Water for varnish removal: Evaluating water-based methods for the removal of dammar varnish from mock-up samples of traditional oil paintings

In the conservation of oil paintings constructed according to traditional painting practices, the uppermost layer of varnish is often removed when discoloured to a degree that obscures the image underneath. To remove the varnish without causing any observable damage to the underlying oil paint, organic solventshave been used via a well-established and systemic approach for selecting and opti mising the single solvent or mixture of solvents. However, due to hazards associated with organic solvents, to the underlying oil paint layers, the practitioner, and the environment, an extensive number of water-based methods have been developed and introduced to the field since the 1990s. The current challenge is to select, modify and evaluate a method for a specific varnish removal scenario among the multitude of options available. To facilitate this decision-making process, promising water-based methods for varnish removal were identified, and their modifiability and comparative advantages and disadvantages were evaluated, also in relation to a typical organic solvent-based approach. Furthermore, as these aims were primarily accomplished via the use of mock-up samples, the transferability of the mock-up-based results to expected outcomes in conservation practice was also evaluated. The key outcomes of this work were the identification of a new type of cleaning system for varnish removal, surfactant-free oil-in-water (OiW) microemulsions (Paper I), the development of a framework for comparative evaluation of methods for varnish removal (Paper II), and the development of a methodology for preparing mock-up samples for research on the removal of coatings from paintings and for evaluating results obtained via mock-up samples (Paper III).