vivid with distance from the origin. The limits of

METHODS

Reflectance Spectrophotometer

Much has changed since an article about artists’ colors published in this Journal 40 years ago simulated a reflectance curve by subtracting the absorbance spectrum of a red object

Figure 1. Color coordinates a* and b* plotted for the powdered pigments (filled circles) listed in Table 1 plus two pigment mixtures: cadmium green and chrome green (see the text for composition). Points near the origin represent pigments that are dull-colored (e.g., indigo and Prussian blue), whereas those farther away are vivid (e.g., cadmium yellow). Open circles correspond to the paints swatches shown in Figure 2. Matched pairs of pigments and paints are connected by a solid line. The filled and open circles are colored according to the RGB values computed from the L*a*b* coordinates of the pigments and paints. B

DOI: 10.1021/acs.jchemed.8b00845 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Figure 2. Paint swatches on a canvas-paper support. Chrome yellow is a synonym for lead yellow. All the pigments are ground in linseed oil, except for malachite, which is mixed with diluted egg yolk. The oil blots result from paints too rich in oil. Art work by Sharon Greene. Used with permission.

from the spectrum of natural daylight.10 The costly integratingsphere diffuse-reflectance accessories for UV−vis spectrophotometers available at that time have been joined by low-cost, hand-held devices that use fiber-optic and diode-array detectors. The spectrophotometer used here (X-Rite i1Basic Pro) with powdered pigments and paint swatches is a self-contained, mouse-shaped device controlled by third-party software.11 Light from a tungsten filament is delivered to a port (diameter of 4.5 mm) on the underside of the device via an optical fiber; the light reflected from the sample is collected at the same port by another fiber-optic bundle and dispersed by a diffraction grating over a diode-array detector. The software displays the reflectance spectrum, measured at 10 nm intervals from 380 to 730 nm, and the L*a*b* color coordinates. As an aid to visualizing a sample’s location in the color space, the data are also plotted in the a*−b* plane. It takes only a couple of minutes to calibrate the device and view the data for the first sample; data for additional samples are acquired in seconds.

Pigments and Paints

The artists’ pigments prepared by students in the paints course are listed in Table 1 along with references to the method of preparation (see the Supporting Information for general notes about making pigments). Different pairs of students record the reflectance data for the powdered pigments each week. The L*a*b* coordinates (Table 1) and reflectance spectra are posted to an online forum. Students are asked to discuss these data in their reports in relation to the observed color of the pigment. As this procedure is independent of the identity of the samples, it is easily adapted to any experiment involving the synthesis of a colored compound. The plot of the collected pigment data in the a*−b* plane (Figure 1) gives the class as a whole (including the instructor) a sense of accomplishment. The distribution of colors or color gamut is comprehensible at a glance. The sets of L, a*, and b* values vary by a few percentage points from class to class because of the size distribution of the particles in the powder,21 C

DOI: 10.1021/acs.jchemed.8b00845 J. Chem. Educ. XXXX, XXX, XXX−XXX

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pigment; lightness, L*, is also affected. The color shift is related to the relative refractive indices of the pigment and binding medium, the pigment concentration, and the particle size.21,24,25 The L*a*b* coordinates of paint swatches of the same pigment vary slightly from class to class, principally because of the concentration of pigment in the linseed-oil or diluted-egg-yolk medium. Reflectance curves of selected paints from Figure 2 are shown in Figure 3. Considering the reflectance curve of the malachite paint swatch, a student in a photographic-processes course was inspired to write at length: Anyone looking at malachite with friends might be surprised to hear them calling it different colors. I personally would call it a blue, an “eggshell blue”, or a cyanbut one could just as easily call it a green, or perhaps more accurately, a “sea green”. Its spectral plot shows that it is indeed a greenits strong peak reflectance around 530 nm accounts for thisbut also shows us why we may call it different things. Looking more closely, we see that the color also reflects a good deal of blue light in the frequencies [a slip for “wavelengths”] below its peak. Blue and green mix to create cyan, a color many call “blue”, so it is unsurprising that it seems to hover cautiously on this boundary. Malachite’s strong peak reflectance of about 55% accounts for its bright nature, while its slow falloff around that accounts for its strong color focus yet slight desaturation. A notable absence of red light, especially above 650 nm, similarly contributes to its cyan nature. After noting the individuality of verbal descriptions of color, this student turns to the spectrum to explain why this might be true and goes on to read the lightness and saturation from the height and shape of the curve. An instructor could hardly ask for more. These examples of student writing along with others in the Supporting Information demonstrate that students have achieved the learning goals of introducing reflectance measurements into the laboratory. Students have advanced their understanding of color, a topic important to many scientific fields, by learning how to describe color in terms of the attributes hue, saturation, and lightness; interpret the CIE L*a*b* color coordinates; and interpret reflectance spectra.

incomplete drying, and (in the cases of carmine lake, cobalt violet, and malachite) differences in composition due to the pHs of the solution from which they precipitated. Students go on to make paints with the pooled class pigments (see the Supporting Information for details). A set of paint swatches collected by a student is shown in Figure 2. At least two papers in this Journal describe students making paints from artists’ pigments in a traditional general-chemistry course (malachite and verdigris)22 and in a novel multidisciplinary general-chemistry course (Egyptian blue).23

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RESULTS With the reflectance curves and L*, a*, and b* coordinates for the pigments in hand (Table 1), students can go beyond a

Figure 3. Reflectance curves for paint swatches selected from Figure 2. The reflectance is recorded at 10 nm intervals. All but ultramarine were made from pigments prepared in class. The lines are colored according to the RGB values calculated from the L*a*b* coordinates of the paints.

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qualitative description of color. A student interpreting the L*a*b* coordinates of cadmium yellow in his laboratory report covers the essentials: The value of a* > 0 means that the hue is more red than green and b* > 0 means that the hue is yellow rather than blue. The value of L* is between 50 (gray) and 100 (white), which is true, because it was consistent with the lightness of the pigment. Hence, the color of the pigment can be described as more orange with more yellow than red. That the student draws on the relative magnitudes of a*and b* to describe the sample’s color as orange is noteworthy in light of the fact the laboratory experiment was titled “Synthesis of Cadmium Yellow”. Along with the a*b* coordinates of powdered pigments, Figure 1 plots the coordinates of the paint swatches shown in Figure 2. (The L*a*b* coordinates are tabulated in the Supporting Information.) In some instances, a paint’s coordinates are noticeably shifted from those of the powdered

CONCLUSION

Reflectance tends to get less curricular attention than other types of visible spectroscopy, which is incommensurate with its importance to visual perception. The first steps toward correcting this imbalance are to make a modest investment in a portable, fiber-optic reflectance spectrophotometer and to add measurement of color parameters to existing laboratory experiments that involve the preparation of colored compounds. Because the measurements are easy and quick, this is a very practical addition. Undergraduate students at any level will learn, probably for the first time, that color is a measurable property that can be situated in a coordinate system or color space. Reflectance spectra, from which the color coordinates are computed, complement the absorbance−transmittance spectra familiar to students. Incorporating these simple measurements enlarges students’ color vocabulary and deepens their scientific understanding of color. D

DOI: 10.1021/acs.jchemed.8b00845 J. Chem. Educ. XXXX, XXX, XXX−XXX

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(12) Douma, M. Cadmium yellow/red, 2008. Pigments through the Ages. http://www.webexhibits.org/pigments/indiv/recipe/ cdyellowred.html/ (accessed Feb 2019). (13) Deveoglu, O.; Torgan, E.; Karadag, R. Identification of Dyestuffs in the Natural Pigments Produced with Al3+, Fe2+ and Sn2+ Mordant Metals from Cochineal (Dactylopius coccus Costa) and Walloon Oak (Quercus ithaburensis Decaisne) by HPLC-DAD. Asian J. Chem. 2010, 22, 7021−7030. (14) Butler, I. S.; Furbacher, R. J. Chemistry and Artists’ Pigments. J. Chem. Educ. 1985, 62, 334−336. (15) Douma, M. Cobalt blue, 2008. Pigments through the Ages. http://www.webexhibits.org/pigments/indiv/recipe/coblue.html/ (accessed Feb 2019). (16) Douma, M. Cobalt violet, 2008. Pigments through the Ages. http://www.webexhibits.org/pigments/indiv/recipe/coviolet.html/ (accessed Feb 2019). (17) McKee, J. R.; Zanger, M. Microscale Synthesis of Indigo: Vat Dyeing. J. Chem. Educ. 1991, 68, A242−A244. (18) Ramette, R. W. Precipitation of Lead Chromate from Homogeneous Solution. J. Chem. Educ. 1972, 49, 270−271. (19) Tanaka, H.; Yamane, M. Preparation and Thermal Analysis of Synthetic Malachite CuCO3·Cu(OH)2. J. Therm. Anal. 1992, 38, 627−634. (20) Orna, M. V. Chemistry and Artists’ Colors. Part III. Preparation and Properties of Artists’ Pigments. J. Chem. Educ. 1980, 57, 267− 269. (21) Brill, T. B. Light: Its Interaction with Art and Antiquities; Springer: New York, 1980. (22) Solomon, S. D.; Rutkowsky, S. A.; Mahon, M. L.; Halpern, E. M. Synthesis of Copper Pigments, Malachite and Verdigris: Making Tempera Paint. J. Chem. Educ. 2011, 88, 1694−1697. (23) Morizot, O.; Audureau, E.; Briend, J.-Y.; Hagel, G.; Boulc’h, F. Introducing the Human Element in Chemistry by Synthesizing Blue Pigments and Creating Cyanotypes in a First- Year Chemistry Course. J. Chem. Educ. 2015, 92, 74−78. (24) Jones, S. R. History of the Artist’s Palette in Terms of Chromaticity. In Application of Science in Examination of Works of Art; Museum of Fine Arts: Boston, 1967; pp 71−77. (25) Feller, R. L.; Kunz, N. The Effect of Pigment Volume Concentration on the Lightness or Darkness of Porous Paints. In Contributions to Conservation Science, A Collection of Robert Feller’s Published Studies on Artists’ Paints, Paper, and Varnishes; Whitmore, P. M., Ed.; Carnegie Mellon University Press: Pittsburgh, 2002; pp 269− 277.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00845. Notes about the reflectance spectrophotometer, laboratory organization and class size, pigments and paints, hazards, and CIE L*a*b* coordinates of the paint swatches along with additional excerpts from student reports (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Simeen Sattar: 0000-0002-1597-0347 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS The author is grateful to Robert Olsen for his numerous suggestions for improving this manuscript and for preparing Figures 1 and 3. The author is also grateful to Mohammad Mubasi Bokhari, Bari Bossis, Adela Foo, Jonathan Mildner, and Isaiah Schwartz for consenting to have their reports quoted and to Sharon Greene for permission to use her artwork.

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REFERENCES

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DOI: 10.1021/acs.jchemed.8b00845 J. Chem. Educ. XXXX, XXX, XXX−XXX