OCT Illuminates Art Forgeries

Scientists in Poland have described how a medical imaging technique has taken on a second life in revealing forgery of an artist’s signature and changes in inscriptions on paintings that are hundreds of years old. A report on the technique, called optical coherence tomography (OCT), appears in Accounts of Chemical Research, a monthly journal published by the American Chemical Society.

The oil painting on the left fluoresces to reveal hidden details (right) when exposed to a new noninvasive imaging technique that uses ultraviolet light. Lower panels show the areas of the artist’s signature. (Image: Waldemar Grzesik, Institute for the Study, Restoration and Conservation of Cultural Heritage, Nicolaus Copernicus University.)

Piotr Targowski of the Institute of Physics at Nicolaus Copernicus University in Torun notes that easel paintings prepared according to traditional techniques consist of multiple layers. The artist, for instance, first applies a glue sizing over the canvas to ensure proper adhesion of later layers. Those layers may include an outline of the painting, the painting itself, layers of semitransparent glazes and, finally, a transparent varnish.

Understanding the stratigraphy – or the order, thickness, composition and origin – of an artwork is important for proper attribution of the piece, for detection of possible forgeries or reproductions, and for planning conservation efforts.

Shown is Saint Leonard of Porto Maurizio, an 84 × 121-cm oil-on-canvas picture that hangs at the Franciscan Church of St. Bonaventure in Pako. Circles mark the regions in which the tomograms were acquired. Rectangles mark the regions of the inscriptions investigated. Figure numbers refer to original paper in Accounts of Chemical Research; 4a-4c are shown top to bottom in the image below. (Photo by Magdalena Iwanicka is used with permission.)

Art conservators and other experts resort to a variety of technologies to see below the surface of a painting and to detect changes, including forged signatures and other alterations. However, many approaches require removal of tiny samples from a painting, thus damaging artistic treasures. Also, many techniques are not sensitive enough to detect finer details.

Targowski and his colleagues describe how OCT, which is commonly used to produce three-dimensional images of the layers of the retina of the eye, overcomes those difficulties. OCT also has been used by art conservators to study varnishes and underdrawings, but the work by Targowski’s group is the first known effort to use OCT data to extrapolate historical information,

OCT tomograms from Saint Leonard of Porto Maurizio: (a) multilayer varnish (image width, 7 mm); (b) semitransparent overpainting (image width, 7 mm); (c) opaque overpainting (image width, 12.3 mm). Yellow arrows indicate the surface of the primary varnish layer; green arrows indicate the primary opaque paint layer; circles show boundaries between original and overpainted areas; rectangle indicates a region in which the overpainting is completely opaque. The bars indicate real distances in media of refractive index of 1.5.

The group used OCT to analyze two oil paintings from the 18th and 19th centuries. In one, Saint Leonard of Porto Maurizio, OCT revealed evidence that the inscription “St. Leonard” was added approximately 50 years after completion of the painting. In the other, Portrait of an Unknown Woman, OCT found evidence of possible forgery of the artist’s signature.

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