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

在历史上和今天，基于天然或合成树脂的保护清漆在清洗和修复后被应用于大多数老大师的画作。这种清漆改变了绘画的外观，使表面饱和，减少光泽度的变化，并提供保护涂层。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）或本体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，显而易见的是，在实际使用中，保护人员通常必须对清漆配方进行调整（例如，树脂、溶剂或添加剂的选择，或树脂作为分层系统或共混物的组合使用），以获得令人满意的涂漆外观。本项目将研究保护漆的实际性能，以便能够更准确地预测保护漆在实际使用中的行为以及如何改善或修改其性能以用于特定应用，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），以允许对现有清漆和实验系统的外观、状况和性能的（非主观）评估。OCT可以生成透明和半透明材料表面和次表面微观结构的3D图像（“虚拟横截面”），并显示出对绘画清漆研究的前景。光学相干断层扫描仪和其他非侵入式便携式设备，以及化学分析与更传统的实验室设备，将被用来调查的影响，不仅树脂的选择，但也实际问题的配方和应用方法的性能（包括表面粗糙度、散射、层厚度、透明度、泛黄、弹性，折射率和玻璃化转变温度）。这项研究将利用画廊画作上实际自然老化的保护漆的独特资源和实验系统，通过研究薄膜形成过程和所得薄膜的光学和物理性质，可以探索配方和应用方法。研究成果有可能直接投入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.