Chapter 1
Paintings

1.1 Cleaning

Historically, artists have protected oil painting surfaces with varnish. This is a system that allows the varnish to be brushed clean or even washed relatively frequently to remove accumulated surface dirt without exposing the paint to risk (1).

Unfortunately, mastic or other traditional soft-resin varnishes do not last indefinitely. After a few decades the varnish becomes yellow and brittle, losing transparency, and the cleaning process is transformed into the more challenging problem of removing the degraded varnish directly from the painting surface.

Even when new, a varnish may change the appearance of a painting. The varnish increases the transparency of any partly coated pigments or low refractive index medium, and also it imparts a new surface, which is frequently glossy. Mostly, artists have accepted such immediate changes in appearance for the future benefits of protection from dirt and from the risks of dirt removal.

By the eighteenth and nineteenth centuries, when state academies controlled much professional painting practice, the need for a varnish became important.

The concept of finish embodied many notions and became an unwritten contract of quality and reliability between academician and purchaser of art. It seems likely therefore that professional artists and their clients or patrons have always considered the application of varnish as a necessity of permanence and that artists have chosen to exploit its properties for both visual and practical benefit.

Many artists, through ignorance or untidy practice, continued painting up to exhibition deadlines and then immediately brushed varnish onto undried paint. A soft-resin varnish, such as mastic, was mixed into a paint to improve the short-term handling properties. Painting was even continued after varnishing. Adding a soft natural resin to oil paint remained popular into the middle of the 20th century (2).

Annual spring cleaning can be simply done by brushing or vacuuming dust from a varnish. However, washing with water is more effective and may need to be done only every decade or two decades. This procedure requires a wetting agent to ensure a good contact with the varnish surface and to trap dirt within the surface of the liquid.

Traditional recipes using potatoes and onions are well known (3). Saliva is still considered effective. Many other materials have been recommended, including borax and urine.

Conventional varnishes are most susceptible to UV radiation, air pollution, and moisture, and as the varnish ages, it becomes more polar and brittle and more soluble in aqueous mixtures. Aqueous methods for cleaning have been described in a monograph (4).

The varnish surface and, eventually, the body of the varnish disintegrate under the action of repetitive cleaning. Wax or poppy oil coatings can be applied to impregnate the varnish surface to extend its life, but opacity and yellowing may destroy its optical qualities (3).

Perhaps two generations will have passed since anyone saw the painting through a clear fresh varnish. The removal of a well-oxidized mastic varnish from a thoroughly dried oil film using spirits of wine has been carried out for centuries (5, 6).

Alternatives to solvents have been favored by Wolbers (7). The cleaning of paint surfaces is done by using surface active agents in water-based systems. This can be effective in removing oxidized varnishes and oil varnishes as well as dirt. The formulations proposed by Wolbers have provided new tools to remove stubborn material more controllably (1).

1.1.1 Special Considerations

With the rapid developments in new cleaning techniques and analytical techniques it is important and necessary for the conservation community to constantly remind itself of the debate surrounding cleaning. In modern times, this debate began with the National Gallery of London cleaning controversy of 1947 (8). A scientific examination for art history and conservation has been published (9-11). The (surface) cleaning and the removal of varnishes are arguably the most controversial and invasive restoration interventions that a painting will undergo.

Doerner, already in 1921 published warnings about the damage that could be caused by solvents and cleaning (2, 8): The origins of the profession of painting restoration in France have been reviewed (12).

There are countless cleaning materials, most of which are the secret of a particular conservator. One cannot believe all the possible types of materials which are applied to paintings. The strongest caustics, acids, and solvents are used without a second thought. Solutions with unknown composition, so-called secret solutions, are recommended to the public, as something anybody without any knowledge can use to clean pictures. Such cleaning methods are often too successful, right down to the ground layers. In those cases, the conservator covers up his sins by retouching.

It is not uncommon that such locations appear cleaner to the unknowing public than the older version. Even to this day there are conservators who, in all seriousness, claim that they have cleaning materials which remove new paint but stop at the real, original layers. The only thing missing is that a bell should ring when the original paint layer is reached.

The use of balsams for cleaning paintings, in particular copaiba balsam, was fashionable until the end of the 19th century. However, the effect of this balsam was devastating and catastrophic, especially on oil paintings (13).

Copaiba balsam is a resin now known for its softening properties that remain active over a long period of time. An original paint layer treated with copaiba balsam is thus much more sensitive and subject to future damage than prior to the intervention. It is to be noted that commercial solutions such as Winsor and Newton Artists' Picture Cleaner still contain copaiba balsam (8).

1.1.2 Oxalate-Rich Surface Layers on Paintings

Oxalate salts have been the subject of extensive research as alteration products on calcareous substrates, e.g., stone and fresco. However, there has been relatively little notice concerning their occurrence on other objects such as easel paintings (14). The conservation of easel paintings has been reviewed (15).

An understanding of these materials is important since they can be responsible for significant changes in the surface appearance of artworks and the solubility of the matrices where the oxalates are formed.

Altered, oxalate-rich surface layers can causes substantial challenges for the visual interpretation of the painted surfaces.

Oxalate-containing layers or deposits have been reported on a variety of noncalcareous substrates, including glass (16, 17), bronze (18-20), human remains such as mummy skin (21), and polychrome wood (22) and easel paintings (23-25).

The oxalate salts of calcium, whewellite (calcium oxalate monohydrate) and weddellite (calcium oxalate dihydrate), are those most commonly encountered on painted surfaces, although copper oxalates have also been identified in paint layers containing copper pigments.

Mostly these compounds have been found in deteriorated organic surface layers. Biological and chemical mechanisms have been proposed for the formation of oxalate films on artworks (26).

In the paintings studied in the Philadelphia Museum of Art, the oxalate minerals may likely derive from a gradual oxidative degradation of organic materials in the surface layers and their reaction with calcium-containing pigments or particulate dirt.

The resistance of the calcium oxalates to organic solvents and other cleaning agents presumably affects their enrichment on the surface (14).

1.1.3 Leaching

The cleaning of unvarnished paintings is one of the most critical issues. Several studies exist regarding different cleaning tools, such as gels, soaps, enzymes, ionic liquids, and foams, as well as various dry methods and lasers, but only a few have been performed on the risk associated with the use of water and organic solvents for the cleaning treatments in relation to the original paint binder (27).

The behavior of water gelling agents during cleaning treatments and the interaction of the following elements have been assessed: Water or organic solvents used for the removal of gel residues with the original lipid paint binder.

The study was conducted on a fragment of canvas painting from the 16th to 17th century of Soprintendenza per i Beni Storici, Artistici ed Etnoantropologici del Friuli Venezia Giulia, Udine, by means of Fourier transform infrared (FTIR) spectroscopy, gas chromatography (GC)/mass spectroscopy (MS), and scanning electron microscope (SEM) (27).

1.1.4 Removal of Dirt

The removal of dirt from an unvarnished paint surface may be very challenging, in particular, when the deposit is patchy and resilient; besides which, fragile unvarnished underbound paint surfaces are sensitive to aqueous solvents.

When the dissolved dirt may have impregnated the paint surface irreversibly, nonsolvent cleaning methods are necessary (28).

Dry surface cleaning uses a large range of specific materials like sponges, erasers, malleable materials, and microfiber cloths. However, these materials have not yet been fully integrated into the current practice of conservators. Only a few studies have focused on the use of dry cleaning materials in conservation. Most of the studies have focused on textile and paper conservation (29-32).

The testing methodology and results of dry cleaning materials on underbound and...