Micro‐Raman spectroscopy (MRS) and surface‐enhanced Raman scattering (SERS) on organic colourants in archaeological pigments

Abstract

Direct identification of organic colourants in heterogeneous matrices of cultural heritage objects with very little or no sampling remains a challenging analytical task. A Raman procedure was investigated for the direct identification of archaeological organic-coloured pigments found in works of art, without solvent extraction. Conventional micro-Raman spectroscopy (MRS) using different excitations and surface-enhanced Raman scattering (SERS) were tested on microscopic samples of paints and cosmetics containing dyes from different sources (animal or vegetal). A tiny lump of purple pigment discovered during excavations in the ancient Minoan city of Akrotiri, on the SantoriniIsland in Greece (1650 B.C.) and Greco-Roman pink cosmetics were studied. In some cases the results were compared with a range of lake pigments made in the laboratory following historical recipes.

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

“…The spectra obtained from HF treated lakes of anthraquinone dyes are more detailed and more intense than in the absence of the treatment, as evident by comparison of the data presented here with those of earlier studies (12).…”

“…To date the SERS identification of dyes in paintings or polychrome objects other than textiles has not been possible. The only exception to the predominance of textile analyses in the published work is the identification of purpurin in a pink powder, thought to be a cosmetic, from the excavation of a first century Roman site in Germany (12); in this case however the colorant was a pure lake, without any binder.…”

The Nature of the Extraordinary Finish of Stradivari’s Instruments

“…The upper varnish of the "Provigny" violin contains translucent red particles, the diameter of which ranges between 1 and 10 mm, and their MRS spectra are typical of anthraquinone dyes. [22] The closest match within our set of reference spectra is carminic acid, which suggests the use of a cochineal insect dyestuff (Figure 3 a). [23] SEM-EDX spectra show that these grains are aluminum-and oxygen-rich, which indicates a red lake pigment on an alumina substrate.…”

Zusammensetzung und Aufbau des berühmten Stradivari‐Lackes

“…Die erhaltenen MRS-Spektren sind für Anthrachinonfarbstoffe typisch. [22] Die beste Übereinstimmung mit unseren Referenzspektren ergab Karminsäure; dies deutet auf die Verwendung von Cochenille hin (Abbildung 3 a). [23] REM-EDX-Analysen belegen, dass die Partikel reich an Aluminium und Sauerstoff sind.…”