Research Centre for Science and Technology of the Arts - CITAR

In this work a series of egg tempera paint and varnish systems have been prepared, artificially aged and irradiated with KrF excimer laser at a wavelength of 248 nm. The samples were prepared with pure pigments and selected mixtures. It was found that, for some pigments, the colour changed upon laser irradiation even at low energy densities, below the ablation threshold while for other inorganic pigmented egg temperas the degree of discoloration is very small at moderate fluence of ∼0.30 J cm?2. The varnish systems did not present signs of discoloration. The thickness, superficial roughness and magnitude of the colour changes of the samples were measured. X-ray diffraction, Raman spectroscopy and UV/visible spectroscopy were used in order to investigate the changes induced by the KrF excimer laser radiation.

There has been a growing interest since the mid-1990s in the use of lasers as an alternative method of cleaning of painted surfaces to surpass the possible problems of irreversible modification caused by chemical or mechanical methods. The objective of this research is to extend the current understanding of excimer laser-induced discoloration effects on pigments mixed in oil medium. The study focuses on the response of samples of pure pigments and their mixtures widely used by painters in the nineteenth century, to different laser parameters. Characterisation of physical changes was performed using optical microscopy, colorimetry, scanning electron microscopy, and atomic force microscopy. It has been found that materials have different sensitivity to irradiation at 248 nm and that even if an ultraviolet wavelength is used, the photo-chemical mechanism may not be the predominant effect. The effect of irradiation on a pigment mixture cannot be predicted from the effect of the irradiation on the individual pigments.

The potential of laser cleaning for paintings as an alternative to traditional cleaning methods has been recognized, and studies using industrial laser equipment have shown some potential. Irradiation at a wavelength of 248 nm has shown to be the most promising for natural resin varnish and overpaint removal, because it is strongly absorbed by triterpenoid varnish functional groups and associated degradation products (Georgiou 1998). However, it has been suggested that a minimal varnish thickness should remain intact to prevent any radiation from being absorbed by the underlying layers. This is because some painting materials have also shown sensitivity to laser radiation (Bordalo 2006). Research into the mechanisms of alterations which may occur due to laser radiation on resins, pigments and oils and its long-term effects has been scarce, and dispersed in its approach because of the multitude of different materials present in the paint layers (Chappé 2003, Sobott 2003, Melessanaki 2006). Thus, the systematic study of laser radiation interaction with paint materials is still important. The aim of this study was to advance the understanding of the effects that KrF excimer laser irradiation has on pigmented oil paint layers.

The nineteenth century was a very prolific artistic era during which the materials used by painters changed radically. During this period, and due to the rapid development of the chemical industry, many new pigments were discovered and were commercialized while other traditional ones ceased to be used by painters. The use of these new materials depended greatly from the painter’s own preference and color taste. Nevertheless, the adoption of new materials was not immediate throughout Europe but gradual since it depended on its commercial availability in each country. Thus, the use of a particular material is not only of high importance for the study of the painter technique but also to the study of the dissemination of the new materials in the Western world. The systematic study of the painting materials and their use in the nineteenth century in Portugal is still to be made since it was a period of great transformation and from which only few painters and paintings have been already materially characterized.
The present paper contributes to the study of the painting materials used by Portuguese painters in the nineteenth century. It describes the comparative microanalytical characterization performed on five paintings currently exhibited at the National Museum of Contemporary Art (Chiado Museum) in Lisbon. The oil paintings, dated from 1855 to 1879, comprise three works by Miguel Ângelo Lupi (1826-1883) and two by João Cristino da Silva (1829-1877), two important Portuguese painters of the Romanticism whose palette has not been previously studied.
The characterization of the painting materials was performed by optical microscopy, Variable Pressure Scanning Electron Microscopy coupled with Energy Dispersive X-Ray (SEM–EDX) microscopy, micro Fourier Transform Infrared Spectroscopy (µ-FTIR), µ-Raman Spectroscopy, and micro X-ray Fluorescence spectrometry (µ-XRF). The study allowed the identification of the painting materials used by the painters such as copper arsenite, ultramarine blue, Prussian blue, azurite, lead white, barium white, zinc white, vermillion (Figure 1), carbon black, red lead, calcium carbonate and ochre among others.
This article allowed not only to characterize for the first time the palette of Cristino da Silva and Miguel Ângelo Lupi but also to contribute to the study of the use of pictorial materials in Portugal in the nineteenth century.

Western art in the nineteenth century was profuse in movements, and artists were more experimental than ever before. The nineteenth century also brought numerous novelties in painting materials and techniques. Traditional painting materials from past centuries were used at the same time as new contemporary materials were discovered and experimented. One of the biggest changes was that painters no longer had to prepare their own materials, both supports and painting materials, since they could purchase them already made from artist’s colourmen.
The paintings’ ground layer is usually hidden from sight but it is not without importance since it has a considerable impact in the stability and final appearance of the painting. The ground layer was traditionally prepared by the painter by mixing animal glue as binder, lead white as white pigment and other white fillers. During the nineteenth century, and with the introduction of new white colour materials, the formulations of ground layers changed significantly and even more due to their industrial application in ready-made canvases. Nevertheless, some painters still applied another layer over the industrial one before painting.
This article describes the comparative characterization of the ground layers of eleven paintings dated from 1852 to 1884 by eight Portuguese painters: João Cristino da Silva (1829-1877), Tomás da Anunciação (1818-1879), António José Patrício (1827 - 1858), Miguel Ângelo Lupi (1826-1883), Marques de Oliveira (1853-1927), Alfredo Keil (1850-1907), Carlos Reis (1863-1940), Francisco Metrass (1825-1861), Luís de Meneses (1820-1878). The paintings are part of the permanent exhibition of the National Museum of Contemporary Art (Chiado Museum) in Lisbon. Even though well-known, some of these painters’ works were never previously subjected to material characterisation.
The characterization of the painting materials was performed by optical microscopy, Variable Pressure Scanning Electron Microscopy coupled with Energy Dispersive X-Ray (SEM–EDX) microscopy, micro Fourier Transform Infrared Spectroscopy (µ-FTIR), µ-Raman Spectroscopy, and micro X-ray Fluorescence spectrometry (µ-XRF). The ground layers present some similarities such as the presence of coarse calcite and lead white. However, they are quite diverse concerning their content with typical variation with the presence of fillers such as barium white and zinc white, and other impurities in different percentages.
The study allowed the characterization of the several ground layers including thickness and materials, and its distinction by painter.