Conservation varnishes at the National Gallery: studying their optical and material properties

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Historically and today, conservation varnishes based on natural or synthetic resins are applied to most Old Master paintings after cleaning and restoration. Such varnishes alter a painting's appearance, saturating the surface and reducing variation in gloss, and provide a protective coating. However, the optical, chemical and physical properties of such varnishes can alter with time, leading to changes in the behaviour of the varnish and the appearance of the painting. Currently, our understanding of such changes is largely based on experimental studies (often involving accelerated ageing experiments) or on the expected properties of the bulk resin or polymer, which do not always accurately reflect the long term behaviour of thin films of conservation varnishes. Furthermore, it has become apparent that in practical usage, conservators often have to make adjustments to varnish formulations (e.g. choice of resin, solvents or additives or the combined use of resins as layered systems or blends) in order to obtain a satisfactory appearance of the varnished painting. This project will investigate the actual properties of conservation varnishes, in order to be able to predict more accurately how conservation varnishes behave in practical usage and how their properties might be improved or modified for particular applications, based on the use of cutting-edge non-invasive optical imaging and spectroscopic techniques including optical coherence tomography (OCT) to allow a (non-subjective) assessment of the appearance, condition and properties of both existing varnishes and experimental systems. OCT produces 3D images ('virtual cross-sections) of surface and subsurface microstructure of transparent and semi-transparent materials and has shown promise for research on varnishes on paintings. OCT and other non-invasive portable equipment, as well as chemical analysis with more traditional laboratory equipment, will be used to investigate the effect of not only of the choice of resin but also the practical issues of formulation and application method on the properties (including surface roughness, scattering, layer thickness, transparency, yellowing, elasticity, refractive index and glass transition temperature) of the resulting conservation varnish. The research will draw on both the unique resource of the actual, naturally aged conservator-applied varnishes on the Gallery's paintings and experimental systems in which formulations and application method can be explored by studying film formation processes and the optical and physical properties of the resulting film. The research outcomes have the potential to feed directly into conservation practice at the National Gallery and around the world.