For many years, the study and conservation of materials and paints used in works of art has been an active area of research among museum curators, scientists and restorers around the world. Over the centuries, the employment of a wide variety of materials (organic and inorganic, natural and synthetic) by artists as well as the continuous development of their painting techniques present a variety of challenges when analyzing their works. . Likewise, the spread of forgeries and forgeries in the art market makes the authentication of works of art a daunting challenge.
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Over the past century, there has been growing interest in the preservation of our cultural heritage objects, such as paintings, sculptures and other works of art. Modern multi-analytical methods, including X-ray Photoelectron Spectroscopy (XPS), Attenuated Total Reflection Mode Fourier Transform Infrared Spectroscopy (ATR-FTIR), Multispectral Imaging and Scanning Electron Microscopy coupled to Energy Dispersive Spectroscopy (SEM/EDS) enable art owners, restorers, museums and dealers to extract valuable information needed for pre-conservation diagnostics and authentication of valuable paintings.
Moreover, these multi-analytical tools are indispensable for evaluating new conservation treatments and their effect on the internal structure of paintings.
Artworks with complex layered structures
A painting is an art form represented by an image made with various pigments superimposed on a support. This support layer is the base of the paint and serves to protect it from mechanical shock and damage. Various materials can be used as a support if they have sufficient durability. These can include rock, paper, fabric (canvas), metal, plaster and others.
A typical painting contains several layers. The base coat serves as an interface between the support and paint layers. This layer is usually made of gesso, a chalk-containing compound, forming a smooth surface suitable for applying paint.
Paint layers contain the most information about paint composition. Artistic paint is a mixture of finely powdered pigments (responsible for color) and matrix material (or binder) holding the pigment particles together. Pigments can be organic or inorganic compounds found in nature as minerals or prepared artificially. Among the most used binders are walnut, linseed, poppy seed and distemper (egg yolk). Additionally, paints may also contain thinning agents to make the paint more uniform and to adjust its transparency to the desired degree.
The top varnish layer is transparent and serves to protect the outer layers of paint or to improve the visual perception of the paint layers below. The varnish is generally composed of natural or synthetic resin.
Limits of non-invasive characterization
The complex layered structure of the paintings as well as the various materials used by the artists make the characterization of such works a real challenge. In recent years, experts have developed multi-analytical methods that combine various non-invasive and invasive techniques for the characterization of these stratified specimens.
Noninvasive characterization typically relies on a combination of techniques such as ultraviolet (UV) fluorescence, infrared reflectivity (IRR) and energy dispersive X-ray fluorescence (EDXRF) spectroscopy. The information obtained can provide details about the artistic technique used to create the painting, the distribution of pigments and the characteristics of the top layer of the painting (for example, the presence of a non-original varnish).
However, non-invasive diagnostics cannot answer questions related to the stratigraphy of the painting and the chemical composition of the materials used by the artist. Nevertheless, the non-invasive characterization step can reveal hidden flaws and greatly facilitate the selection of suitable sampling areas for further analysis.
Reveal the artist’s signature through the analysis of the chemical composition
Accurate elemental analysis can be achieved via micro-invasive spectroscopic techniques such as XPS, ATR-FTIR and SEM/EDS. In this case, the characterization is carried out on microscopic sections extracted from specific areas of the painting. These spectroscopic techniques make it possible to detect the presence of certain elements (such as metal ions, silicon, phosphorus, etc.) and to confirm the use of certain pigments by the artist (for example, Prussian blue containing iron or chromium oxide green containing chromium).
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Spectroscopic data can be compared to some previously documented data (such as the spectra database or historical records of pigment manufacture and availability). Thus, based on the organic and inorganic materials identified in the cross sections, a time frame for the creation of the painting can be established.
Additionally, results may reveal similarities between the pigments and binder used in a painting and the palettes and techniques of other well-known artists, allowing experts to determine authorship. This is particularly relevant to 20th century art, where the wide variety of materials used and the continuous technological evolution often prove difficult to establish compatibility with artists’ techniques.
A multi-analytical approach improves the conservation of works of art
Multi-analytical characterization can also guide experts when evaluating new cleaning and conservation treatments through the characterization of surface contaminants and degradation products and their selective removal. Most modern and environmentally friendly cleaning methods are designed to achieve the best performance and a good degree of solubilization while ensuring selective removal of aged varnish through multi-analytical characterization of the cleaning process. These cleaning agents represent an alternative to the more toxic organic solvents that are still used in the field of conservation, allowing the safeguard of human health, works of art and the environment.
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References and further reading
Macchia, A. et al. (2022) Multi-Analytic Survey of Oil Painting”The seller of Cerini” by Antonio Mancini and Definition of the best ecological cleaning treatment. Sustainability, 14(7), 3972. Available at: https://www.mdpi.com/2071-1050/14/7/3972
Wiggins, MB, et al. (2019) Multi-analytical study of historic copper-based pigments and their weathering products. Applied Spectroscopy, 73(11),1255-1264. Available at: https://journals.sagepub.com/doi/10.1177/0003702819856606
Gavrilov, D. et al. (2014) A Review of Imaging Methods in Fine Art Analysis: Thermographic Imaging Method in Art Analysis. Canadian Journal of Physics, 92(4), 341-364. Available at: https://cdnsciencepub.com/doi/10.1139/cjp-2013-0128