The Optical and Material Properties of Conservation Varnishes for Paintings
Around the time of the 19th century, a transparent varnish coating would typically be applied to the surface of a painting as a liquid film and left to dry. These coatings had the function of both providing some protection to the paint layer and more importantly improving the appearance of the painting and saturating the colours.
Traditionally, the varnish solutions used would comprise of naturally sourced resins, such as dammar or mastic, dissolved in a solvent. These varnish coatings however, undergo chemical reactions and deteriorate over time, becoming more hazy and yellow with age.
OCT system examining an easel painting from the National Gallery's collection
Conservators may need to remove and replace the varnish layer on a painting, through the process of cleaning, in order to preserve the artist’s original intended appearance. However, varnish removal presents a degree of risk to the painted surface, with the loss of other original materials, such as pigments or glazing layers, also likely during the removal process. Diagnostic methods that can support conservators during varnish removal are therefore very valuable.
The aims of this project are:
to develop non-invasive techniques to extract stratigraphic information on the layered structure of paintings
to identify the spectral features of semi-transparent layers using a combination of Optical Coherence Tomography (OCT) and spectral imaging.
Addressing the Challenge
Optical Coherence Tomography (OCT) has been the subject of interest within the heritage science community for the over 20 years. Its potential use as monitoring tool for the conservation treatment of artworks and historical objects is well recognised. Of particular interest to this work is its abilities to give non-invasive micron scale measurements on the stratigraphy of transparent and turbid multi-layered structures such the varnish and paint layers on Old Master Paintings, enabling the monitoring of the removal of degraded varnish coatings during the cleaning process.
In many cases, however, difficulties may arise when attempting to distinguish glazing layers on paintings (coloured, translucent layers containing pigments with refractive indices very close to the binding medium) from layers of aged varnish due to similarities in their appearance and scattering properties within OCT scans.
To counteract this, a combined approach has been developed, where the thickness of a discrete material layer can be assessed using OCT measurements, and then diagnostic reflectance spectra for that layer can be extracted using spectral imaging. Alongside this, the development of accompanying instrumentation to allow for the collection of OCT and spectral imaging within the same region is being undertaken, giving a direct one-to-one relationship between the two data sets recorded.
Ultra High Resolution OCT Scanning of the painting ‘A Vase of Wild Flowers’ (probably 1870-80) by Adolphe Monticelli at the National Gallery in London.
Below: Results obtained during Ultra High Resolution OCT Scanning of 'A Vase of Wild Flowers'. Multiple degraded varnish layers are visible above the paint layer, with the paint layer itself being applied on top of a thick varnish layer.
Making a Difference
The analysis approach being developed during this project, with the combined use of two complementary techniques, aims to provide a useful non-invasive diagnostic tool for conservators. This will aid both investigation of the stratigraphy of a painting and monitoring of the cleaning process for varnished objects. This approach could also be applied to objects which have historically had glues and other coatings applied during previous conservation treatments, in order to restore the integrity of the object.
Nottingham Trent University, United Kingdom (Lead Research Organisation)
Charles III University of Madrid (Collaboration)
The National Archives, Richmond, United Kingdom (Collaboration)
The National Gallery, United Kingdom (Student Project Partner)
Prof. Haida Liang (Nottingham Trent University)
Dr Lucas Goehring (Nottingham Trent University)
Dr Catherine Higgitt (National Gallery)
Marika Spring (National Gallery)
Partick Atkinson (Nottingham Trent University)
AHRC Collaborative Doctoral Award
The National Gallery