A strategy for cleaning
With the documentation and examination of the painting now well underway, the time has approached to develop a strategy for cleaning the painting, that is, the removal of the discoloured varnish and restorer’s overpaints. While every cleaning must follow the particular demands of each individual painting, the steps we will take for the Crossing of the Red Sea are typical for the treatment of many old master paintings.
Before cleaning can commence, tests will need to be carried out to ensure that it can be done with the minimum of risk. Here are some of the steps we’ll take:
Tests under magnification
The first stage will involve testing of the solubility of the varnish and potentially the paint films. During an examination it usually clear what type of varnish lies on the surface and what type of paint medium has been used by the artist. With that knowledge a suitable varnish removal solution is selected and tried out on a tiny area of the surface of the painting, usually near an edge. This is generally done with the aid of a magnifier or microscope to see how the varnish and paint respond to the cleaning solution. If problems occur the choice is clear: stop or otherwise find another an alternative cleaning mixture, or perhaps a different method of cleaning. In most cases, when the varnish and paint layers are properly understood it does not take long to arrive at a suitable method.
Small windows on different areas of the painting
Once the cleaning tests indicate that the old varnish can be safely removed, the next stage in the process will involve carrying out slightly larger tests in different passages of the painting. The aim of this part of the process is to determine how the cleaning solution works in different parts of the painting.
The surfaces of paintings are not uniform in their composition; certain traditional pigments – for instance lead white – often form stronger and more resistant paint films than others, such as vermilion red or azurite blue. Other colours might be more vulnerable because the artist may have added a resin (similar to the varnish medium) to the oil paint medium, making it potentially vulnerable.
Furthermore, some parts of the painting might contain a different paint medium altogether: In the seventeenth century it was customary for some painters to use a medium other than the usual linseed oil for specific pigments. It was not uncommon for blues to be painted with walnut or poppy oils, which were less prone to yellowing than linseed. Other painters used a glue-like medium for blues. Investigations into Poussin’s materials have shown that he generally used linseed oil but did employ walnut oil for some blues and occasionally added pine resin to his paint. For this reason, all potentially vulnerable areas of paint need to be tested. Once this has been done it will be time to commence the removal of old varnish and restorer’s overpaints.
With testing out of the way it is now time to commence the varnish removal. It comes as a surprise to some people to learn that the cleaning can be a relatively quick part of the restoration process. With the Crossing of the Red Sea we have anticipated that the cleaning time required will take up only one month of the twelve we have set aside for the treatment, though that will be subject to revision if we encounter any unexpected problems.
Like the testing, the varnish removal itself will take place in stages:
First stage of varnish removal
If the solubility tests suggest the cleaning will not present any particular problem, the first part will involve a first broad cleaning across the painting. The aim of this step is to remove the bulk of the discoloured varnish and restorer’s retouchings, leaving fine residues and hard-to-remove retouchings for a later secondary cleaning. Doing it this way gives you an opportunity to assess the cleaning in a gradual and considered way and allows for adjustments to be made you go along. For example the conservator can vary the proportions of the solvents used as it the varnish thins down.
Second pass of varnish and overpaint removal
The second pass allows the painting to be cleaned to a more refined level of consistency across the surface and allows for any problematic areas to be cleaned on their own terms. Quite often older restorer’s retouchings were painted in oil, making them more difficult to remove. Removing them during a second stage of cleaning enables them to be treated locally.
Sometimes old retouching paint and varnish is resistant to solvent cleaning. When this occurs, they may need to be cleaned mechanically. This is often done manually with the aid of a scalpel, delicately scraping or chipping the non-original components away from the original. This is almost always done under magnification, either with a magnifying loup or else under a stereomicroscope. It can be very time-consuming.
The later stages of cleaning are about ensuring that the painting has been cleaned to a consistent level of finish. Throughout the process the conservator ought to have developed a clear idea of what he or she wants to achieve with the cleaning and what is the best that can be reached given the variables involved: the condition of the original paint layers; how vulnerable they may be to the cleaning solution, the feasibility of removing the varnish and overpaints, the aesthetic implications of removing the non-original components, and so on.
Ideally the final balancing stage of cleaning provides the opportunity for final subtle adjustments to be made to reach the appearance required.