Spectroscopic study of mixtures of green copper pigments in fresh and aged model samples. Case studies on masterpieces from the Spanish Golden Age

Abstract

Precise knowledge of the palette employed by historic painters is fundamental to preserving their works of art and can help pinpoint the period in which their paintings were created, their provenance and au- thorship. Raman and IR spectroscopy is successful in identifying pigments via their molecular spectrum, nevertheless, to distinguish the pigments in a binary mixture is challenging, particularly when the mixed pigments are the same colour and Cu-containing. This is precisely the aim of the current research: to evaluate the effectiveness of the molecular spectroscopy techniques μRS and μFTIR-ATR to discriminate binary mixtures of three green pigments, all containing copper, specifically copper resinate, malachite and verdigris. To that end, two sets of model samples were prepared with pure pigments and their bi- nary mixtures in variable weight concentrations from 20% to 80% for each component. The 15 model samples obtained were mixed with linseed oil in the fixed proportion of 3:1 (w/w) of powdered pigment to binder and spread in a homogeneous layer on a support of commercial grade canvas. Five replicates of μFTIR-ATR and μRS spectra, respectively, were recorded after 0 h, 24 h, 48 h, 72 h and 144 h of exposure within the controlled conditions of a climatic chamber. The aim is to establish the minimum percentage of a Cu-containing pigment in a binary mixture that can be detected by μRS and/or μFTIR-ATR spec- troscopy. The product of this ample study is a database of the spectroscopic signature of each pigment in the specific mixtures (in the given proportions and artificial ageing times) for reference purposes in the study of the green areas of selected masterpieces. The oil paintings used in the research ( The Nativity, The Visitation , and The Assumption ) are of Alonso Cano’s work, a well-known artist of the Spanish Golden Age (the Siglo de Oro in Spanish). With this study, the knowledge of the mixture of green pigments em- ployed in Cano’s palette was expanded, making considerable progress in the ambitious National Projects in which our Research Group, ever since 2003, has been attempting to understand Cano’s technique.