Chemicals and Methods for Conservation and Restoration

Paintings, Textiles, Fossils, Wood, Stones, Metals, and Glass

Johannes Karl Fink

• Publisher: Wiley-Scrivener
• Year: 2017
• Print ISBN: 9781119418245
• eBook ISBN: 9781119418887
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

Before the 1970s, most information concerning the conservation and restoration of paintings, wood, and archaeological artefacts were focused on the history of the artefacts, previous attempts of conservation, and the future use of these artefacts. The technical methods of how the restoration and conservation were made were dealt with only very briefly. Today, sophisticated methods of scientific analysis such as DNA are common place, and this encourages conservators and scientists to work together to work out the development of new methods for analysis and conservation of artefacts.

This book focuses on the chemicals used for conservation and restoration of various artefacts in artwork and archaeology, as well as special applications of these materials. Also the methods used, both methods for cleaning, conservation and restoration, as well as methods for the analysis of the state of the respective artefacts. Topics include oil paintings, paper conservation, textiles and dyes for them, archaeological wood, fossils, stones, metals and metallic coins, and glasses, including church windows.

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 substantia...

Table of contents

• Cover
• Title page
• Copyright page
• Preface
• Chapter 1: Paintings
• Chapter 2: Textiles
• Chapter 3: Archaeological Wood
• Chapter 4: Fossils
• Chapter 5: Stones
• Chapter 6: Glass
• Chapter 7: Archaeological Metals
• Index
• General Index
• End User License Agreement