On the use of OCT to examine the varnish layer of paintings cleaned with an Er:YAG laser

Optical Coherence Tomography (OCT) was used to visualize and quantify the varnish layer of paintings. And to verify the success of efforts by conservators to remove the varnish using laser ablation. An Er:YAG laser (λ = 2.94μm) with a repetition rate of 15Hz and an optical power of ImW was used to remove the varnish. A spectral domain OCT system with a Michelson topology was constructed using a broadband super-luminescent diode (λ = 840±25nm). The OCT system provided an 8μm resolution, and a field of view of 5×5mm. Samples, including pigment, varnish and substrate, approximately 1mm2 in size, were removed from the oil painting, San Giorgio Maggiore by Martin Rico (1833–1908). Varnish thickness obtained from OCT was validated by similar measurements obtained from SEM analysis. Other paintings, including a late 18th century landscape, signed Thomas Gainsborough, were imaged with OCT to compare neighboring regions before and after laser treatment and to examine the layering of the artist's signature in an effort to determine its authenticity. In conclusion, the non-invasive OCT technique is an efficient tool for measurement of varnish layer thickness, for imaging over-paint under varnish layers, and for assessing the effectiveness of laser assisted varnish removal.