60 Seconds on... sparsity-based terahertz reflectometry
Credit: David Citrin. This image shows reflected raw terahertz signals measured across the painting.
Using terahertz scanners and advanced signal processing techniques, developed for petroleum exploration, an unprecedented insight into the layers of 17th Century artwork has been gained.
Using a commercial terahertz scanner, the research team studied Madonna in Preghiera by the workshop of Giovanni Battista Salvi da Sassoferrato, loaned from the Musée de la Cour d’Or, Metz Métropole, France. Described in the paper as typical of the period and having previously resisted attempts to analyse its stratigraphy (the order and distribution of layers of paint) – due to the layers measuring only tens of micrometres – it is an oil painting on canvas, mounted on a wooden stretcher.
Researchers at the Georgia Institute of Technology, USA, detailed the new technique in Global mapping of stratigraphy of an old-master painting using sparsity-based terahertz reflectometry, published in Scientific Reports.
The scanner’s electromagnetic wave generator emits signals that penetrate layers of paint. Different pigments and physical structures, such as imperfections in the layers, are revealed by wavelengths as they reflect some of the beam back to the scanner. Oil paint and newer varnish do not fluoresce under UV – meaning recent retouchings appear as darker patches on the surface.
The collected data was processed by a computer-based signal processing technique – sparsity-based time-domain deconvolution – and a three-dimensional map of the image was constructed using the signals reflected from each layer. The team was able to identify layers including the canvas support, ground (the background surface), imprimatura (the initial colour stain painted on the ground), underpainting, pictorial and varnish layers. A formerly unknown layer of varnish restoration was also detected.
To read more on this topic see the upcoming January issue of Materials World.
To read Global mapping of stratigraphy of an old-master painting using sparsity-based terahertz reflectometry, visit go.nature.com/2Bts2vL
