Characterisation of the varnish-object interface using white light confocal profilometry

An ongoing concern of conservators when restoring paintings and polychrome objects is the possibility of irreversible damage to the original paint layers. This is the case when removal and replacement of aged varnishes is being considered. Although much work has been conducted to determine the effect of the removal of a varnish on the condition of the (painted) surface, there is still some uncertainty in the conclusions because it has, until recently, been impossible to characterize the original surface under the varnish in situ. A promising solution to this problem is the use of white light confocal profilometry. An initial study has been conducted using this technique to measure the roughness of pencil lines and oil paint under a varnish. Measurements were made on areas up to several mm 2 in size, at lateral resolutions down to 1 μm, and vertical (roughness) resolutions of 0.1-0.3 μm. It was found that the surface roughness of surfaces under relatively thick varnishes can be measured at relatively low magnification (20x). For thin varnish layers typical of paintings, higher magnifications are required. White light confocal profilometry has thus been shown to be a useful tool for in situ studies of the (sub)surface properties of (painted) objects.