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

历史上和今天，保护漆基于天然或合成树脂应用于大多数清洗和修复后的绘画大师。这种清漆可以改变一幅画的外观，使表面饱和，减少光泽的变化，并提供一层保护涂层。然而，这种清漆的光学、化学和物理性质会随着时间的推移而改变，从而导致清漆的行为和绘画的外观发生变化。目前，我们对这种变化的理解主要是基于实验研究（通常涉及加速老化实验）或散装树脂或聚合物的预期性能，这并不总是准确地反映保护清漆薄膜的长期行为。此外，很明显，在实际使用中，维护人员经常不得不调整清漆配方（例如，树脂、溶剂或添加剂的选择，或树脂作为分层系统或混合物的组合使用），以获得令人满意的清漆外观。该项目将调查保护清漆的实际性能，以便能够更准确地预测保护清漆在实际使用中的表现，以及如何在特定应用中改进或修改其性能，基于使用尖端的非侵入性光学成像和光谱技术，包括光学相干断层扫描（OCT），以允许（非主观）评估外观。现有清漆和实验系统的状况和性能。OCT产生透明和半透明材料的表面和地下微观结构的3D图像（“虚拟横截面”），并在绘画清漆的研究中显示出前景。OCT和其他非侵入性便携式设备，以及更传统的实验室设备的化学分析，将用于研究树脂的选择以及配方和应用方法对所得到的保护清漆的性能（包括表面粗糙度，散射，层厚度，透明度，黄化，弹性，折射率和玻璃化转变温度）的实际问题。该研究将利用画廊画作上实际使用的自然老化的清漆的独特资源和实验系统，通过研究胶片的形成过程和所产生的胶片的光学和物理特性来探索配方和应用方法。研究成果有可能直接用于国家美术馆和世界各地的保护实践。

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.