REPORT FOR ANALYTICAL CHEMISTS
Analytical Methods in Archaeology
\
Figure 1 . Mosaic plaque made of colored glasses fused together—possibly made in Alexandria (Size 2.0 X 1.8 X 0.8 cm)
LECTURE GIVEN on October 4,
·**- 1798, b y t h e distinguished Swedish chemist M a r t i n Heinrich Klaproth to the R o y a l Academy of Sciences and Belles-Lettres in Berlin m a y be regarded as marking the starting point of the use of analytical methods in archaeology. I n this lecture Klaproth (1) reported t h e results he had obtained in the analysis of samples of ancient glass. This pioneer work stimulated m a n y other chemists t o carry o u t t h e analysis of antiquities during t h e first half of the nineteenth century. These developments have been admirably reviewed by Caley {2) who gives credit to Gobel (3) for recognizing brass as a R o m a n alloy, and to Wocel (4) for being t h e first chemist to a t t e m p t a correlation between chemical composition and the date and place of origin of a n a n t i q u i t y ; he also draws attention t o a valuable series of papers by Fellenberg (5) on the composition of prehistoric Central European bronzes, and to a book published in 1869 b y Bibra (6), which still remains as a classical source of information on the composition of a variety of antiquities. Another imp o r t a n t contribution which marked the earliest definite liaison between antiquities and analytical chemistry was made by A. H . L a y a r d when he published his book, "Discoveries among the Ruins of Nineveh and Babylon in 1853." This was t h e first archaeological book which included a n appendix giving analytical information about excavated objects. These early analytical researches which were being carried out in the
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ANALYTICAL CHEMISTRY
Α. Ε. Α. W e r n e r Keeper, Research Laboratory, T h e British Museum London, England
nineteenth century were undoubt edly of great interest t o the ar chaeologist, but they tended to be of a rather spasmodic nature and were limited in their scope. The in tensive application of analytical methods in archaeology became possible only with the development of refined methods of analysis on a micro scale, and with the establish ment of museum laboratories, in which a unique opportunity was provided for chemists to apply their specialized knowledge to studying problems about the composition of antiquities in close collaboration with scholars and archaeologists. The problems facing the chemist engaged in the analysis of archaeo logical objects are in general quite different from those in the conven tional research laboratory or in the industrial laboratory, and it is therefore appropriate to consider first of all, in general terms, what are the particular requirements of analytical techniques as applied to archaeology. These m a y be con veniently considered under the fol lowing headings: • Type of Analysis. Here two main categories may be recognized. The first is what m a y be referred to as systematic analysis—i.e., the analysis of groups of antiquities in order t o obtain analytical results on a large number of samples which are of statistical significance, and thus m a y provide information about the sources of origin of raw m a t e rials used in the making of an tiquities, trade routes of the con temporary supply of a given raw material, significant changes in an cient methods of fabrication; or
Figure 2. The Lycurgus Cup (Roman, ca. 4 t h 5th century A.O.)
VOL. 40, NO. 2, FEBRUARY 1968
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REPORT FOR ANALYTICAL CHEMISTS
possibly t h e relative dating of certain classes of antiquities. The second category is t h a t of specific analysis—i.e., the analysis of single objects or small groups of objects in order to obtain information which m a y help to solve special problems relating to the objects in question, including questions of authenticity. • Type of Object. The nature of the object to be analyzed will have an important bearing on the amount of sample which can be taken for analysis, and hence on the choice of the method of analysis to be adopted. I t will be only in exceptional cases t h a t samples large enough for the classical methods of gravimetric analysis will be available; usually the permissible sample may be of the order of only a few milligrams. This means t h a t sensitive physico-chemical methods
Table I.
Oxide
WO
DBT
Si0 2 CaO K20 MgO Al 2 0 3 Fe 2 0 3 Sb 2 0 5 PbO CuO Cu 2 0 CoO MnO Na 2 0
58 5.9 0.3 4.9 2.2 0.4 8.9* 0.2 0.0 — 0.0 0-.05 19
66 4.8 0.5 0-.5 2.2 1.5 1.3 0.1 0.3* — 0.13* 0-.05 23
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ANALYTICAL CHEMISTRY
Results of Electron Microbeam Probe Analyses
Fused mosaic plaq ue DBO LBO RO
YO
X
69 3.7 0.4 0-.5 2.5 0.9 5.8* 0.4 3.7* — 0.0 0-.05 13
53 3.4 1.3 0-.5 1.8 1.9 2.2* 21.5 0.1 — 0.0 0-.05 14
67 5.0 0.4 0.5 2.0 0.4 1.0 0.2 — — — — 23
62 4.2 0.3 0-.5 1.8 2.1 8.1* 0.2 0.8* — 0.05* 0-.05 21
WO—White opaque DBT—Dark blue transparent DBO—Dark blue opaque
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Figure 3. Alleged Roman bronze figure, Shown to be made of pure zinc
of analysis have to be employed, and t h a t special care must be t a k e n to obtain a proper sample, suitable for a given method of analysis. I n certain circumstances objects may be so precious t h a t it m a y not be possible to take any sample at all, and it will therefore be necessary to use so-called nondestructive methods of analysis. • Nature of Analysis. The choice of the method of analysis must be related to the type of problem to be solved. I t m a y be desirable simply t o obtain analytical information about the major elements present in an object, or, on the other hand, it m a y be necessary to determine the concentrations of minor or trace elements in order, perhaps, to establish a correlation between a given pattern of trace impurities and a particular geographical source of origin of a class of antiquity. This means t h a t a number of different analytical techniques have constantly to be used and adapted to particular problems. I n the field of the specialized analysis of antiquities, the chief interest lies not in the actual methods of analysis used, but in the way in which they are used to obtain the necessary analytical data so as to provide information relevant to the different problems of interest to the scholar and to the archaeologist.
54 8.6 0.6 2.3 2.2 1.8 0.8 29.0 — 1.7* 0.0 0.3 0-5
LBO—Light blue opaque RO—Red opaque YO—Yellow opaque
Roman millefiori fragrr ent WO YO RO C 59 5.8 2.4 0.8 3.2 0.4 6.0* 1.6 0.06 — — 1.3 19
59 3.9 1.8 0.8 4.5 2.2 2.8* 16* 0.28 — — 0.8 23
55 4.3 1.8 0.8 4.0 2.9 1.3 4.5 — 1.1* — 1.5 23
65 9.0 2.7 0.8 ~3 1.4 0.8 0.0 0.02 — — 2.7* 15
C—Colorless transparent X—Hypothetical parent glass *—Coloring or opacifying agent
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ANALYTICAL CHEMISTRY
1
T h e range of problems which can be investigated has been greatly extended as a result of the development of modern physico-chemical methods of analysis, which are sufficiently sensitive to give reasonably accurate results using samples of only a few milligrams. A comprehensive survey of the subject would be beyond the scope of the present article; instead selected examples have been t a k e n to illust r a t e the basic principles involved. W e will start by considering some problems involving systematic analysis, in which the techniques t h a t have proved to be of most general application are those of emission spectrography, atomic absorption, X - r a y diffraction, and X - r a y fluorescence analysis. One problem which is of particular interest to archaeologists is the sources of metallic ores used for the production of ancient metal objects. Some investigators have thought t h a t t h e sources of ancient ores could be traced by finding a correlation between the t r a c e impurities found in ancient metals and those present in the various geographical deposits. There are, however, two difficulties inherent in this proposal. First, m a n y ore deposits worked in ancient times m a y be exhausted so t h a t ancient ore material will not be available for comparison, and, second, m a n y ancient metal objects —particularly from later civilizations—were m a d e by remelting scrap metal t h a t m a y have come from a variety of original sources. However, in special cases such correlation between ore and metal m a y be possible. For example Otto and W i t t e r (7) carried out a systematic spectrographic analysis of prehistoric copper and bronze objects from Central Europe, and were able t o demonstrate the presence of a p preciable proportions (in the range 0.3 to 2 per cent) of arsenic, a n t i mony, bismuth, and silver, thus indicating the use of the tetrahedrite ore of the German deposits which characteristically contains all these impurities. A similar a t t e m p t to differentiate the geographical sources of the metal of prehistoric copper and bronze objects over a m u c h wider area in E u r o p e has been m a d e by Junghans, Sangmeister,
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