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Visions of Cultures Past

Photonics Spectra
Dec 2009
Caren B. Les, caren.les@laurin.com

Surface-enhanced resonance Raman scattering has provided a rapid and unobtrusive way of analyzing organic pigments and glazes in works of art. Analysis of organic colorants could lead to a deeper understanding of past cultures and societies, providing key information to art historians in terms of attribution and provenance, relating works of art and historical interests such as following trade routes.

Peregrinations1.jpg
A refined spectroscopy technique called surface-enhanced resonance Raman scattering was used to analyze the red painted area on this ancient Egyptian artifact titled “Fragment of a Quiver,” Accession No. 28.3.5, at the Metropolitan Museum of Art. Photos courtesy of the Metropolitan Museum of Art.

Typically, only extremely small samples can be removed from works of art, and the identification of colorants is complicated by the protein-, gum- or oil-binding media present in pigment and glaze samples.

The technique can identify organic colorants in samples smaller than 25 μm in diameter, according to Marco Leona, an investigator at the Metropolitan Museum of Art in New York. He refined the Raman spectroscopy method with microwave-reduced monodisperse silver colloid and a nonextractive hydrolysis sample treatment procedure. Conventional Raman spectroscopy is not considered suitable for the identification of most organic dyes, which are generally fluorescent even at 785-nm excitation, and the resulting background obscures their Raman spectrum, he said.

The method identified colorants at the microscopic level in archaeological objects, polychrome sculptures and paintings at the museum.

For example, Leona analyzed microscopic bits of red pigment on a fragment of a painted leather Egyptian quiver, estimated to be from the Middle Kingdom, circa 2124 to 1918 B.C. His discovery that the color was madder lake, a red dye, provides the earliest evidence thus far of a culture that possessed the complex chemical knowledge needed to extract the dye from a plant and turn it into a pigment.

The quiver fragment predates by at least 700 years any previous indication for the use of madder in Egypt, according to Leona’s research report, which was published in the journal Proceedings of the National Academy of Sciences (PNAS) in September 2009.

Leona’s method also identified kermes, a dye made from the bodies of insects, as the red coloring in the painting “St. John the Baptist Bearing Witness” from the workshop of Francesco Granacci in the early 1500s in Florence, Italy. Before the dyestuff cochineal was imported from the New World, kermes and other insect dyes were popular in Europe. They were used much more commonly than madder in the preparation of glazes, which are translucent red or crimson paints prepared by dispersing lake pigments in oil, according to the report.

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Analyses of the red coloring on the sculptures “Morgan Madonna” (“Virgin and Child in Majesty”), Accession No. 16.32.94 (shown), and the “Montvianeix Madonna” (1967.153, not shown), both at the Metropolitan Museum of Art, identified it as lac dye, providing strong evidence that the sculptures may have originated from the same workshop.

The technique documented the red coloring in the polychrome wood sculpture “Morgan Madonna” (“Virgin and Child in Majesty”), dated 1150 to 1200, as the insect dye lac, which originated in South Asia and which may have been imported to southern Europe by Muslim traders. The sculpture originally was housed in a church near the Provence region of France. Leona said that this is the first direct evidence of lac dye in European art before the 15th century.

The technique also identified lac dye in the French Romanesque sculpture the “Montvianeix Madonna,” (1150 to 1200), which was thought to be similar in style and from the same region as the Morgan Madonna. The identification of lac dye in both sculptures provides strong evidence that they originated from the same workshop.


GLOSSARY
raman spectroscopy
That branch of spectroscopy concerned with Raman spectra and used to provide a means of studying pure rotational, pure vibrational and rotation-vibration energy changes in the ground level of molecules. Raman spectroscopy is dependent on the collision of incident light quanta with the molecule, inducing the molecule to undergo the change.  
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