Optical coherence diagnostics for painting conservation

One of the most important and sometimes controversial stages of the conservation process is surface cleaning: decisions have to be made regarding partial or complete removal of varnish. Technical considerations include selection of a method that allows a great deal of control in the cleaning process, so that undesired layers can be removed without damage to the underlying ones. Traditional cleaning methods include mechanical or chemical removal, and restorers and conservators work would be considerably helped by the knowledge of the varnish thickness. Up to now thickness measurement has been performed in an invasive way by means of stratigraphy, a well known painting structure investigation technique since half a century. In this work we present an application of Optical Coherence Tomography (OCT), a well-established technique for biomedical applications, for non-destructive measuring of the varnish film thickness during the cleaning process of an ancient painting, the Ritratto Trivulzio by Antonello da Messina. OCT images of three differently cleaned areas on the painting surface were acquired and the results were compared with a spectral characterization of the same areas.