OCT provides new tool for art historians

Lynn M. Savage, Features Editor, lynn.savage@photonics.com

Scientists in Poland have described how optical coherence tomography (OCT) – a technique most often used in medical imaging – can be used to reveal the forgery of an artist’s signature or changes in inscriptions on 100-year-old paintings.

Shown is “Saint Leonard of Porto Maurizio,” an 84 x 121-cm oil-on-canvas picture that hangs in the Franciscan Church of St. Bonaventure in Pako, Poland. Circles mark the regions in which the tomograms were acquired. Rectangles mark the regions of the inscriptions investigated. Figure numbers refer to the 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.

Piotr Targowski of the Institute of Physics at Nicolaus Copernicus University in Torun, Poland, 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.

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. However, many techniques require removal of samples from a painting, thus damaging artistic treasures no matter how tiny the removed portion. In addition, many analytical 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, overcomes those difficulties. OCT 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 it 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; the 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.

The group reported its work in the Dec. 31, 2009, issue of Accounts of Chemical Research.