Microanalysis of organic pigments and glazes in polychrome works of art by surface-enhanced resonance Raman scattering

Abstract

Scientific studies of works of art are usually limited by severe sampling restrictions. The identification of organic colorants, a class of compounds relevant for attribution and provenance studies, is further complicated by the low concentrations at which these compounds are used and by the interference of the protein-, gum-, or oil-binding media present in pigment and glaze samples. Surface-enhanced resonance Raman scattering (SERRS) was successfully used to identify natural organic colorants in archaeological objects, polychrome sculptures, and paintings from samples smaller than 25 microm in diameter. The key factors in achieving the necessary sensitivity were a highly active stabilized silver colloid, obtained by the reproducible microwave-supported reduction of silver sulfate with glucose and sodium citrate, and a non-extractive hydrolysis sample treatment procedure that maximizes dye adsorption on the colloid. Among the examples presented are the earliest so far found occurrence of madder lake (in a 4,000 years old Egyptian object dating to the Middle Kingdom period), and the earliest known occurrence in Europe of the South Asian dyestuff lac (in the Morgan Madonna, a 12th century polychrome sculpture from Auvergne, France).

Fig. 1.

The main anthraquinone dyes. Substitution of H for the sugar group at position 3 in carminic acid leads to kermesic acid.

Fig. 2.

Analysis of Egyptian painted leather quiver fragment. (A) Fragment of a quiver; Accession No. 28.3.5; Middle Kingdom, around 2124–1981 BC (H. 11 cm; W. 13 cm). MMA 1911–1912, Tomb MMA830, Thebes, el-Khokha, Upper Egypt; Rogers Fund, 1928. (B) Polarized reflected light photograph of sample removed from red painted area before HF treatment (Scale bar, 20 μm). (C) SERRS spectrum of sample from Middle Kingdom leather quiver. Solid line, spectrum of sample from red painted area; dashed line: spectrum of a second C. BC pink pigment from Corinth, Greece, previously identified by HPLC as a madder lake (mostly purpurin). Spectra were normalized and vertically shifted for ease of comparison, but no smoothing or baseline correction was used.

Fig. 3.

Analysis of a painting by the workshop if Francesco Granacci. (A) St. John the Baptist Bearing Witness (detail). St. John the Baptist Bearing Witness. Accession no. 1970.134.2; workshop of Francesco Granacci, Florence (ca. 1510). 75.6 × 209.6 cm. Purchase, Gwynne Andrews, Harris Brisbane Dick, Dodge, Fletcher, and Rogers Funds, funds from various donors, Ella Morris de Peyster Gift, Mrs. Donald Oenslager Gift, and Gifts in memory of Robert Lehman, 1970. (B) SERRS spectrum of red glaze sample from St. John the Baptist Bearing Witness. Solid line, spectrum of sample from red glaze; dashed line: spectrum of a reference sample of kermesic acid. Spectra were normalized and vertically shifted for ease of comparison, but no smoothing or baseline correction was used.

Fig. 4.

Analysis of the Morgan Madonna. (A) The Morgan Madonna. Virgin and Child in Majesty; Accession No. 16.32.194; 1150–1200 CE; (H. 79.5 cm; W. 31.7 cm; D. 29.2 cm). Auvergne, France; Gift of J. Pierpont Morgan, 1916. (B) SERRS spectrum of sample from the Morgan Madonna. Solid line, spectrum of sample from red glaze; dashed line: spectrum of a reference sample of lac dye glaze in hide glue. Spectra were normalized and vertically shifted for ease of comparison, but no smoothing or baseline correction was used.