Typically, varnish coatings were applied to Old Master paintings to improve their visual appearance, saturating the colours. However, over time, varnishes deteriorate and discolour, obscuring or altering the appearance of the artwork, necessitating their removal (usually through the use of a solvent) and replacement. Varnish removal presents a degree of risk to the paint surface and may result in loss of original materials or other damage. Thus, diagnostic methods that can support conservators during varnish removal are very valuable.
Optical Coherence Tomography (OCT), a white light interferometry technique based on the Michelson interferometer, has been proposed as an ideal tool for monitoring the cleaning and treatment of artworks and historical objects. This is owing to its ability to provide non-invasive and accurate measurements across the entire surface of an artwork of the stratigraphy of transparent and turbid multi-layered structures.
In many cases, however, difficulties may arise when attempting to distinguish degraded glaze layers on paintings (coloured, translucent layers containing pigments with refractive indices very close to the binding medium) from layers of aged varnish due to similarities in their appearance and scattering properties in OCT scans. In order to mitigate the possibility of damage through the accidental removal of glaze layers, we propose a simple and entirely non-invasive technique to identify the spectral features of a semi-transparent layer using a combination of OCT and spectral imaging in the visible range.
Building on preliminary studies 1,2, the technique utilises an algorithm to automatically obtain the thickness distribution of the surface layers from the OCT volume of a region with relatively uniform pigmentation and colour. This information is then coupled with the spectral data from the same region to derive the spectral characteristics of each translucent surface layer. Such characteristics may then be used to the discriminate between aged varnishes and glaze layers. The potential of this approach has been demonstrated on mock-up samples and paintings from the National Gallery in London.
 Lange, R., Liang, H., Howard, H. and Spooner, J., “Optical coherence tomography and spectral imaging of a wall painting,” SPIE Newsroom (2011).
 Liang, H., Lange, R., Howard, H. and Spooner, J., “Non-invasive investigations of a wall painting using optical coherence tomography and hyperspectral imaging,” Proc. SPIE 8084, 80840F (2011).