In the case of Poedua, we want to remove the varnish layer because it is very discoloured and has been applied extremely unevenly, which has become more apparent as the discolouration has progressed. Similar to the effects of surface dirt, the discoloured varnish present has diminished the tonal ranges of the painting.
The first step is to characterise the varnish and work out how many layers are present. There are several techniques available to us including, exposing the painting to ultraviolet light and taking cross-sections of the paint and varnish layers. Looking at the painting under UV lamps causes specific auto-fluorescence in different materials which helps us with identification. The varnish layer on Poedua fluoresced a bright green colour, indicating a natural resin such as dammar or mastic. This wasn’t surprising to us since both are common varnishes that we might expect to find on a work such as this.
Cross-section analysis involves the sampling of a microscopic piece of the painting, usually taken from an area of existing damage. This sample is set in a polyester resin and ground down with fine sand paper. The cross-section is then examined using a compound microscope where we can see the building up of paint layers by the artist, surface coatings and later additions. From the cross-section we took from Poedua we can see that there is only one varnish layer apparent.
Varnish layers become increasingly difficult to remove the older they are. This is due to alterations in their chemical structure that occur over time. The solvents that conservators use to remove varnishes are carefully tested in a similar manner to the surface cleaning testing. We use a variety of solvent mixtures and different preparations and combinations including enzymes, soaps and gels. Their effectiveness builds further on our knowledge about the varnish layer we are dealing with. Solvents work by penetrating the varnish layer and swelling the film, bringing the resin into solution and allowing it to be removed from the surface. After initial testing, we select the most appropriate solvent or solvent mixture and test this in the various areas of the composition. Constant care and attention is crucial during this process because the varnish layer is usually the last layer before the paint film is encountered.