Paintings_ Varnish Degradation Processes

Understanding Degradation Processes in Painting Varnishes

Overview

Traditional painting varnishes were made using naturally sourced resins such as dammar or mastic, which were applied to the surface of a painting using a solvent.

Over time, changes can occur within the varnish layer. Varnish blanching, for example, is a whitening phenomenon caused by moisture penetration. Cleaning solvents, introduced during historic cleaning or modern conservation and painting restoration processes, can penetrate varnish and paint layers to induce swelling or structural changes. It is therefore important to understand the mechanisms of these degradation processes in order to control or prevent them in the future.

Analysis at the STFC ISIS Neutron and Muon Source. The OCT probe and SANS Neutron beam can be seen side by side monitoring water invasion of a mastic varnish layer.

This project is part of a joint PhD between the ISAAC Lab and the Science and Technology Facilities Council's ISIS Neutron and Muon Source to demonstrate the feasibility of using Optical Coherence Tomography (OCT) and Small Angle Neutron Scattering (SANS) simultaneously to assess varnish blanching. OCT monitors microscale swelling and optical changes in real time, while SANS provides nanoscale information on pore growth and water uptake, revealing distinct reversible and irreversible stages of the degradation.

The aims of this project are:

to understanding degradation processes in painting varnishes using a non-invasive, multiscale approach
to support safer conservation strategies for cultural heritage

Addressing the Challenge

In November 2024, we took our in-house developed ultra-high resolution OCT instrumentation to the STFC ISIS Neutron and Muon Source in Oxfordshire, to study the blanching of varnish layers and solvent invasion during cleaning of varnish on paintings. The dynamics were studied by the quasi-simultaneous monitoring of the nano scale with small angle neutron scattering, and the microstructure and thickness with the OCT. By integrating these methods, we observed that pore formation and layer swelling occur on distinct timescales: water first infiltrates the superficial layer of the varnish, and swelling becomes apparent as pore growth progresses. The optical change arises from the growth of the pores to a size that effectively scatters visible light, causing the varnish to become opaque.  Additionally, it has been noted that a visible but reversible process happens within the first 50-80 hours after immersion, while irreversible blanching is first noticed after 3 days of soaking in water. The reversibility of the process can be considered a key factor for conservation and restoration practices; however, more in-depth studies need to be carried out to assess possible invisible damage to the naturally restored transparency during the reversible process.

Making a Difference

This research is demonstrating the feasibility of using OCT and SANS simultaneously to assess varnish blanching, highlighting the potential of this integrated analytical technique in cultural heritage conservation. The ability to examine material degradation without damaging artworks is essential for developing safe and sustainable conservation methods, ensuring the safeguarding of valuable heritage materials. In addition, we offer external users the opportunity to test the combined OCT–SANS setup, extending its powerful application to a wider range of research in the cultural heritage field and beyond.