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

Lead Research Organisation: Nottingham Trent University
Department Name: School of Science & Technology


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.


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Title Noninvasive depth-resolved material characterisation using OCT and spectral imaging 
Description Using the combination of spectral imaging and Optical coherence tomography to identify spectral features within the layer structure of oil paintings in order to distinguish varnishes from glazes. 
Type Of Material Improvements to research infrastructure 
Year Produced 2019 
Provided To Others? No  
Impact interest from conservators in adopting the technique. 
Description OCT of pannotype photography 
Organisation The National Archives
Country United Kingdom 
Sector Public 
PI Contribution operation of the optical coherence tomography for the imaging of the layer structure of pannotype photographs from The National Archives. interpretation and processing of the OCT data. Spectral imaging of Pannotype photography. Layer identification from spectral imaging and OCT segmentation using the spectral features and layer thicknes.
Collaborator Contribution Providing the samples and the associated conservation context of the samples. Defining the research questions.
Impact OCT of pannotype photography: demonstration of successful application of OCT in investigating the layer structure of pannotype photography. Leading to future implementation of the technique in the investigation of such photographs. Disciplines involved: Conservation, Physics.
Start Year 2019
Description Optical Coherence Tomography of Tertiary Droplets 
Organisation Charles III University of Madrid
Country Spain 
Sector Academic/University 
PI Contribution Operation of the OCT for the scanning of water-ethanol droplets submerged in anise oil for the investigation of the internal flows. Designing of experiments for the OCT of such droplets.
Collaborator Contribution Measurements of the growth and movement of such droplets. Context and theory for the flows and growth of such droplets
Impact Optical Coherence Tomography of Tertiary Droplets:Demonstration of the use of OCT for the monitoring of the internal flow within such droplets which will allow the implementation of the technique for future work
Start Year 2019
Description Science and Heritage Interdisciplinary Research Workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact 61 people within cultural heritage attended a workshop, within the research teams laboratory, demonstrating the imaging techniques implemented in the analysis of cultural heritage, stimulating interest in the research and collaborations with other organisations.
Year(s) Of Engagement Activity 2019