Academic journal article Heritage Science

Examination of Historical Paintings by State-of-the-Art Hyperspectral Imaging Methods: From Scanning Infra-Red Spectroscopy to Computed X-Ray Laminography

Academic journal article Heritage Science

Examination of Historical Paintings by State-of-the-Art Hyperspectral Imaging Methods: From Scanning Infra-Red Spectroscopy to Computed X-Ray Laminography

Article excerpt

Authors: Stijn Legrand [1]; Frederik Vanmeert [1]; Geert Van der Snickt [1]; Matthias Alfeld [1,2]; Wout De Nolf [1]; Joris Dik [3]; Koen Janssens (corresponding author) [1]

Introduction

Historical paintings are considered to be among the most precious cultural heritage artefacts and have been the subject of intensive studies for decades. Scientific studies on such artefacts are highly relevant, in order to optimize the preservation of the paintings for coming generations and/or to gain more profound insights in their creation process [1, 2, 3]. This review focusses on the examination of easel paintings, i.e., painted renditions realized on a moveable substrate. Easel painting consists typically of a support, ground, paint and varnish layers, applied on top of one another. Canvasses and wooden panels are the most popular supports, but also other materials such as thin copper plates, paper, stone and glass have been used. Often the pictorial layers are very thinly painted out, making some of them semi-transparent. Micrometers below a painting?s surface, a wealth of information may be present about the creative process followed by the artist while making the work of art. Many painterly effects can critically depend on the layer build-up: e.g., the translucent shine of colorful tissues, the suggestion of shadow in flesh tones or the convincing illusion of an object?s texture may be realized by deliberately including the optical contribution of lower layers. Additionally, knowledge about the stratigraphy of a painting often is highly relevant in conservation when stability problems such as paint discoloration or delamination are studied. Thus, the study of a painting, its composition and stratigraphy is a common research theme shared by curators, conservators and conservation scientists. However, this information, comprised of structural and compositional aspects, is usually not easy to obtain in a non-invasive manner. Next to the visible surface layers, subsurface layers may include underdrawings, underpaintings, and adjustments made in the course of painting. Together, all these layers determine the current appearance of the work of art. In a growing number of cases conservators have discovered abandoned compositions underneath paintings, illustrating the artist?s practice of reusing a canvas or panel. Imaging methods that can ?read? this hidden information without any damage to the artwork are therefore valuable for art-historical research [2] while also being very useful during restoration activities.

The standard methods for studying the inner structure of painted works of art are X-ray radiography (XRR) and infrared reflectography (IRR), penetrative illumination techniques that are optionally complemented with the microscopic analysis of cross-sectioned samples. Both methods are full field imaging methods, employing image plates or cameras that are sensitive in the appropriate range of the electromagnetic spectrum for recording the image data (see Figure?1A). Since these methods all have their limitations, recently, a number of new approaches based on X-ray and Infra-red radiation for imaging the buildup of hidden paint layer systems have been put into practice; some of these methods make use of scanning pencil beams over the painting while recording data either in transmission or reflection mode (see Figure?1B, C). Two major motivations can be discerned for the development of these more advanced versions: (a) the desire to know more about the creative process and/or the artist?s way of working that have led to a given work of art and (b) the need to assess and predict the current and future condition of a work of art. Motivation (a) is essentially of art-historical nature and seeks to reconstruct (better) the past/history of an artwork while motivation (b) is more strongly linked to preventive conservation and to conservation technology, and therefore mostly concerned with the future of the artwork [1]. …

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