Optical coherence tomography: a non-invasive technique applied to conservation of paintings

Abstract

It is current practice to take tiny samples from a painting to mount and examine in cross-section under a microscope. However, since conservation practice and ethics limit sampling to a minimum and to areas along cracks and edges of paintings, which are often unrepresentative of the whole painting, results from such analyses cannot be taken as representative of a painting as a whole. Recently in a preliminary study, we have demonstrated that near-infrared Optical Coherence Tomography (OCT) can be used directly on paintings to examine the cross-section of paint and varnish layers without contact and the need to take samples. OCT is an optical interferometric technique developed for in vivo imaging of the eye and biological tissues; it is essentially a scanning Michelson's interferometer with a 'broadband' source that has the spatial coherence of a laser. The low temporal coherence and high spatial concentration of the source are the keys to high depth resolution and high sensitivity 3D imaging. The technique is non-invasive and noncontact with a typical working distance of 2 cm. This non-invasive technique enables cross-sections to be examined anywhere on a painting. In this paper, we will report new results on applying near-infrared en-face OCT to paintings conservation and extend the application to the examination of underdrawings, drying processes, and quantitative measurements of optical properties of paint and varnish layers.

Optical Coherence Tomography in archaeological and conservation science - a new emerging field

“…It was shown that the high sensitivity of OCT enables the interface between different varnish layers on a painting to be seen [5]. OCT has been used for dynamic monitoring of the wetting and drying of different varnish, the examination of the difference between the roughness of the dried surface for the different types of varnish [8,15] and the monitoring of the aging and solvent cleaning of varnish [9]. Measurements of refractive indices of varnish and paint samples using OCT were given in [8,9].…”

“…OCT has been used for dynamic monitoring of the wetting and drying of different varnish, the examination of the difference between the roughness of the dried surface for the different types of varnish [8,15] and the monitoring of the aging and solvent cleaning of varnish [9]. Measurements of refractive indices of varnish and paint samples using OCT were given in [8,9]. To exploit the fast imaging capability of Fourier-domain OCT, it has been demonstrated that a fast imaging OCT is capable of monitoring real time laser ablation of varnish layers [10] and real time tracking of canvas deformation due to environmental changes such as humidity [11].…”

Optical Coherence Tomography: its role in the non-invasive structural examination and conservation of cultural heritage objects—a review

“…In the second case, evidence of forgery of the signature was uncovered. Processes occurring during drying of the varnish layer were investigated by Liang et al [35]. They pointed out that the varnish Paraloid B-72, when drying, begins to follow the roughness of the substrate.…”

“…The advantage of coherent detection in OCT was utilised by Liang et al [14,34,35,37,38] and by Adler et al [19] for (Fig. 5).…”

High precision dynamic multi-interface profilometry with optical coherence tomography

“…OCT monitoring of a Paraloid B72 varnish drying on a regular rough surface showed that the varnish follows the roughness of the substrate as it dried [21]. Subsequently, OCT imaging of a Ketone varnish and a Paraloid B67 varnish on a painting showed directly that the low molecular weight Ketone varnish was better at leveling the surface roughness of the painting [6].…”