Cleaning

Drops of wax on the surface of a painting

The cleaning of a painting

The cleaning of a painting involves:
– the removal of superficial drips and splatters, etc.;
– the removal of dirt deposited on the surface and that over time has become partially imbedded in the surface;
– the removal of over-painting and altered or disturbing in-painting;
– the removal (or better stated, the thinning out) of the altered protective layers (varnishes applied over one another and at times the original varnishes) that inhibit or highly limit the correct reading of the work.

The cleaning of a painting

It is easy to comprehend why cleaning is the most delicate and risky operation that can be undertaken on a work of art because of its completely irreversible nature. It is no wonder then that in regard to this particular intervention there have been fierce discussions. In addition to having brought to light the possible damage connected with this intervention, these polemics have made it clear that there do not – nor can there – exist precise norms upon which to grapple. On a case by case basis, one must rely on the competence and the sensitivity of the individual who actually undertakes the cleaning and who will always be – even though comforted by every possible assistance that science can offer – ultimately responsible for the intervention. The problems do not lie solely in the overcleaning or in the maintaining of the patina of a work (two aspects of the cleaning process that are central to the “cleaning controversy”), but are connected to problems that relate to the composition and the structure of the work itself.

Superficial stains before the cleaning

As we have seen, the painting does not always consist of a layer of paint covered by a coat of varnish applied as a protective layer. If this were the problem, it could be easily simplified if not resolved. In reality, the varnish may have not only been applied by the artist himself, but may have been intended to produce a specific effect even over time. In addition, the artist may have applied the varnish to the work even before it was completed. The varnish facilitates the application of glazes and we have knowledge of painters who would “re-touch” their finished works. When colored varnishes were used – which were meant to impart a general tonality and harmony to the work – it is not possible to tell them apart from the painting. In this case, science can not provide any assistance, because the analysis could indicate the chemical identity of the materials used for the glazes as well as for the final coat.

Cleaning of paintings using a solvent

Not withstanding all these problems, we can in any case generalize by saying that the cleaning intervention is carried out by bearing in mind the solubility differences between the original paint and that that must be removed. This is true in the sense that the intervention is substantially carried out by dissolving the substances that must be eliminated. Immediately following this operation the solvation products must be mechanically removed.
A difference in solubility parameters occurs when the materials are different from one another, or, there is a different degree of ageing in like materials. In any case, distinctions are made between weak solvents and strong solvents, but these qualifications can only make sense with reference to a specific substance. Take the case of water for example, it has no solvent power on oil painting, but easily dissolves a watercolor, a gouache or a lean tempera.

Phase of restoration of a painting, cleaning

On the other hand, the composition of varnishes is extraordinarily varied. In the ancient treatises, varnishes based on egg white (glare), waxes, drying oils, gums, and resins dissolved in essential oils are mentioned. It is therefore always necessary to do preliminary tests on the painting to guaranty the harmlessness of the operation. Solvents that can cause not only immediate, but also, damage over time are excluded. For this reason solvents with a long retention time such as glycols, butylamine, carbon tetrachloride, etc. are not used. Water-based solvents must be utilized with great care because many materials could be damaged by the water component. In the use of solvents particular attention must be paid to their relative toxicity and precautions must be taken in order to safeguard the health of the restorer (M.A.C.).

Cleaning of a painting under the microscope

From a methodological point of view, the cleaning intervention should be a gradual process that can be helped along by the use of a binocular microscope, various systems that hold the solvents in suspension, and, mechanical removal with the tip of a scalpel. Additionally, a differentiated approach to what needs to be removed is advised.
In light of the risks faced by the restorer in using of solvents (all organic solvents should be considered toxic), and given their limited selectivity in relation to various materials, cleaning with alternate methods and substances has been investigated and developed over the past twenty years (see enzyme, surfactant, laser).

Manfredi Faldi – Claudio Paolini

Technical note:

During the cleaning operations of a painting paraffin oil  is often the cause of annoying whitening of the pictorial surface. Paraffin oil does not block the action of the various solvents on the paint layer but, by diluting them, it simply slows down their effect. Moreover, sometimes it also facilitates the penetration of the solvent diluted in it into the innermost layers or can act as a vehicle for it. It is therefore often easier to remove the solvent used during cleaning operations by passing a completely dry swab over the surface, always with extreme caution.