Study and conservation of museum objects: Use of classical analytical techniques

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  • Nuclear Instruments and Methods in Physics Research 814 (1986) 1-9 North-Holland, Amsterdam

    Section I. Introduction: ana~~t~c~~ problems in art and archaeolo~

    STUDY AND CONSERVATION OF MUSEUM OBJECTS: USE OF CLASSICAL ANALYTICAL TECHNIQUES

    Ch. LAHANIER

    Laboratoire de Recherche des Mu&es de France, Pa&s du Louvre, 75041 Paris Cedex OI. France

    F.D. PREUSSER

    J. Paul Gerry Conservation Instirute, P.O. Box 2315, Santa Monica, CA 90406, USA

    L. VAN ZELST

    Conseruakm Analyrical Laboratoq>, Museum Support Center, Smithsonian Institution, Washingron, DC 20560, USA

    The study and conservation of museum collections calls for the application of scientific methodology in the examination, analysis and dating of objects. Because the nature of these objects makes sampling undesirable, and often even impossibte, museum laboratories ~ntinuousIy look out for new nondestructive techniques. which can be adapted for their use. Chemical analysis of museum objects, i_ncluding trace element analysis, can provide information regarding provenance as well as the technology employed in the preparation of the materials and the manufacture of the objects. A very sensitive and precise analysis may thus provide a chemical fingerprint of an object, a school or a cultural group. On the other hand, detailed and precise chemical analysis can also provide insight into the processes involved in the deterioration of the materials of museum objects. Analytical requirements for the various types of materials, of interest in museum research are reviewed along with the classical analytical techniques widely used in

    museum research.

    1. ln~~~etion

    The analysis of objects in museum collections can be required to address a variety of questions, relating to the areas of conservation, authenticity, history of technol- ogy, or archaeometry. In these contexts, analyses may serve for materials identification, characterization, or study of their ageing behaviour.

    Laboratories engaged in the preservation and study of cultural patrimony have to pay at least as much attention to the specific, but highly urgent needs for analytical and other scientific support of the conserva- tion effort, as to the enrichment of our knowledge of the past through the study of the objects which are our material witnesses of earlier civilizations.

    In what follows we review the main requirements, from the museum stand point, on analytical techniques which may be used in museum research. We also consider briefly the classical techniques already in wide use. This review may serve as a starting point to assess the interest of the more recent MeV ion beam based techniques in museum science, to which these proceed- ings are devoted.

    2. Analysis in the service of the collation of the cultural heritage

    The conservation of works of art and archaeological objects consists of three main areas: diagnosis, preserva- tion and restoration.

    Diagnosis requires the technical and analytical ex- amination of the object in question. Goals of this ex- amination are: - to determine the causes of deterioration and the

    mechanisms involved; _ to determine changes in appearance and physical

    properties due to ageing processes; - to determine which parts of an object are original

    and which are later additions. The determination of the chemical composition of

    efflorescence on stone, glass, ceramics, leather, paper and many other materials, corrosion layers on metallic objects, incrustations on excavated objects, surface de- posits on wall paintings, and color changes in painted surfaces are only some examples of problems posed by the objects iii our care.

    While in many cases the information needed can be

    0168-583X/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

    I. ANALYSIS PROBLEMS

  • 2 Ch. Lahanier et al. / Study and conservation of museum objects

    obtained by examination of the object with a micro- scope and various kinds of electromagnetic radiation (X-rays, UV, visible, IR, ultrasonic, microwave) it often becomes necessary to determine the chemical composi- tion of various parts or layers of the object. Depending on the individual case this may involve: - qualitative and/or quantitative analysis of organic

    components; - qualitative and/or quantitative analysis of inorganic

    components: main elements and phases, minor elements and phases, trace elements, isotope ratios.

    Methods applied to the examination and analysis of cultural objects should be nondestructive whenever pos- sible. This means that any technical examination of an object and the analysis of its chemical composition should be done without taking samples from the objects and without changing its composition and appearance. If samples must be taken they should be as small as possible and the preferred analytical methods are those that leave the samples intact for future studies with other methods. Even with the available methods that allow the analysis directly at the object (without sam- pling), it frequently becomes necessary to take samples. Since most of the nondestructive methods are limited to the surface of an object (l-4000 pm depth), often corrosion layers will make it impossible to get accurate information on the chemical composition of the object. Furthermore, many objects are built up in layers and it may be essential to obtain information on lower layers which will then require sampling. Meanwhile, some methods allow depth-profiling and it will be essential that these methods be developed further.

    While methods for the nondestructive analysis of the inorganic components of art historical and archaeologi- cal objects are much more developed than those for the analysis of organic components, further progress is needed. In the field of organic analysis one will have to look for methods which have not been applied yet to the examination of works of art and archaeological objects. The modification of existing methods for organic analy- sis to make then applicable as nondestructive methods is one of the challenges of the future.

    Another limitation of many available methods is the fact that the objects often have to be brought to the equipment. This is frequently prohibited by the size of the object, its value or the distance entailed. Addition- ally, economic considerations also frequently prohibit certain kinds of analyses. Modification of existing equipment or development of new equipment which can be used in situ is another field of development on which we will have to concentrate in the upcoming decade. The development of portable analytical equipment with

    low operational costs could represent a major breakthrough in the fields of conservation science and archaeometry.

    3. The chemical composition as criterion for characteri- zation, classification and authenticity

    The scientific analysis of museum objects, which are composed of a wide variety of materials, ideally requires the availability of methods which are simultaneously: nondestructive, respecting the physical integrity of val- uable and irreplaceable objects; fast, so that large num- bers of objects from archaeological excavations and from museum collections, the latter often with little known archaeological context, can be analyzed com- paratively; uniuersal, i.e. applicable to many materials and objects of any dimension; versatile i.e. able to give both highly localized analyses of microscopic areas, and average bulk analyses of heterogeneous materials; sensi- tive and multielemental to give a maximum of informa- tion.

    The chemical elements in ores, clays or rocks are often characteristic, by their concentrations or isotope ratios, of the geology of the area of provenance. Many of these same elemental relationships continue to exist in the objects made from these raw materials, and hence can serve as a criterion to order them into chemically consistent groups, and to distinguish them from objects produced in a different region. Combination of exten- sive geological prospecting, and the analysis of source materials, with the analyses of museum objects, has made it possible in many cases to identify ores or quarries involved in the manufacture of these objects.

    Of equal importance, chemical differentiations and separations which occur during refinement and prepara- tion of materials, as well as deliberate additions (e.g. alloying), will affect the elemental composition of the materials in the object. Hence, analytical results can contribute to the understanding of the technology in- volved in the production of materials and objects.

    The characterization of materials through a chemical fingerprint requires the use of sensitive analytical techniques. A sensitivity gain of a few orders of magni- tude allows the detection of many more elements, which can serve as classification criteria. The interest of analyses of major and minor elements as well as impuri- ties, present in trace quantities, is evident from the number of scientific publications over the last twenty years, abstracts of which have appeared in the Art and Archaeology Technical Abstracts.

    The results of a selection from this work will serve to illustrate the typical concentrations which have been measured and the variations in chemical classification criteria.

  • Ch. Lahanier et al. / Study and conservation of museum objects 3

    4. Chemical composition of the most important archaeo- logical materials

    Intensive archaeometrical studies have been made on ceramics and copper with its alloys (bronze, brass, etc.). Other studies have been directed at glass (including enamels and mosaics), stone (limestone and marble, obsidian, chlorite and steatite), noble metals (gold, silver and

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