Pigment Synthesis for the Exploration of Binding Media Using a Lead

Nov 15, 2016 - Generating enthusiasm among nonscience majors in a laboratory course is a difficult task. Often, students are asked to perform a precip...
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Pigment Synthesis for the Exploration of Binding Media Using a Lead-Free Alternative to Chrome Yellow Anne C. Gaquere-Parker,*,† Patricia S. Hill,‡ Michael P. Haaf,§ Cass D. Parker,∥ N. Allie Doles,† Amanda K. Yi,† and Todd A. Kaminski† †

Department Department § Department ∥ Department ‡

of of of of

Chemistry, Chemistry, Chemistry, Chemistry,

University of West Georgia, Carrollton, Georgia 30118, United States Millersville University, Millersville, Pennsylvania 17551, United States Ithaca College, Ithaca, New York 14850, United States Clark Atlanta University, Atlanta, Georgia 30314, United States

S Supporting Information *

ABSTRACT: Generating enthusiasm among nonscience majors in a laboratory course is a difficult task. Often, students are asked to perform a precipitation reaction, only to collect and then safely dispose of the solid without detailing composition, properties, or uses. In an effort to keep the students engaged, this laboratory exercise presents an innovative way to use the product of a precipitation reaction. The reaction chosen produces basic zinc(II) chromate, a yellow pigment that can be combined with various paint binders such as beeswax, gum Arabic, egg yolk, linseed oil, and acrylic medium. The students then test their paints on a canvas and analyze their different physical properties. This exercise can be tailored to match different undergraduate levels, ranging from nonscience majors to chemistry majors and be used as a part of a STEAM (science, technology, engineering, arts, and mathematics) activity. A brief discussion on historical paint binders and their chemistry is also included. KEYWORDS: First Year Undergraduate/General, Laboratory Instruction, Hands-On Learning/Manipulatives, Dyes/Pigments, Applications of Chemistry, Precipitation/Solubility

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INTRODUCTION Combining chemistry and art has recently received much interest from chemical educators and is increasingly becoming part of college curricula across the country as part of an effort to develop more STEAM (science, technology, engineering, arts, and mathematics) activities. As judged by the surge in chemistry and art related papers in chemical education journals, many introductory chemistry courses for nonscience majors as well as upper-level chemistry electives for science majors have been developed.1−10 Chemistry faculty are also introducing artbased chemistry experiments as part of a more traditional curriculum.11,12 Recently the synthesis of malachite and verdigris, two green copper-based pigments of historical importance, has been described as a chemistry laboratory activity where students may create tempera paint by mixing their obtained pigments with egg yolk.13 However, there is no description in the literature of a laboratory experiment covering the synthesis of a pigment and its use with historically important paint binders. Chrome yellow is a lead-based pigment that was traditionally produced by the precipitation of lead(II) chromate from potassium or sodium chromate and lead(II) nitrate.14 In this experiment, students have the opportunity to synthesize a lead-free alternative to chrome yellow by substituting lead with zinc, learning about solubility rules, precipitation reactions, and molecular and ionic equations. Then they test the product with historically © XXXX American Chemical Society and Division of Chemical Education, Inc.

important paint binders such as linseed oil, egg yolk, beeswax, acrylic, and gum Arabic to observe changes in physical properties. With this activity, students will learn what paints are made of and how the different types of paint differ. They will learn about the chemical structures of each binder, and how the polymeric structures act to hold the pigment to the painting surface. The prevalence of pigments used in different time periods will be emphasized, and whether the sources of each medium are natural or synthetic. They will realize that, even though there are many different types of binders, the pigments used are often the same, since a common misconception is that acrylic paint is completely different from oil paint including the colorant.

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BACKGROUND AND THEORY In 1797 Vauquelin discovered a new element in the mineral crocoite which contains lead(II) chromate. Because of its strongly colored salts, the newly discovered element was named chromium after the Greek word “chroma”, which means color. Vauquelin also reported the reactivity and preparation of many chromium-containing compounds, including lead(II) chromate from potassium chromate, even describing the color of the final Received: June 28, 2016 Revised: October 5, 2016

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product as a function of the acidity of the solution.15 Finally he connected chemistry and art by mentioning the use of lead(II) chromate in paintings and other chromium derivatives in ceramics. Lead(II) chromate, called chrome yellow by artists, was widely used in 19th century paintings as a source of yellow color. At the same time, zinc was used as a substitute for lead, to give a pigment called zinc yellow, which belongs to the group of zinc chromates. Zinc yellow was occasionally used as a pigment, including some famous paintings like Seurat’s A Sunday Af ternoon on the Island of la Grande Jatte, Van Gogh’s Starry Night, and Munch’s The Scream. Unfortunately it was not known at the time that the pigment would decompose into a brown color.16 In a similar way, the degradation process of lead chromate in paintings by Vincent Van Gogh was recently studied by X-ray spectromicroscopy and accounts for the darkening of some of the original yellow areas of paint.17 Afterward it was mostly used as an anticorrosion coating on World War II aircraft and for similar industrial purposes. Nowadays, zinc phosphate is used as an anticorrosive pigment,18 and the nontoxic bismuth vanadate is now one of the preferred yellow pigments.19 There are four zinc chromate pigments described in the literature with the following composition: ZnCr2O7·3H2O, K2O·4ZnCrO4·3H2O, K2CrO4· 3ZnCrO4·Zn(OH)2·2H2O, and ZnCrO4·4Zn(OH)2.20 In order to keep the focus of this activity for undergraduate students on precipitation reactions, solubility rules, and physical properties of binders, the authors chose to have the students synthesize a basic form of zinc(II) chromate: ZnCrO4·Zn(OH)2, with a 1:1 stoichiometry, while still yielding a yellow pigment usable for painting. The synthesis will be performed in two steps. First, an aqueous solution of potassium chromate reacts with an aqueous solution of zinc sulfate to produce zinc chromate as a precipitate and aqueous potassium sulfate according to the following chemical equation:

Figure 1. Raman spectrum of synthesized pigment (excitation at 532 mn).

Figure 2. Anatomy of a painting.

create what is called an encaustic. Because of the low melting point of beeswax, the paint must be used hot. Early watercolors used natural gums as binders, such as the gum from the acacia tree, known as gum Arabic. Until the middle ages, an aqueous solution of egg yolk was used to make what is called tempera paint, which was slowly replaced by oil-based paints. A common misbelief is that the Van Eyck brothers introduced oil painting in the early 1400s; however, some examples of oil paintings produced in Northern Europe a couple of centuries earlier exist. The Van Eyck brothers did perfect the technique of mixed media before oil painting became the favorite medium of Italian Renaissance artists, seeking greater depths and contrasts. The exact nature of the oil as a binder for oil paintings varies depending on the geographical location of the artist, and linseed oil, walnut oil, poppy seed oil, hemp oil, and safflower oil prove to be the favored ones.24 Today acrylicbased paints are commonly used. Additional information may be found in books dedicated to the chemistry of art materials listed in the references.25−27 In this experiment, students synthesize a basic zinc(II) chromate, a yellow compound, and mix it with different paint binders, beeswax, gum Arabic, egg yolk, linseed oil, and acrylic, giving them the opportunity to explore some of the different physical properties of their paint, like artists who learn to choose the correct binder, by experimentation, depending on the transparency or hiding power desired for their paint. The transparency, and in turn the hiding power, of a paint depends on the concentration of pigment in the binder, the particle size of the pigment, and the difference of the refractive indexes between the pigment and the binder. With everything else being equal, the greater the difference between the refractive indexes, the more light scattering occurs, thus causing the paint to appear less transparent, providing a greater hiding power. For instance, a glaze typically should be highly transparent, so a binder with a similar refractive index as the pigment is chosen. Zinc yellow is reported to have a refractive index, n20/D, between 1.84 and 1.90.28 Relevant information about the binders have been compiled as shown in Table 1.

K 2CrO4 (aq) + ZnSO4 (aq) → ZnCrO4 (s) + K 2SO4 (aq)

Second, half of the zinc(II) chromate reacts with an aqueous solution of sodium hydroxide to produce a precipitate of zinc(II) hydroxide and aqueous sodium chromate as shown below. The final compound after filtration of the precipitate is basic zinc chromate, a yellow solid, ZnCrO4·Zn(OH)2. ZnCrO4 (s) + 2NaOH(aq) → Zn(OH)2 (s) + Na 2CrO4 (aq)

Because complete analysis of the pigment is beyond the scope of this article, Raman spectroscopy was used to characterize the pigment obtained. The Raman spectrum is comparable to that of zinc yellow, which strongly suggests that a form of zinc chromate is present21−23 (Figure 1). A painting is made of different layers, as shown in Figure 2. The support, which may be wood, canvas, stone, or paper, is prepared with a ground layer, called gesso, that is usually made of calcium sulfate (gypsum) mixed with oils or glues. The paint layer itself contains the pigment and the binder. The varnish is optional and is traditionally found on oil paintings to provide a protective layer and a glossy finish. Throughout the ages, artists have used different binders for their pigments to be usable as paints. In cave paintings, naturally occurring pigments were mixed with, among other things, saliva, blood, and animal fats. In the Egyptian antiquity period, records show the use of beeswax as a common binder to B

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Table 1. Information about Binders

a

Paint Binder

Monomer (or Chemical Structure)

Time Period Introduced

Refractive Index, n20/D

Acrylic resin Beeswax Egg tempera Gum arabic, solid Gum arabic, aq sol (10%) Linseed oil

Methyl methacrylate Ester of fatty acids and long chain alcohols Amino acids (albumen) Galactose Galactose Triglycerides

Mid-1940s Antiquity Antiquity−dark ages Antiquity Antiquity Late middle ages

1.5a 1.485−1.505b 1.346c 1.476d 1.344c 1.4795b

See ref 29. bSee ref 30. cSee ref 25. dSee ref 31.

In addition to transparency, the flexibility of the binding media and the durability of the paint can be observed, leading students to speculate about the type of support (canvas, fresco, wood panel, etc.) that might be most appropriate for each binder. For example, due to the brittle nature of the material, encaustics have been used primarily on rigid surfaces such as wood or natural stone surfaces, while oil, acrylic, and tempera paints are a more appropriate choice for a more flexible surface like canvas. Another support which was not tested as part of this experiment but could be used is plaster: Students could also use wet plaster (buon fresco) or dry plaster (fresco secco) as a support, and in that case, they would apply plaster on a ceramic tile. This activity lends itself to discussions on solubility rules, nomenclature of inorganic compounds, stoichiometry and percent yield calculations, and refractive index, while important laboratory techniques are taught. It could be modified by adding stoichiometry calculations to make it suitable for both an introductory chemistry course for nonscience majors and chemistry majors. Although it describes a zinc-based yellow pigment, one may use other natural or synthetic pigments as well. In more advanced laboratory classes, the discussion could also include carbohydrate, protein, and/or polymer chemistry, needed to describe the chemical makeup of the binders and explain the curing process that takes place when oil binders dry. It could also be used in an art course or an interdisciplinary course that integrates art, history, and chemistry.

laboratory session, when it can be weighed and used with different binders to make paints. Binders and Paint Preparation

Four binders are prepared, beeswax, gum Arabic, egg tempera, and linseed oil, while the acrylic medium is purchased ready to use. Preparation of Beeswax for Encaustic. A small cube of beeswax, equivalent to one-half of a teaspoon, is placed in a small disposable aluminum container, placed above a hot water pan. Due to the high flammability of beeswax, it should be melted using a boiling water bath. Preparation of Gum Arabic Binder. Gum Arabic binder can be purchased already prepared from art supplies stores. It can also be prepared as a 30% w/v solution by dissolving 30.0 g of gum Arabic in 1.0 L of hot deionized water. The mixture is stirred and left to stand overnight or until the next laboratory session before being strained through cheesecloth to remove residual lumps. The solution can be kept in a refrigerator until ready to use. Alternatively, 3 drops of clove oil can be added to the solution, thus allowing it to be kept at room temperature for several months. Preparation of Linseed Oil Binder. A 1.0 mL portion of linseed oil is stirred with 2.0 mL of turpentine in a 10 mL beaker. The beaker is covered with a watch glass and kept under a fume hood until ready for use. Preparation of Egg-Based Binder for Tempera Paint. An egg is cracked, and the egg sac that contains the yolk is carefully separated from the white albumen which is discarded. While making sure the yolk sac does not break, it is placed on a paper towel and gently dried. The egg sac is transferred to a 50 mL beaker and the yolk sac punctured, allowing the yolk to be poured into a 10 mL graduated cylinder while holding the sac with a glass stirring rod. The volume of the yolk is recorded and transferred in another 50 mL beaker, and the same volume of distilled water is added. The egg yolk−water mixture is stirred and covered with a watch glass until ready to use. Preparing the Paints. The dried yellow pigment, after grinding with a mortar and pestle, is combined with an approximately equal amount of the selected binder in a well of a well plate or painter’s palette, except for the beeswax since it rapidly cools down and is a solid at room temperature. In that case, the yellow pigment is added to the melted beeswax while it is still in the hot water bath. For all binders, the mixture is stirred with a paintbrush, and additional pigment or binder can be added to obtain a texture similar to that of commercial paint, while being aware that higher concentrations of pigment and smaller particle sizes will decrease the transparency of the paint. Testing the Paints. Students paint with their freshly made paints and, once dried, compare their physical appearance. Even layers of paint should be applied, thick enough to cover the support, yet thin enough not to crack easily upon drying. Using

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EXPERIMENTAL SECTION Zinc sulfate, potassium chromate, sodium hydroxide, and gum Arabic were purchased from Fisher Scientific. Beeswax, acrylic medium, linseed oil, and turpentine were purchased from a local art supply store. Synthesis of ZnCrO4·Zn(OH)2

In a 100 mL beaker, 4.855 g (2.500 × 10−2 mol) of K2CrO4 is dissolved in 25.0 mL of distilled water; alternatively, students can be provided with a 1.00 M solution already prepared. While the reaction mixture is stirred with a glass stirring rod, 25.0 mL of a 1.00 M aqueous solution of ZnSO4 (2.50 × 10−2 mol) is added forming the yellow-orange precipitate, ZnCrO4. To the beaker is added 4.2 mL of 6.0 M NaOH (2.52 × 10−2 mol) to make the solution basic, and the color changes from yelloworange to bright yellow, yielding ZnCrO4·Zn(OH)2. In aqueous solution, the chromate ion, which is yellow, is in equilibrium with dichromate, an orange ion, as follows: 2CrO4 2 − + 2H+ → Cr2O7 2 − + H 2O

Therefore, adding sodium hydroxide helps shift the equilibrium toward chromate ion formation.33 The mixed precipitate is collected by vacuum or gravity filtration and stored in an open vial to dry until the next C

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regarding the chemical nature of the binders and how they hold the pigment on a surface, without going into great details that are beyond the scope of this work. The role of the binder is to “trap” the pigment onto the support by creating a 3D network. Students saw that each binder will accomplish this in a different way depending on its chemical composition. The 3D network is achieved either by physical entanglement of macromolecules like proteins and lipids in tempera, lipids in waxes, carbohydrates in gum Arabic, and synthetic polymers in acrylic medium, or by radical cross-linking of unsaturated fatty acid chains in oil. This polymerization by cross-linking depends on the number of carbon−carbon double bonds in the fatty acid chains and leads to the difference between nondrying, semidrying, and drying oils.

regular copy paper as a support is quite inexpensive, although the oil-based paint does not provide a satisfactory sample on paper. Instead students could paint on mixed-media paper or small canvas, both available at art supply stores.

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HAZARDS All reagents should be handled by students wearing gloves, goggles, and laboratory coats and working under a fume hood. Sodium hydroxide is corrosive; zinc sulfate is an irritant. Potassium chromate is also an irritant and a strong oxidizer; all solutions should be handled with care. Both zinc(II) chromate and potassium chromate contain chromium(VI) and are toxic and confirmed human carcinogens.32 Special care should be taken when grinding the pigment.

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SUMMARY A chemistry experiment showing the synthesis, isolation, and use of a safer version of a historical yellow pigment with paint binders is described. Selected details regarding historically important paint binders along with some of their chemical properties are provided, allowing instructors to incorporate these facts in their teaching to make this activity especially relevant to nonscience majors. This experiment is usually performed during two 2 h laboratory classes, the first of which focuses on precipitation reactions, solubility rules, filtration, and pigment synthesis. Since the synthesis itself is short and concise, the instructor can include either a lecture on pigments and binder if this experiment is a standalone chemistry and art experiment, or have the students predict and test solubility rules with many other aqueous solutions or synthesize another pigment described in the literature like malachite. The second 2 h class is dedicated to calculating the percent yield, preparing some binders, as well as making, testing, and comparing the paints.

CLASS TESTING This experiment has been performed with a class size ranging from 35 to 55 students. Student learning outcomes are measured on the basis of their laboratory report and accompanying data sheet. Most students are intrigued by this experiment as shown by the pictures they take with their mobile devices. In most cases, they have never had the opportunity to prepare their own paint before and are usually not aware of the different types of paint binders that exist besides oil and acrylic. The yield obtained by the students averaged 80% (SD 4, N = 40) whereas an experienced chemist obtained 95%. Photographs of paint samples on paper (Figure 3A) and on canvas (Figure 3B) are shown below for comparison.

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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.6b00479. List of chemicals needed to perform the experiment (PDF, DOCX) Student handout (PDF, DOCX)

Figure 3. Paint samples on paper (A) where the oil stain on the paper can be noticed when using linseed oil. Paint samples on canvas (B) where the beeswax paint had started to dry on the paintbrush making it more difficult to spread.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Students noticed the stain left by the oil paint on the paper only and realized that beeswax is the most difficult medium to work with, since it solidifies so rapidly on the paintbrush. This led to a discussion on encaustics and its meaning in Greek “to burn in”. Students also related to contemporary artists who rely on heat guns and hot irons to manipulate the wax or heat the support. Students were directed to the International Encaustic Artists’ Organization for other relevant information. Egg tempera, gum Arabic, and acrylic are probably the easiest media for students to use. Students especially enjoyed learning about tempera paint and its use in medieval art that they can still enjoy in art museums today. In addition, oil-based paint, prepared with linseed oil as a binder and turpentine as a solvent, led to important conversations regarding the structure of organic molecules and solvent safety in the art studio. Furthermore, the instructor provided additional information

Notes

The authors declare the following competing financial interest(s): Drs. Anne C. Gaquere-Parker and Cass D. Parker are the coauthors of the textbook "Chemistry and Art", Kendall Hunt Publishing, Dubuque, IA, 2014, cited in reference 27.

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ACKNOWLEDGMENTS The authors wholeheartedly wish to thank the National Science Foundation for financial support (TUES 1043847 and DUE 9752769). Also, the authors want to thank the reviewers for helpful and thorough comments. Finally, Jill Stallings from the Department of Chemistry at the University of West Georgia is acknowledged for her continuous professional and friendly support. D

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(23) Burrafato, G.; Calabrese, M.; Cosentino, A.; Gueli, A. M.; Troja, S. O.; Zuccarello, A. ColoRaman project: Raman and fluorescence spectroscopy of oil, tempera and fresco paint pigments. J. Raman Spectrosc. 2004, 35 (10), 879−886. (24) Mayer, R. The Artist’s Handbook of Materials and Techniques, 5th ed.; Viking: New York, 1991. (25) Orna, M. V.; Goodstein, M. P. Chemistry and Artists’Colors, 3rd ed.; ChemSource.Inc: New Rochelle, NY, 2013. (26) Orna, M. V. The Chemical History of Color, Springerbriefs in Molecular Science; Springer: Dordrecht, 2012. (27) Gaquere-Parker, A.; Parker, C. Chemistry and Art; Kendall Hunt Publishing: Dubuque, IA, 2014. (28) Museum of Fine Arts (Boston) website, http://cameo.mfa.org/ wiki/Zinc_yellow (accessed June 2016). (29) Museum of Fine Arts (Boston) website, http://cameo.mfa.org/ wiki/Acrylic_resin (accessed June 2016). (30) Sigma-Aldrich Chemistry Catalog, http://www.sigmaaldrich. com/chemistry.html (accessed June 2016). (31) Museum of Fine Arts (Boston) website, http://cameo.mfa.org/ wiki/Gum_arabic (accessed June 2016). (32) http://ntp.niehs.nih.gov/ntp/roc/content/profiles/ chromiumhexavalentcompounds.pdf (accessed September 2016). (33) Otero, V.; Carlyle, L.; Vilarigues, M.; Melo, M. J. Chrome yellow in nineteenth century art: historic reconstructions of an artists’pigment. RSC Adv. 2012, 2, 1798−1805.

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