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 alloys), lead, tin, antimony, gemstones and paper.

    4. I. Ceramics

    Of all types of archaeological materials, ceramics have received the largest amount of attention in the way of chemical analysis. From the archaeological perspec- tive, ceramics are generally the most plentiful artefact- ual remains, and hence constitute the material by which archaeologists establish cultural and temporal char- acterizations. Hence, they are also the obvious choice for compositional analysis as an aid to the elucidation of archaeological questions.

    4.1. I. Composition The principal component minerals of earthenwares

    are clays, quartz, feldspars, mica and calcite. Some of these may have been introduced as deliberate additions, as in the case of tempers, mixed into the clay to give it better working properties. Sometimes organic tempers (straw or other plant materials) have been added.

    The geochemical formation process of clays has a homogeneizing nature, and therefore the trace element composition of a particular clay deposit tends to be quite homogeneous throughout the deposit. At the same time, this composition reflects that of the parent material from which the clay derived, and thus is characteristic of the geological origin. Depending on the geological variability of the area, very careful and precise analysis can make it possible to differentiate geographically quite close clay sources.

    Table 1 shows the elements typically determined in archaeological ceramics, and illustrates the range of concentrations for each.

    4.1.2. Analytical techniques While clay deposits, as mentioned above, are on the

    whole rather homogeneous, the ceramic material, com- posed of a number of different mineralogical con- stituents, is by nature an inhomogeneous material. Therefore, it is always necessary to determine for any type of ware the minimum sample size which will yield representativity for those elements analyzed. Even then, a natural variation over the elemental concentrations determined in different samples requires the analysis of a number of samples which is relatively large compared to the number of elements analyzed, in order to obtain a statistically significant characterization of the ware. The

    Table 1 Compositions of ancient ceramics

    Major and minor elements

    Concentrations in percent

    Si 25-40 AI 3-30 Ca 0.01-20 Fe 0.2-15 K 0.1-7 Mg 0.1-5 Na 0.05-4 Ti 0.01-1.5

    Trace elements

    As

    Concentrations

    in ppm

    l-250

    B 25-350

    Ba 80-4000

    Br l-25

    Cl 45-3000

    Ce 20-160

    co l-100

    Cr 50-2000

    cs l-150

    CU l-600

    DY l-20

    EU l-5

    Ga 10-55

    Ge 0.5-4

    Ho 0.5-2

    Hf l-20

    I 0.5-8

    La l-100

    LU 0.2-2

    Mn 60-10000

    Nb 20-60

    Nd 20-60

    Ni lo-600

    P 150-6000

    Pb 5-1500

    Rb l-1000

    S 100-2000

    Pr 5-20

    SC l-100

    Sb 0.1-2

    Sm l-20

    Sr 10-2000

    Ta l-10

    Tb 0.5-1.5

    Th 5-50

    u 2-5

    V l-300

    Y 20-100

    Yb l-10

    Zn 10-350

    Zr 10-500

    importance of sophisticated multivariate statistics in the interpretation of the analytical results cannot be over emphasized. While sometimes attempts have been made

    I. ANALYSIS PROBLEMS

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

    to define so-called diagnostic or significant elements, it is in general not possible to predict which elements will be the characteristic ones in the differentiation of par- ticular groups of ceramics. Moreover, not only the absolute concentrations of the individual elements, but also their internal correlations are characteristic for a group. Hence, an analytical technique which allows for the simultaneous determination of a large number of element concentrations is preferable. For each specific problem, subsequent statistical data analysis will then indicate which combination of elemental concentrations may help to yield an answer to the question posed.

    Table 2 Compositions of ancient copper alloys

    cu Sn Zn Pb

    matrix 0.1 ppm-30% 1 ppm-30% 1 ppm-30%

    Two techniques have seen widespread application in multielement analysis of ceramics: Neutron activation analysis and X-ray fluorescence spectrometry. Of these, X-ray fluorescence serves mainly to determine major and minor elements, while neutron activation analysis yields data on a large number of trace elements, espe- cially the transition metals and the rare earths. The two techniques are largely supplementary in nature, and can indeed be used to great advantage in combination.

    PIXE has in common with X-ray fluorescence the property that a nondestructive surface analysis can be performed. This property is of great use in the study of surface decoration, lustre wares and rare and precious porcelains, which obviously cannot be sampled.

    4.2. Metals

    The copper alloys, and in particular the ancient bronzes, have been the subjects of. numerous research projects. Work on the precious m&s, gold, silver, and its alloys, generally requires the use of nondestructive techniques. Lead, tin and antimony, while they have not yet been the subject of many extensive analytical stud- ies, have equal potential value as indicators of trade and cultural exchange patterns in antiquity.

    Ag Al AS Au Ba Bi Ca Cd co Cr Fe In K Mn MO Ni P Pd Pt Rb S Sb Se Si Sr Te Ti V Zr

    1 ppm-5% 1 ppm-5% 1 ppm-10% < 500 ppm 0.1-500 ppm 0.01 ppm-5% 20 ppm-5% 0.1-100 ppm 1 ppm-5% l-2000 ppm 10 ppm-5 % 0.1-100 ppm 10 ppm-3% l-5000 ppm 0.1-500 ppm 1 ppm-5% 1 ppm-0.5% 0.1-20 ppm 0.1-100 ppm 0.05-1000 ppm 10 ppm-3% 0.01 ppm-10% l-2000 ppm 1 ppm-0.5% 0.5-500 ppm 0.1-500 ppm 1 ppm-0.1% l-100 ppm 0.1-100 ppm

    4.2A. Copper alloys

    4.2A. 1. Composition

    technology, procurement sources for the raw materials and trade and exchange patterns. The interpretation of trace element concentrations as a means to characterize materials sources is severely complicated however through the changes effected by the reduction and alloy- ing processes.

    While objects made out of these metals, prepared by melting, are obviously more homogeneous than ceramics, often segregations take place after the solidification of the metal. These can be major, with a scale of the entire object, as for example in the case of leaded bronzes, where the insolubility of lead in the bronze matrix results in the formation of lead inclusions, but also of a smaller scale, such as compositional variations over dendrites or along grain boundaries. Another complica- tion in the case of ancient objects lies in the corrosion, which often penetrates deep into the metal, and the products of which have an elemental composition tot- ally different from it.

    Surface analyses can also contribute important infor- mation on joining techniques (soldering, brazing), and surface treatments, such as surface enrichments or patinations, hardening, gilding (leaf or amalgam), appli- cations of thin layers of other metals (silver, tin, lead), and inlays.

    Table 2 gives an overview of the elements commonly analyzed in ancient copper alloys, and the range of concentrations encountered. Analysis of major and minor elements generally allows classification not only according to alloy type, but also by characteristic im- purities.

    As in the case of ceramics, the composition of the 4.2A.2. Analytical techniques metal can provide information on ancient metallurgical Mass spectrometry seems the most sensitive multiele-

  • Ch. Lahanier et al. / Study and conservation of museum objects 5

    ment technique used in analytical studies of ancient bronzes: elements can be determined with this method which, not~thstanding its rather poor precision (308), seems quite succesfully applicable in these studies.

    Optical emission spectroscopy, in its more classical version which was used in most published bronze stud- ies, is less sensitive although it offers better precision. It can be expected that the newer plasma source emission spectro~aphs will change this picture significantly. The use of a laser microprobe allows, through successive shots at the same place, to establish a depth profile for a number of elements. Such information is useful in the evaluation of patinas.

    Neutron activation analysis is less often used, both because of the strong activities induced in some major elements and the inabilitiy to activate others (e.g. lead).

    When metallurgical specimens can be prepared, the electron microprobe is very useful in determining varia- tions in composition as a function of depth beneath the surface, as well as for the analysis of inclusions or inhomogeneities aiong grain boundaries. The proton microprobe, with its much larger sensitivity, should provide similar information with much greater detail.

    4.2 B. Silver

    Analysis of silver objects serve to yield inf~~rmation analogous to that already discussed under copper alloys. In addition, silver is an important coinage metal. Prob- ably no ancient objects have been the subject of so many and intensive analytical studies as coins. These studies concern especially the degree of refinement or adulteration (debasement) of the coin metal, and hence the relative values of historic valuta. Also, because coins can carry mintmarks, which can locate the place of their manufacture, they may be of use in source characteriza- tion studies.

    Strict control of alloy composition, of self-evident importance for coinage metal, also became the norm for silver alloys used to manufacture decorative art objects (e.g. Brittania, Sterling). Analysis of alloy composition may therefore help in establishing period and country of manufacture,

    The silver alloys, especially the common Ag-Cu alloys, can present problems with regard to heterogene- ity. Surface enrichment of silver (and the natural gold impurity), either intentional or resulting from burial, often makes accurate analyses through direct, nonde- structive surface methods, such as XRF, impossible. Surface enrichment layer thickness can extend down to about 200 pm: hence even a more penetrating exci- tation such as in the case of proton activation analysis stilt can yield seriously erroneous results.

    Phase separations in silver alloys also complicate analyses of very small samples. Thermal neutron activa-

    Table 3 Compositions of ancient silver

    As

    AU

    Bi Br

    Ca

    Cd

    CO

    cu

    Fe

    Ga

    Ge

    *g I

    In

    Ir

    Mn

    MO

    Na

    Ni

    Pb Pd Pt Re RU Sb Se Sll

    Te Ti V W Y

    ZFI

    Zr

    O.l- 100 ppm

    0.1 ppm-5%

    1 ppm-2%

  • 6 Ch. Lahanier et al. / Study and conservation of mweum objects

    Proton and neutron activation analyses have also been used extensively, often nondestructively, with activation of the whole object, as especially in the case of coins. Neutron activation analysis also has been used with extremely small samples. For the determination of some ultra trace elements, such as the noble metal Ir which can be present at concentrations down to 1 ppb, chemical separations are necessary after the activation.

    4.2C. Gold

    4.2C. I. Composition Gold is a quite homogeneous material, except when

    alloyed with copper in excess of 25%. Ancient gold contains significant amounts of silver, which in the case of the natural alloy electrum can be of the same order as the gold content.

    Refinement techniques for gold, by cementation or cupellation, had by Roman times been improved to a level where gold with a fineness comparable to modern industrial gold (about 99.95%) could be obtained. Only the noble metals are not affected at all by the refine- ment, but stay present in the gold in unchanged relative concentrations. Therefore they offer the best approach for provenance studies. Platinum concentrations in Ro- man gold coins, for example, indicate a change in gold source around 346 B.C. However, the concentrations of most noble metals are rather low and make analysis by nondestructive techniques extremely difficult.

    The high value of gold resulted in large scale remelt- ing, especially of coins. This, of course, highly com- plicates the interpretation of elemental composition pat- terns for provenance studies. With gold jewelry, where the esthetic value surpasses the intrinsic one, this may happen less often; however, the possibility must be considered that often remelted coin metal was used for the manufacture of the object itself.

    While this noble metal does not corrode easily under normal conditions, often a red surface accretion is observed on ancient gold objects which seems to be the result of a surface alteration. While this product has as yet not been identified satisfactorily, its appearance is often accepted as an argument in favor of authenticity.

    Solder joints are other indicative areas. Modern gold solders contain significant amounts of cadmium; the presence of such elevated amounts of cadmium is there- fore generally regarded as an indication of a modern repair or worse.

    4.2C.2. Analytical techniques Because especially on small, delicate objects such as

    jewelry sampling is impossible, and trace element analy- sis of the significant noble metals is generally also very complicated with nondestructive techniques, little such work has been done on these objects so far.

    Analyses on coins with proton or alpha activation

    analysis have produced nondestructive determinations at quite high sensitivities.

    4.3. Gemstones

    While artificial gemstones can be made at high tem- peratures and elevated pressures, obtaining the same chemical composition and physical structure as of natu- ral ones, differentiation can be made through detection of the presence of natural trace impurities. Where, how- ever, sampling is out of the question in situ techniques could be very useful in addressing authenticity ques- tions. Moreover, trace element analysis and analyses of inclusions could be of great interest for provenance studies. Analyses of small remains of polishing com- pounds again could help with authenticity studies.

    4.4. Sifice0ri.r materials

    4.4. A. Glass

    4.4A. I. Composition Glass, an isotropic, amorphous material, is com-

    posed of a silica matrix with fluxes (plant ashes, natron, lead oxide, calcium carbonate, borax), while other ele- ments are introduced with the coloring agents (e.g. Cu, Fe, Co, Ni, Zn, Sn, Sb, Pb, Mn), decolorants (Sb, As. Mn), opacifiers (Sn, Sb, Pb, As) and fining agent (As).

    The technology has evolved over time: mold cast in Mesopotamia and Egypt, then mold blown (1st century B.C. in Syria), free blown in Roman times, flat window glass (3rd Century A.D.) and stained glass in Medieval Europe. Besides these different forming processes, there are other technological variations in decoration (engrav- ing, carving, etching, gilding, painting). Glass can easily be affixed to another substrate and serve as a decorative element, as in ceramic glazes or enamels on metal substrates.

    Over times and geographic localities the composi- tion, with regard to fluxes, decolorants, opacifiers and colorants, has shown great variations. This is reflected in the range of values from a small selection of pub- lished analyses shown in table 4. While of course very fragile, glass tends to be chemically reasonably stable. Degradation starts on the surface, with devit~fication and hydration processes, which among others can result in the characteristic visual effect of iridescence, often encountered on archaeological glass.

    4.4A.2. Analytical techniques Much analytical work has been done using tech-

    niques such as neutron activation analysis, optical emis- sion spectroscopy, atomic absorption spectrometry, X- ray fluorescence, and electron microprobe analysis. The results have been especially useful in the study of tech- nology involved in glass production, as it has evolved in

  • Ch. Lahanier et al. / Study and conseruation of museum objects 7

    Table 4 Compositions of ancient glasses

    Major elements

    Si Al Ba Ca Fe K

    Mg Na P Pb Sn

    Concentrations in percent

    20-40 0.2-7 0.01-10 0.1-20 0.01-12 0.01-20 0.01-5 0.1-20

    0.05-5 0.01-30 0.01-7

    Minor and Concentrations trace elements in ppm

    f% OS-10,000 AS 5-5000 Au 0.01-0.15 Br 1-8 Cl 100-2~~ Ce l-15 co 0.5-2000 Cr 10-1200 cs 0.1-1.5 cu 100-60000 Eu 0.02-0.5 Hf 0.2-l La 0.8-2 Mn 0.01-15000 Ni 10-2000 Rb l-500 S 10-5000 Sb 0.540000 SC 0.3-I .5 Sm 0.2-0.3 Sr 10-2000 Th 0.3-2 Ti 1000-5000 V IO-200 Zn 10-15000 Zr 10-500

    different areas over time. Much more work remains to be done in this area, however.

    Ion beam techniques may contribute in this field by the improvements which they can offer for the analysis of light elements. Nondestructive applications will be limited by the severe surface effects on ancient glass. However, for the same reason they would be extremely helpful in conservation science studies of the alteration processes on the surface, both when these are a result of exposure to natural conditions or to air pollutants.

    4.4B. Obsidian

    This natural glass of volcanic origin contains, next to silicon, major amounts of sodium, aluminum, potas- sium, calcium and iron, as well as minor quantities of titanium and manganese. The trace elements are excel- lent indicators of geographic provenance, and, because

    Table 5

    Compositions of obsidians

    Major elements

    Si Al Ca Fe K Na

    Ti

    Concentration in percent

    25-40 5-12 0.1-6 0.1-10 0.5-6 l-5 0.04-2

    Minor and Concentration trace elements in ppm

    As l-250 Ba Br Ce co Cr

    cs cu Eu F Ga Cd Hf In La LU

    Mg Mn Nb Nd Ni

    Pb Pr Rb Sb SC Sm Sr Ta

    Tb Th Tm U Y Yb Zn Zr

    5-2500 0.1-8 10-350 0.1-2 =7 O-50 I-30 0.4-4

    300-5000 5-70 O-38 l-50 = 0.04 20-150 0.2-3 l-2500 100-3000 I-300 S-120 2-25 1-85 I-50 40-700 0.2-2 0.2-10 3-30 I-1250 I-20 0.3-5 I-100 f 0.3 l-5 O-200 I-50 lo-600 15-2200

    I. ANALYSIS PROBLEMS

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

    this material was a major trade item, trace element analysis can, analogously as with ceramics, be used for studies of trade relations and exchange patterns.

    Many techniques have been used in numerous stud- ies of artefacts from both the Old and New World, with neutron activation, X-ray fluorescence and PIXE analyses as the most suitable ones, because of the rela- tive speed and the multielement nature, for projects involving large amounts of samples. Of these, neutron activation analysis offers the greatest sensitivity and largest range. Typical composition ranges, as reported in some representative publications, are summarized in table 5.

    The hydration of obsidian, although the kinetics depend on local environmental conditions which are difficult to evaluate, can nevertheless be studied for dating purposes. NRA has been used to great advantage for such analyses.

    4.4C. Stone

    Notwithstanding the obvious interest in characteriza- tions of especially sculptural stones, such as marble, only a limited amount of work has been published in this area. Trace element characterizations of these materials do not always succeed, due to their hetero- geneity. Nevertheless, some very succesful research has been published, such as on characte~zations of scuipt- ural limestone and Mezoamerican jade.

    The heterogeneity that the metamorphical formation of marble induces is very large; indeed this heterogene- ity has been used to match broken pieces on the similar- ity of the distribution patterns. Consequently other techniques, especially analysis of the ratios of stable isotopes of carbon (*C/i3C) and oxygen (6O/8O}, are often preferred for provenance studies of this im- portant material. Even so, some successful studies of marble provenance through neutron activation analysis of trace element concentrations, followed by sophisti- cated multivariate statistical analysis of the data, have been reported.

    The size of sculptures often prohibits the transport of the objects to a laboratory; in general samples must be removed. Nondestructiveness may therefore not be the most urgently required property for techniques used in studies of this material except if the analytical tool is inside the museum.

    4.5. Paper and ink

    The chemical composition of paper from different countries and time periods reflects a technological evolution with regard to the materials used for produc- tion (wood, plant fibers, textile fibers) and to the physicochemical treatment of the pulp: addition of fillers (ashes, sodium carbonate, kaolin, talc, calcium

    Table 6 Composition of some ancient and modem papers

    Element Concentration in ppm

    Na 30-4200 M!iC 200-9000 Al 48-17000 Cl 50-9400 K 16-12000 Ca 5~-~4~ SC 0.01-12 V 0.02-4 Cr 0.1-32 Mn 0.5-210 Fe 21-3300 CO 0.02-4 CU Z-110 Zn 5.8-450 AS 0.04-1.7 Br 0.4-75 Sb 0.01-5 Ba 2-170 La 0.06-6.1 Sm 0.006-1.5 AU 0.012-60

    Hg 27-45

    carbonate), bleaching (CaOCl,, NaOCl), washing, etc. Table 6 shows some typical ranges of elemental con- centrations. Very little seems to have been done in the way of systematic studies of drawing media (pencil, crayon, chalk, metal point. etc.).

    A study performed in Japan with neutron activation analysis succeeded in differentiating modern Japanese, Korean and Chinese papers, as well as 18th and 19th Century Japanese and European papers.

    PIXE has been used for a microanalysis ((?I < 1 mm) of the printing ink composition on a few hundred pages of a Gutenberg bible in the collection of the Edward Laurence Doheny Memorial Library. The results enabled the reconstruction of the work organization in the printshop. With a beam intensity of less than a nanoampere it was possible to do the analysis without damage to the paper.

    4.6. Amber

    Although infrared absorption spectrometry has proven an inexpensive and sufficient tool for the pur- pose, trace element analysis. through neutron activation analysis has also been used to differentiate Baltic and Sicilian amber on Au and Na levels.

    4.7. Paintings

    Generally paintings consist of a support (linen, wood panel, metal panel), coated with a ground layer (e.g.

  • Ch. Lahanier et al. / Study and conservution of museum objects 9

    calcite or gypsum applied in a glue medium), on top of which the paint layers consisting of natural or artificial pigments in an organic binder (oil, tempera, synthetic resins) have been applied. Often the painting is coated with a varnish (natural or synthetic resins). The com- positions of all these materials vary with period and locality.

    Elemental analysis is especially useful for a inferred identification of pigments. Energy dispersive X-ray fluo- rescence is often used for this purpose, because this allows a nondestructive analysis. Most other work in technical studies of paintings does not entail analysis of

    elemental composition, but rather of chemical com- pounds, both inorganic and organic.

    A possible cont~bution from the use of ion beam techniques could be in the analysis of individual pig- ment particles in a prepared sample, studying for exam- ple the minor and trace element patterns in certain pigments from different periods and locals. Some suc- cessful work along these lines has been done by neutron activation anatysis of lead white. The study of sections with microprobes could yield information on the layered structure of paintings.

    I. ANALYSIS PROBLEMS

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