Analytical studies on commercial artists’ colour charts from Das Deutsche Farbenbuch (1925)—identification of synthetic and natural organic colourants by Raman microscopy, surface-enhanced Raman spectroscopy and metal underlayer ATR-FTIR spectroscopy

Abstract

Historical colour charts provide a rich and often well-dated reference materials source for studying the chemical composition of all kinds of commercial brands of artists’ paints. This article presents the results of an extensive analytical study of more than 80 paint hues from 11 colour charts that are included in the German standard book Das Deutsche Farbenbuch by H. Trillich (1925, Part II). Our research focused on the identification of synthetic organic pigments, whose quickly increasing significance for artists’ paints in the early twentieth century is impossible to evaluate by documentary source research alone. A stepwise procedure combining different non- or minimally invasive vibrational spectroscopy techniques—Normal Raman and Surface-Enhanced Raman spectroscopy as well as Metal Underlayer Attenuated Total Reflection Fourier-transform Infrared Spectroscopy—allowed the identification of 18 different organic colourants in artists’ watercolours, tempera and oil colours from six German manufacturers. In addition, micro-X-Ray Fluorescence spectroscopy was applied to determine the elemental pattern of substrates, fillers, and admixed inorganic pigments. In addition to a few traditional natural organic colourants (dark and rose madder lake, cochineal lake), most of the identified compounds comprised synthetic organic pigments or synthetic dyes from various chemical classes (indigo, anthraquinone, monoazo, ß-naphthol, xanthene, triarylcarbonium, nitroso, and azine compounds). Some of these have not or only rarely been reported in artists’ paints so far. Since the identified organic colourants have mainly poor to fair (only sometimes good) fastness to light according to modern standards and partially also to solvents typically used in conservation treatments, it is evident that works of art from this period should be treated keeping in mind the possible presence of such colourants, when planning both interventive treatments and preventive measures.

Traditional and modern pigments in Gino Severini's Swiss murals

“…Considering the time of use, first half of 20 th century, it is probably synthetic alizarin and not the natural one. In fact, Raman investigation detected only pure alizarin and no other components, and this is typical of the synthetic one 20 . This finding most likely corresponds to the purchased “laque de Garance” listed in the invoices of painting materials for Semsales decoration 10 .…”

“…In fact, Raman investigation detected only pure alizarin and no other components, and this is typical of the synthetic one. 20 This finding most likely corresponds to the purchased "laque de Garance" listed in the invoices of painting materials for Semsales decoration. 10 Indeed, in the early 20 th century the term "laque de Garance" was used to indicate both natural and synthetic red lakes.…”

An Introduction and Recent Advances in the Analytical Study of Early Synthetic Dyes and Organic Pigments in Cultural Heritage

“…Recently, metal underlayer attenuated total reflection Fourier-transform infrared spectroscopy (MU-ATR) has been proposed as a new methodology to enhance the FTIR signal by depositing a micro-extract on a gold-coated glass; a commercial FTIR microscope with a fixed incident angle is then used [101]. This technique was successfully used on the extracts of reference pigments, colour charts and dyed fibres [22,101,102], but it proved to be much less efficient when directly applied on a gold-coated sample. Finally, the brilliance, monochromaticity and high focalisation of synchrotron radiation have been recently exploited in SR-µ-FTIR imaging techniques for the determination of organic pigments in historical artworks [103].…”

“…It can be applied deposing the silver nanoparticles directly on the artwork, or on a micro-sample (as-is or on an extract), resulting in a micro-invasive/micro-destructive technique. Examples of successful use of SERS encompass the identification of SOPs in commercial artists' colour charts [22], paintings [92], inks [66,104,118], dyes on textiles [94,122] and colourants in Japanese woodblock prints from the Meiji period [75,123,124]. Additionally, an interesting study has been recently carried out on the photostability of acrylic and alkyd paints, showing that Raman is effective in revealing the degradation phenomena occurring in the binder but not those relative to the pigment [125].…”

“…Nevertheless, reference materials and databases built by analysing them are essential for the identification and study of synthetic colourants. Researchers interested in this historic period tend to exploit original sources of reference materials, such as dye sample books [20][21][22][23][24][25], or historic collections of labelled dyes and pigments in powder form [11,26]. However, these historic sources of reference samples can lack full reliability, as discrepancies between the dyes reported in the labels and the ones used to produce the samples were observed [20,21].…”