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ANALYTICAL CHEMISTRY
aid to the archaeologist is isotopic analysis. This has recently been applied to a study of ancient lead by Brill and Wampler {30), who have carried out isotopic analyses on lead ores from ancient mining areas, and on samples of lead from archaeological objects, selected to represent a broad range of materials, periods, and sites. The relative abundances of the four stable isotopes of lead, 204 Pb, 20e Pb, 207 Pb, and 208 Pb were measured using a thermal-emission mass spectrometer. It was found that the isotopic compositions of the archaeological objects tended to fall within three definite groups, which also included galena ores from the best known ancient lead-mining areas, namely Laurion, Southern Spain, and Roman Britain. Although this is in the nature of a preliminary survey, the results obtained have been very encouraging. The technique also has many advantageous features. First, only very minute samples are required; second, any lead-containing artifact, e.g., corrosion products, slags, pigment, glass, or pottery glaze, can be analysed, and third, the isotopic composition—unlike chemical composition—is unaffected by any process to which the lead is subjected in the fabrication of an object, apart from mixing together leads from two different sources. The samples which have been considered so far have been concerned largely with the systematic analysis of groups of objects to obtain analytical results of statistical significance to provide a corpus of information about the composition of various antiquities of different periods. Analysis may also be used, however, to solve specific problems relating to individual objects—or to groups of objects. One such investigation has been carried out recently on the Lycurgus Cup in the British Museum (Figure 2) ; this is an elaborately cut glass cup which exhibits a striking dichroic effect, since it shows a deep magenta color by transmitted light, but has a turbid greenish appearance when viewed by reflected light. The object of the investigation was to obtain analytical information which would make it possible to explain this di-
chroic effect. By a happy chance a chip of glass was found already dislodged when the modern silver-gilt mount on the broken foot of the cup was removed. Two analytical studies were carried out, one by Chirnside and Proffitt {31), and the other by the Corning Museum of Glass. Using a variety of microchemical techniques such as spectrophotometry, flame emission photometry, and X-ray fluorescence spectrometry, it was possible to carry out a complete quantitative analysis on a total sample of less than 9 mg. The glass was found to correspond to a soda-lime-silica glass having a composition consistent with glass of the Roman period, but an unusual feature was the presence of about 0.05 per cent of silver and 0.004 per cent of gold. It is the presence of these minute quantities of silver and gold which is primarily responsible for the dichroic effect, although, as Brill {32) showed, the actual formation of the observed colors depends upon other factors, such as time and temperature of heating, control of furnace atmosphere, and the proportions and oxidation states of certain elements in the glass. These factors were investigated by Brill, who prepared a series of experimental glasses having the Lycurgus composition and succeeded in producing a glass bowl which duplicated the dichroism of the Lycurgus Cup. This is a striking example of how the information provided by analysis can lead to the "re-creation" of an ancient technique. Another ancient technique which has also been investigated by analysis is the nature of a black material called niello, which was used as an inlay on gold and silver objects to fill in an incised decorative design. By X-ray diffraction analysis, Moss {33) showed that the niello used up to about the 10th century A.D. was acanthite (silver sulphide, Ag2S), but that after this date the material used was stromeyeritc (a mixed sulphide of silver and copper, Ag 2 SCu 2 S), sometimes containing also galena (PbS). This change corresponds to an alteration in the technique used for applying niello; acanthite decomposes just above its melting point and had therefore to
