Color and Chemistry in Art

Color & Chemistry in Art: Research by Summer Chemistry Fellows 

“I fell in love with the colors.” 

That’s a common refrain among chemists, particularly those in the subfields of inorganic and physical chemistry. Color is one of the many reasons people find the field fascinating. The entire color spectrum is accessible in the lab, and can be produced in over a dozen different ways, from absorption and emission to refraction and scattering. Nanoparticles, dyes, conductive polymers, semiconductors, and even proteins can make up the color palette of a modern chemistry lab. 

But why? Where does color come from and how can it be manipulated? To answer such questions, we must dive deep into our understanding of the molecules and materials themselves. Over the course of ten weeks, Amherst College students in Summer Undergraduate Student Fellowship (SURF) conducted cross-disciplinary chemistry and art-historical research with a group of artworks from the Mead Art Museum. The resulting illustrated essays, by Timothy Zhao ’23 and Arjun Nanda ’23, seek to explain the origin of color in several materials, not in the lab, but in art. Juhyun (Grea) Lee ’23, Mead Art Museum collections intern, contributed to the art-historical research in each essay.

Readers will soon see that describing how molecules yield their color can be complicated. Atomic theory and quantum mechanics are important tools for understanding color at a fundamental level. If it’s been a while since you’ve thought about quantum mechanics, or if you’d prefer not to think about it, fear not, let’s just start at the beginning. 

Light can be described as a wave, and that wave can have various wavelengths, say from one peak to another. The range of possible wavelengths can be organized into the electromagnetic spectrum. Even as you read this now you’re being bombarded by infrared (IR) light, and if you’re outside, you may be exposed to ultraviolet (UV) light. You’re also likely connected to Wi-Fi, which is supported by radio waves. Color as we know it, or visible light, comes from a very narrow sliver of this spectrum. We see color when light from some source hits our eye. Cone cells on the back of your retina sense the wavelengths of light and send a signal to your brain, which you interpret as brown, red, blue, etc. The reason we can’t see UV light, IR light, or radio waves is because we don’t have the biology for it. Our eyes have evolved to see visible light in the range of ~350–750 nanometers. But seeing a color is the end of that light ray’s life in a way. You’re its final destination. To truly understand the origin of color we must back up. 

All the pigments and dyes you’ll see in the illustrated essays produce color by absorption. That is to say, light from some source (the sun, a lamp, a candle, etc.) emits light and travels to the pigment. At this point, based on the chemical makeup of the material in question, some of the wavelengths may bounce back to your eye for you to register it as color. If all wavelengths bounce back to your eye, you see white; if none of the wavelengths return to your eye, you see black. If you’re staring at a white wall right now, what you’re really seeing is the reflection of the light in the room. Turn off the light and it will become black. Shine a red light on it and it will become red, as that is the only wavelength in the room. Understanding color means understanding the source, the material, and the eye.

Enter the artist. An artist uses a palette of chemicals and constructs them into a final artwork in an endless variety of ways. When light strikes the art object, you see the image with each molecule reflecting back to the viewer exactly the intended wavelength to convey the artist’s intent. Several examples will be discussed here with the goal of explaining why a given pigment would absorb red and green to appear blue, or absorb blue to appear orange. While the answers can become quite technical, each example can be simplified to this: follow the electrons. In these examples, how electrons in a material interact with light is the key to their observed color. It is how we arrange those electrons as scientists and artists that makes things truly spectacular. 

Chris Durr

Assistant Professor of Chemistry and Chemistry Project Adviser

August 2020

Fig. 1. The pigment Egyptian blue in its powdered state.

Fig. 2. The chemical structure of Egyptian blue. Photo: Nick Greeves. CC BY-NC-SA 2.0

Egyptian Blue

The chemical formula for Egyptian blue is CaCuSi₄O₁₀. The synthesis of Egyptian blue is shown by this chemical equation at a temperature between 800℃ and 900℃:

Cu₂(CO₃)(OH₂) + 8 SiO₂+  2CaCO₃   2 CaCuSi₄O₁₀ +  3CO₂+ H₂O

Malachite   Sand  Lime        Egyptian Blue 

A flux made from a mixture of sodium sulfate, soda (Na₂Co₃), and sodium chloride was used to help the synthesis.

To understand the blue color, we must first understand that the electrons in an atom exist in orbitals. Each orbital can hold up to two electrons, and depending on the identity of the atom and its local environment, they can have various energies. 

The blue color of Egyptian blue can be explained by crystal field theory. A qualitative example of the copper d-orbital splitting in Egyptian blue can be seen in Figure 3. Here, the spatial arrangement of the oxygen atoms around copper causes an energy difference in the metal orbitals. When photons of the appropriate energy are absorbed, electrons can move from one level to another; the wavelengths that reflect back to our eye is the color we perceive. Because electrons are traversing from one metal d-orbital to another, these are known as d-d transitions. The energy difference and the precise ordering of these levels depend on many factors including the nature of the metal and the surrounding environment. Even slight differences in makeup can lead to drastically different colors, as seen in another pigment of interest: verdigris.  

Fig. 3. A qualitative crystal field diagram of copper in Egyptian blue. Image: Courtesy of Christopher Durr

Picture description: Unknown. Egyptian. Coffin, ca. 1077–943 BCE. Painting, plaster, and wood. Gift of Dr. Stephen H. Weeks. P.1905.1 and P.1905.2. Photo: Mead Art Museum

Egyptians are credited with the invention of this synthetic color pigment, so-called Egyptian blue. As a symbol of health, happiness, and fortune, blue assumed an important place in the decoration of devotional and ceremonial objects in ancient Egypt society. Egyptians extracted blue pigments from minerals such as azurite and lapis lazuli. 

In accordance with ancient Egyptian beliefs about the afterlife, the interior sides of this wooden coffin in the Mead’s collection are decorated with hieroglyphs, figures, and other iconography that symbolize the soul’s journey within the realm of the dead. Several bands of human-like figures represent Egyptian deities who guide the dead. The three most clearly visible rows on the inside consist of a combination of human and animal-headed figures. At the very top of the coffin sits a pharaoh, with a bird’s wing stretched above his head. 

The bright colors that the artisans used to paint the case are mainly red, blue, green, and occasionally white. Blue is the most prominent, decorating the bodies and headdresses of the various figures on the interior and exterior walls, as well as the thick lines that separate the different sections of the coffin’s sides. The predominant blue coloring illustrates the significance of this pigment in ancient Egypt.

Art Bibliography

“Ancient Egyptian Coffins and Sarcophagi .” Google Arts & Culture. Google, n.d. https://artsandculture.google.com/usergallery/ancient-egyptian-coffins-and-sarcophagi/SgIinGKKfuSLLg. 

“Egyptian Art.” Essential Humanities, July 2010. http://www.essential-humanities.net/world-art/egyptian/.

Picture of the Statue of Liberty, New York, showing the green verdigris of the statue.

Verdigris is a synthetic pigment that ranges from a blue to a bluish-green color. There are two general forms of verdigris, basic and neutral, consisting of copper acetate with different combinations of H₂O and CuOH. Neutral verdigris has the chemical formula Cu (CH₃COO)₂·H₂O and was the preferred pigment in the Middle Ages. Verdigris is synthesized by hanging a copper plate over hot vinegar in a sealed pot and then scraping the green product off the plate.

In many respects verdigris is similar to Egyptian blue. Both obtain their distinctive color from copper ions surrounded by oxygen and take advantage of d-d transitions. They are distinct from one another because the metal resides in a different chemical environment. Where the copper in Egyption blue is surrounded by the oxygen atoms of a silicate, the copper in verdigris is acetate (CH₃COO₂^–). This subtle yet important difference causes a precise ordering of the metal d-orbitals and thus produces a unique color. Verdigris has a peak absorption in the range of 474nm and 672nm, which corresponds to the blue and red range respectively.

It should be noted that over time verdigris starts to turn brownish-red when exposed to oxygen. This is because the copper reacts to form oxide compounds such as CuO and Cu₂O. 

Fig. 4. The chemical structure of verdigris. Photo: Dalton Transactions 46 (2017): 14847–58

Picture description: Attributed to Master of Santa Coloma, Andorran, 12th century, Saint Sylvester, fresco possibly from the Church of Santa Coloma, 1150–99, Fresco, transferred to canvas, Museum purchase 1941.9. Photo: Petegorsky/Gipe Photography

Fortunately, the Mead’s fresco of Saint Sylvester was able to preserve most of its original colors. This life-size portrayal of the saint was originally part of a complex wall decoration in the Romanesque Church of Santa Coloma in Andorra, dating to the twelfth century. Sylvester was the Pope and head of the Roman Catholic Church, from 314 to 335. A circular halo, a cross, and an extended open palm symbolize his sanctity and the pope’s authority.

The Master of Santa Coloma, the celebrated muralist who is thought to be the maker of this work, used a restricted palette for his wall paintings. His method of applying pigments to fresh and damp plaster required using inorganic pigment compounds rather than natural dyes; this allowed the color pigments to withstand the passage of time. The artist most likely employed smoke-derived black for the contours, ocher for the yellow halo and the coat’s golden-like decorations, and cinnabar’s red for facial details. The blue-green of the saint’s ornate robe is likely verdigris, the man-made extract pigment that was often used during this period since antiquity.

Art Bibliography

Anthony, Edgar Waterman. “II. Technique.” Essay. In Romanesque Frescoes, 61–65. Princeton University Press, 1951. 

“Apostles from Sant Romà De Les Bons.” Museu Nacional d’Art de Catalunya. Museu Nacional d’Art de Catalunya, n.d. https://www.museunacional.cat/en/colleccio/apostles-sant-roma-de-les-bons/mestre-de-santa-coloma-dandorra/015783-000. 

The Editors of Encyclopaedia Britannica. “St. Sylvester I.” Encyclopædia Britannica. Encyclopædia Britannica, inc., April 30, 2020. https://www.britannica.com/biography/Saint-Sylvester-I. 

Palomares, Susanna Vela. Magister Sancta Columba: La Pintura romànica Del Mestre De Santa Coloma i La Seva època. Viena Art, 2003.

Fig. 5. The pigment Naples yellow in its powdered state. Photo: ColourLex, https://colourlex.com/project/naples-yellow/

The chemical formula for Naples yellow is Pb₃(SbO₄)₂. It was first used in ancient Egypt, with evidence of the pigment’s synthesis dating back to 1400–1300 BCE. Naples yellow fell out of popularity, however, as a preference for pigments such as lead-tin yellow grew, until around 1500 CE. 

While the distinctive colors of Egyptian blue and verdigris come from transitions between one metal orbital and another, the intensity of Naples yellow derives from a transition between electrons on the oxygen to the metal itself. This is known as ligand to the metal charge transfer, or LMCT. Color derived from charge transfer occurs when an electron is momentarily transferred from one atom to another. This can be between two metals, such as in sapphire, or between a metal and a ligand, as seen here. The energy required to induce such a charge transfer is absorbed by the material, and we perceive whatever wavelengths remain. For a ligand to metal transfer to occur, the metal must be nearly devoid of electrons itself. The increase in positive charge facilitates the transfer of electrons from the oxygen once excited by light. In the case of Naples yellow, we have a charge transfer between the ligand oxygens and the metal center requiring all wavelengths of light except that of yellow. 

Fig. 6. The chemical structure of Naples yellow. Photo: CC BY-SA 4.0 User: Smokefoot

Picture description: Justus Sustermans, Flemish, 1597–1681, Bust of a Young Man in a Cuirass, ca. 1620–25, Oil on canvas, Gift of Dr. Frank L. Babbott (Class of 1913), 1957.34. Photo: Petegorsky/Gipe Photography

This rendering of a young man wearing a black cuirass—a type of chest armor—is evocative of the portraits of nobility that Justus Sustermans created throughout his illustrious career in Italy. Although the title of the work is vague and not dated, further research has uncovered records suggesting that the confidently posed and ceremonially dressed sitter might well be the young Ferdinando II de’ Medici, painted around 1622. As the Grand Duke of Tuscany, Ferdinand II was passionate about sciences and technology and became a patron of a newly established academy of sciences in Florence.

Against the dark background and black shining armor, a triangle of intense colors stands out: the silvery white of the intricate lace of the Duke’s fancy collar, the orangish red of the satin band around the man’s upper left arm, and the prominent yellow of the richly folded golden commander’s sash, the symbol of the ducal family’s military regalia. This golden color is likely created by the Naples yellow pigment. Painters used this artificial yellow pigment since the Renaissance, meaning that it was most likely part of Justus Sustermans’s palette at the time he executed the portrait.

References

Manuscript letter, dated September 4, 1898, from Clemente Bressanelli, Mantua, Italy to Y. A. Holzer, Florence (Mead Art Museum, Object Files, 1957.24)

Fig. 7. Vermilion. Photo: Beneski Museum of Natural History

The color vermilion can be explained through band theory. So far in our discussion of color substances we have been talking about discrete orbitals and their local environment. Materials like vermilion gain their fascinating properties by invoking the many orbitals of a solid at one time. Consider the simplified example in Figure 8. With a small number of orbitals in a line we may see only a few discrete energy levels electrons can shuttle between upon absorbing light. But consider the implications of many orbitals in a repeating pattern. In this case we no longer form single orbitals but rather bands of orbitals. One band at low energy, the valence band, is filled with electrons, and the other at high energy, the conduction band, is generally empty. The difference between the two is called the bandgap, which is the hallmark of semiconductor materials like vermilion. While semiconductors are ubiquitous in the age of computers, their first use was as pigments.

Vermilion owes its brilliant red color to the energy of its bandgap: ~2.0 eV. In this case we observe the color red or orange because the bandgap is tuned to absorb blue and green light, leaving red light to be reflected back to the viewer. These bandgaps act in a similar way to orbitals and exclude certain wavelengths that do not provide enough energy for the electron to make the jump.

Fig. 8. A diagram relating how one atomic orbital can be turned into bands when repeated infinitely. Semiconductors, like vermilion, contain a relatively small bandgap and get their color from photons being absorbed when an electron is excited from the valence band to the conduction band. Image: Courtesy of Christopher Durr

Fig. 9. An image showing the chemical structure of cinnabar, the naturally occurring pigment that is used to synthesize vermilion. Yellow represents sulfur (S), and gray represents mercury (Hg). Photo: CC Benjah-bmm27

Picture description: Willem Kalf, Dutch,  1619–1693, Still Life, ca. 1660–70, Oil on canvas, Purchased, 1955.308. Photo: Petegorsky/Gipe Photography

Vermillion produces a vibrant orange-red color that stands out among darker tones in this baroque painting. Through the visual effects of a dimly lit background and studiously placed highlights, Willem Kalf created an elaborate composition of luxury objects and exotic fruits, hinting at the great popularity of global trade that the Dutch successfully invested in during the seventeenth century. A silver tray lies on a patterned Turkish rug partially spread over a wooden table. On this tray, a tall goblet looms in the center of the painting, its gleaming surface reflecting the dark background and amber wine poured in the glasses next to it. The Chinese porcelain bowl at the forefront accentuates the lavishness of the scene; the orange-toned fruit (an orange or pomegranate) in the bowl makes a striking sight. Based on historical, geographical, and scientific information, it is probable that Willem Kalf used vermilion to produce this particular shade of orangish red.

Art Bibliography

Editors of Encyclopaedia Britannica. “Willem Kalf.” Encyclopædia Britannica. Encyclopædia Britannica, Inc., July 27, 2020. https://www.britannica.com/biography/Willem-Kalf. 

Grisebach, Lucius. “Kalf [Kalff], Willem.” Grove Art Online. Oxford University Press, 2003. https://www.oxfordartonline.com/groveart/view/10.1093/gao/9781884446054.001.0001/oao-9781884446054-e-7000045533?rskey=Vz8zdU.

(mono-azo β-naphthol colorant), Naphthol Red (Acid red 26)

Fig. 10. The pigment red azo dye in its powdered state

Azo dyes are organic molecules that can be tailored to absorb specific wavelengths of light yielding a specific color. Azo dyes get their name from a double-bonded nitrogen group  (―N=N―) known in organic chemistry as an azo group.  This acts as the chromophore, or color-producing group, of the dye. The azo group can be bonded to a multitude of other organic substituents to modify the color. Some of these groups include naphthalenes (the ingredient typically found in mothballs), benzene rings, aromatic heterocycles, and enolizable aliphatic groups. 

The exact chemical explanation for the color of azo dyes can best be explained through molecular orbital theory. The electrons in an azo dye can be thought of as waves that surround the entire molecule. These electrons exist at different energy levels similar to those described previously. When the molecule interacts with light, specific wavelengths can be absorbed and the electrons can be redistributed into different orbitals but ultimately settle back down into their original state. To change the wavelength of light absorbed, chemists can change the shape of the molecule, which will change the color. Chemists do this through synthetic organic chemistry, which makes azo dyes highly versatile.

The color of azo dyes comes from the excitation of electrons from the highest occupied molecular orbital (HOMO, 𝜋) to the lowest unoccupied molecular orbital (LUMO, 𝜋*) (Figure 11). Wavelengths of light required to induce this momentary redistribution of electrons around the molecule are absorbed, and we then observe the reflected light.

Fig. 11. Certain wavelengths of the appropriate energy are absorbed by the dye, exciting an electron from the HOMO to the LUMO, resulting in the observed color. (Molecular orbitals calculated with Spartan ‘18).  Image: Courtesy of Christopher Durr

Picture description: Utagawa Kunimasa IV, Japanese, 1848–1920, Brocade Picture of the “Pavilion above the Clouds” Sugoroku (Ryounkaku kikai sugoroku), 1890 (Meiji 23), Woodblock print with collage elements, Museum purchase, 2008.48. Photos: Petegorsky/Gipe Photography

The red color used in this print is likely an azo dye. The print was produced in 1890, when the preference of Japanese printmakers began to shift away from using naturally occurring carmine in favor of red azo dyes. The shade of red in this print is also very similar to the shade of naphthol red dye, which further strengthens the idea that the red is in fact an azo dye. To determine the true nature of the color, analytical experiments would need to be carried out, including Raman spectroscopy.

The Ryounkaku (“Pavilion above the Clouds”), also known as the Junikai (“Twelve Stories”), was a symbol of the Westernization that was rapidly taking place in Japan during the Meiji era (1868–1912). As Japan’s first skyscraper, the building introduced the country’s first elevator, housed shops selling imported goods, and showcased Western cultural events. This work is not only an architectural depiction but also a board for the Japanese game of Sugoroku. The artist’s decision to combine a popular board game with a traditional printmaking technique may have offset the sense of unfamiliarity associated with the drastically different cultural trends embodied by the structure and made it easier to distribute—and to introduce the building to the public. 

The use of colors that were either rare or expensive, such as purple, together with colors that already had cultural significance in Japan, such as red and white, had a similar effect of bridging the gap between old and new. Although red, one of the most prominent colors in the print, was traditionally used to represent sacrality and festivity, here it visualizes technological progress and signals the profound changes occurring in Japanese society on the threshold of the twentieth century.

Art Bibliography

Cesaratto, Anna, Yan-Bing Luo, Henry D. Smith, and Marco Leona. “A Timeline for the Introduction of Synthetic Dyestuffs in Japan during the Late Edo and Meiji Periods.” Heritage Science 6, no. 22 (April 3, 2018). https://doi.org/10.1186/s40494-018-0187-0.

Morse, Sam. “Brocade Picture of the ‘Pavilion Above the Clouds’ Sugoroku (Ryounkaku Kikai Sugoroku).” Collections Database. Five Colleges and Historic Deerfield Museum Consortium, 2012. http://museums.fivecolleges.edu/detail.php?museum=all.

Yim, Jaey. “Cyberpunk in Asia: Reflections on Dystopia in a Time of Coronavirus.” Mount Holyoke College Art Museum. Mount Holyoke College Art Museum, August 31, 2019. https://artmuseum.mtholyoke.edu/virtual-engagement/cyberpunk-asia-reflections-dystopia-time-coronavirus.

The chemical formula of indigo is C₁₆H₁₀N₂O₂, an organic compound that gives a purplish-blue hue. 

Fig. 12. Solid indigo. Photo: CC BY-SA 3.0

Fig. 13. The chemical structure of indigo.

Indigo is an organic molecule similar to the azo dyes previously described, and its color can similarly be accounted for by molecular orbital theory. Here, however, the chromophore is not an azo group but rather the long conjugated system punctuated by nitrogen and oxygen. This perfectly flat molecule contains a HOMO and LUMO perfectly situated in energy to absorb low-energy red light, thus allowing a deep blue to return to the eye. One ubiquitous use of indigo is in the dying of blue jeans. In this process, indigo proceeds through several color changes as the electronic structure alters, eventually ending in the blue color we see here.

Picture description: Yomi Tiamiyu, Nigerian, born 1969, Adire Quilt, ca. 1998, Cotton adire cloth with indigo dye. Museum purchase, 2002.253. Photo: Stephen Petegorsky

The indigo pigment, one of the oldest clothing dyes ever used, was often extracted from the leaves of certain plants during the Middle Ages. In this particular work, the pigment is extracted from the leaves and stems of a shrub indigenous to West Africa, Lonchocarpus cyanescens. 

Adire, which means “to tie and dye” in Yoruba, generally refers to indigo-dyed cotton cloth. Traditionally among  Yoruba people, textiles were produced exclusively by women for domestic use. They applied this plant-based resist-dye technique to the cloth and passed the technology, skills, and pattern designs down generations. By the late 1970s, however, increased imports of synthetically colored fabrics gained wide popularity in Yoruba society. Carrying on the tradition of adire became a statement made by individual craftswomen, local workshops, and artists about their culture and identity.

Yoruba artists of all genders would incorporate the traditional adire textiles in their practice. Yomi Tiamiyu created this quilt with patterns that fit the Yoruba aesthetic, while also embracing the form and techniques of quilting, a textile tradition of the African diaspora. Furthermore, he incorporated pieces from old adire cloth, making his work a symbol of the interconnectivity between the past and the present, the traditional and the contemporary.

Art Bibliography

“Adire – Indigo Resist Dyed Cloth From Yorubaland, Nigeria.” Victoria and Albert Museum: The world’s leading museum of art and design. Victoria and Albert Museum, July 26, 2013. http://www.vam.ac.uk/content/articles/a/adire-indigo-resist-dyed-cloth-from-yorubaland-nigeria/. 

Pemberton, John, Ulli Beier, and Rowland O. Abiodun. “Yoruba Textiles: Technology and Artistry.” Essay. In Cloth Only Wears to Shreds: Yoruba Textiles and Photographs from the Beier Collection, 19–25. Mead Art Museum, 2004. 

“What Is Adire Textile?” THE CRAFT ATLAS, February 25, 2020. https://craftatlas.co/crafts/adire. 

Atomic and molecular orbitals were calculated and visualized using Spartan ‘18 distributed by Wavefunction, Inc.

“A-Baker-04-Pigments.Pdf.” Accessed August 11, 2020. http://d-scholarship.pitt.edu/11954/1/a-baker-04-pigments.pdf.

Allen, R.L.M. “The Chemistry of Azo Dyes.” In Colour Chemistry, edited by R.L.M. Allen, 21–36. Studies in Modern Chemistry. Boston, MA: Springer US, 1971. https://doi.org/10.1007/978-1-4615-6663-2_3.

Benkhaya, Said, Souad M’rabet, and Ahmed El Harfi. “Classifications, Properties, Recent Synthesis and Applications of Azo Dyes.” Heliyon 6, no. 1 (January 31, 2020). https://doi.org/10.1016/j.heliyon.2020.e03271.

Berke, Heinz. “The Invention of Blue and Purple Pigments in Ancient Times.” Chemical Society Reviews 36 (February 1, 2007): 15–30. https://doi.org/10.1039/b606268g.

Cesaratto, Anna, Yan-Bing Luo, Henry D. Smith, and Marco Leona. “A Timeline for the Introduction of Synthetic Dyestuffs in Japan during the Late Edo and Meiji Periods.” Heritage Science 6, no. 1 (April 3, 2018): 22. https://doi.org/10.1186/s40494-018-0187-0.

Chemistry LibreTexts. “6.8B: Band Theory of Metals and Insulators,” June 15, 2015. https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Map%3A_Inorganic_Chemistry_(Housecroft)/06%3A_Structures_and_energetics_of_metallic_and_ionic_solids/6.08%3A_Bonding_in_Metals_and_Semicondoctors/6.8B%3A_Band_Theory_of_Metals_and_Insulators.

Chemistry LibreTexts. “Crystal Field Theory,” October 2, 2013. https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Modules_and_Websites_(Inorganic_Chemistry)/Crystal_Field_Theory/Crystal_Field_Theory.

Clair, Kassia St. The Secret Lives of Colour. John Murray Press, 2016.

Editors of Encyclopaedia Britannica. “Indigo.” Encyclopædia Britannica. Encyclopædia Britannica, Inc., February 18, 2019. https://www.britannica.com/technology/indigo-dye.

García-Fernández, Pablo, Miguel Moreno, and José Antonio Aramburu. “Origin of the Anomalous Color of Egyptian and Han Blue Historical Pigments: Going beyond the Complex Approximation in Ligand Field Theory.” Journal of Chemical Education 93, no. 1 (January 12, 2016): 111–17. https://doi.org/10.1021/acs.jchemed.5b00288.

marieflemay. “Verdigris.” Traveling Scriptorium (blog), January 17, 2013. https://travelingscriptorium.library.yale.edu/2013/01/17/verdigris/.

Keegan, Graham. “Indigo Vat Basics.”. Accessed September 4, 2020. https://www.grahamkeegan.com/indigo-vat-basics.

Nassau, K. The Physics and Chemistry of Color: The Fifteen Causes of Color, 2nd Edition, 2001, Wiley, New York.

Orna, Mary Virginia. “Chemistry and Artists’ Colors. Part II. Structural Features of Colored Compounds.” Research-article. Division of Chemical Education, April 1, 1980. World. https://doi.org/10.1021/ed057p264.

Robin, Melvin B., and William T. Simpson. “Assignment of Electronic Transitions in Azo Dye Prototypes.” Journal of Chemical Physics 36, no. 3 (February 1, 1962): 580–88. https://doi.org/10.1063/1.1732574.

Solomons, Graham. “Chemistry of Blue Jeans: Indigo Synthesis and Dying.” Eltamitz. Accessed August 8, 2020. https://eltamiz.com/files/indigo.pdf.

Soukup, Rudolf W., and Roland Schmid. “Metal Complexes as Color Indicators for Solvent Parameters.” Journal of Chemical Education 62, no. 6 (1985): 4.

St. Clair, K., The Secret Lives of Color, 2016, Penguin, New York.

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Ziarani, Ghodsi Mohammadi, Razieh Moradi, Negar Lashgari, and Hendrik G. Kruger. Metal-Free Synthetic Organic Dyes. Elsevier, 2018.