Art conservation chemistry

Art conservation chemistry The application of chemistry to the technical examination, authentication, and preservation of cultural property. Chemists working in museums engage in a broad range of investigations, most frequently studying the chemical composition and structure of artifacts, their corrosion products, and the ma­terials used in their repair, restoration, and conservation. The effects of the museum environment, including air pollutants, fluctuations in temperature and relative humid­ity, biological activity, and ultraviolet and visible illumination, represent a second major area of research. A third area of interest is the evaluation of the effectiveness, safety, and long-term stability of materials and techniques for the conservation of works of art. Though analytical techniques appear to dominate, many other areas of chem­istry, biology, physics, and engineering, including polymer chemistry, kinetic studies, imaging methodologies, biodegradation studies, dating methcx:ls, computer modeling, metallography, and corrosion engineering, play active roles in conservation science.

Methods of examination may be divided into two classes: those that provide an image of the entire object (holistic examination) or a section of it; and those that provide an analysis at a point on the object, with or without sampling. Nondestructive methods, not requiring sampling, are always preferable. However, modern methods of analysis can be employed on such minute samples that they are in effect nondestructive. Jn some cases, samples must be taken for methods that are in principle nondestructive because the object is too large to fit into a sample chamber. The ability to analyze minute samples introduces the serious concern that the sample may not be representative of the composition of the artifact but may be an inclusion or contaminant introduced by the experimentalist. With specimens from painted surfaces, great care must be taken to identify areas of restoration.

A further concern arises from the differing depths from which signals originate. On a metal surface, ion scattering spectrometry (ISS) would see the initial fraction-of-a­nanometer, predominantly adsorbed species and contaminants. Secondary ion mass spectrometry (SIMS) would begin to penetrate the oxidized area; Auger electron spec­trometry (AES) would examine the bulk of the oxidized layer; and x-ray-induced pho­toelectron spectrometry (XPS) would give data on the bulk sample some 10 nm below the specimen surface.

The most commonly employed holistic method is x-ray radiography, where vari­ations in the density and average atomic number of the sample attenuate an x-ray beam, leaving a negative image on film. Other methods, such as ultraviolet and in­frared reflectance and fluorescence, are used to show areas of compositional difference indicating restoration or variation in the pigments used by the artist.

In the examination of paintings, small samples are taken under the binocular mi­croscope, embedded in transparent resin, and polished to produce a cross section for microscopic examination. This permits a study of the artist’s painting technique and shows how several layers may have been built up to achieve a desired effect. Con­servation studies of the composition and technique embrace the entire spectrum of modern chemical analysis. \

The separation of the fake from the authentic is a small but often spectacular aspect of the technical examination of artifacts. In some cases, direct age determination (den­drochronology for panel paintings, fission track dating for uranium glass, radiocarbon dating for organic materials, thermoluminescence dating for ceramics) is possible.

More commonly, the issue of authenticity turns upon anachronisms in composition or technique when the artifact in question is compared to accepted artifacts of the period. Thus, the greater part of the work in the conservation laboratory concerns the building of databases of analyses of composition, trace-element distributions, and studies of technique.

Many artifacts are sensitive to destructive agents in the museum atmosphere. Rapid changes in relative humidity will cause dimensional changes in wood furniture, poly­chrome sculpture, and panel paintings, leading to cracking and splitting of the wood with loss of painted surface decoration. High relative humidity can lead to mold growth and foxing on books and prints, while low relative humidity will cause photographic prints and films to become brittle.

Oxidation of iron objects, tarnishing of silver plate, and the development of corrosion products on lead artifacts by the action of formic and acetic acids emitted by wooden display cases have regularly been observed in museums.

The common air pollutants sulfur dioxide (S0₂) and ozone (0₃) have been mon­itored at elevated levels in museums, libraries, and archives. These pollutants cause the degradation of leather, spotting of photographic prints, and fading of dyes and pigments. Chemical methods of analysis are used to identify degradation products and to study the kinetics of degradation mechanisms. Specialists in air-pollution monitoring use analytical instrumentation to measure ambient pollution levels in museums.