An improved methodology for the characterization and identification of sepia in works of art by normal Raman and SERS, complemented by FTIR, Py‐GC/MS, and XRF

Abstract

The natural sepia pigment is based on eumelanin, the most prevalent type of melanin, and can be obtained from the ink sac of different members of the Cephalopoda class, such as cuttlefish and squid. The main components of natural sepia are indole derivatives, but pigments prepared following historical recipes are generally more heterogeneous as they may also contain proteins, polysaccharides, and lipids. Sepia is difficult to identify in works of art owing to its heterogeneity, insolubility in most organic solvents, interference by the binding media, and its poorly defined spectroscopic properties. In the present study, three commercial natural sepia pigments, along with a sample extracted from cuttlefish following a historical recipe in an artists' manual, a relatively more pure commercial melanin, and a synthetic eumelanin obtained by the oxidation of dopamine were characterized by normal Raman and surface‐enhanced Raman scattering (SERS), complemented by Fourier transform infrared, pyrolysis–gas chromatography–mass spectrometry, and X‐ray fluorescence, in order to obtain vibrational signatures for the identification of the pigment in works of art. Pyrolysis–gas chromatography–mass spectrometry and X‐ray fluorescence analysis showed that the pigment composition is strongly influenced by the extraction procedure used. Lipids were identified in the chromatograms of all the natural samples analyzed, and the majority of these also showed the presence of cholesterol derivatives. Additional components resulting from the decomposition of polysaccharides were found to be present in the sepia extracted from cuttlefish in our laboratories. Normal Raman and SERS were also used to study two sepia inks applied on paper and subjected to accelerated aging. In the SERS measurements, a hydroxylamine‐reduced silver colloid was used as this substrate is suitable for the identification of other natural heterogeneous black‐brown pigments. The SERS methodology developed was applied to identify sepia in Waiting for Aaron, a drawing by the 19th century artist Robert Frederick Blum. Copyright © 2014 John Wiley & Sons, Ltd.