THE INFRARED SPECTRA OF AMBER AND THE IDENTIFICATION OF BALTIC AMBER

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<ul><li><p>THE INFRARED SPECTRA OF AMBER AND THE IDENTlFICATION OF BALTIC AMBER </p><p>BY C. BECK, E. WILBUR, S. MERET, D. KOSSOVE AM) K. KERMANI Department of Chemistry, Vassar College, Poughkeepsie, New York </p><p>INTRODUCI'ION Until well into the second half of the nineteenth century amber found at </p><p>archaeological sites anywhere outside the Baltic region was considered an import from the North. Several Southern deposits of amber or amber-like fossil resins were known, including those of Sicily. but they were not seriously considered as a source of raw material for ornaments of early Mediterranean cultures. Literary sources supported this view. Amber was brought to Greece by Phoenician traders;' we do not know whence, but since they broughmt tin from the British Isles, they could well have brought amber from not much farther away, viz. from the German and Danish coast and coastal islands of the North Sea. The Roman amber trade with Northern Europe is well attested by the Elder Pliny.2 </p><p>Von Sadowski first established the land routes by which amber reached the Black Sea and the Mediterranean by an ingenious reduction of Ptolemy's longitudes to modern values combined with a thorough knowledge of the topology of Eastern Europe.3 Notable work on the amber trade has since been done by De Navarro4 and by Spekke.5 </p><p>While amber routes were neatly laid out across Europe it may have seemed churlish to raise the question whether all the amber found in the South need at all times have been transported along these paths. Yet that perfectly reasonable question was asked by Capellini6 at the Jnternational Congress of Anthropology in Stockholm in 1874. His suggestion that Terramare and Villanovan artifacts could have been fashioned from local resins rather than of Baltic amber could not be backed by evidence, but in the absence of evidence to the contrary it could not be dismissed. In 1911, Baudoin7 raised the same question about the neolithic amber artifacts of Southern France. As even late in the nineteenth century native amber was so plentiful in the Department des Basses-Alpes that the peasants there used it for fuel.8 his doubts are equally reasonable. </p><p>The problem of determining the provenance of amber by analysis appealed to chemists. Otto Helm, apothecary in Danzig, was the first to make a systematic investigation of ambers of various geographical origin with a view of finding characteristics which would allow him to identify archaeological finds. </p><p>I t had been known since the middle of the 16th century that Baltic amber contains a small amount of a characteristic organic acid which sublimes when the resin is subjected to destructive distillation. This compound, succinic acid, was in fact prepared on a commercial scale by this method. Helm9 improved the quanti- tative determination of succinic acid by working out a wet method involving base hydrolysis followed by precipitation of the barium salt which he either weighed directly or used to liberate the free acid which crystallises as the monohydrate. He found the succinic acid lowest in clear Baltic amber (3.2 to 4.5%). higher in the white opaque variety called bone amber (5.5 to 7.804). and highest in the weather- ing crust (8.2%). Helm further measured melting points (of the whole resin as well as of portions extracted by a number of solvents), densities, solubilities, and elemental composition.i0 By such means, but mainly on the basis of succinic acid </p></li><li><p>A R C H A E O M E T R Y 97 </p><p>content,il Helm identified a number of important amber finds as having come from the Baltic littoral, among them the beads which Schliemann had found in the shaft graves at Mycenae.12 </p><p>Helm later13 realized that non-Baltic European fossil resins from Galicia, Hungary, Rumania, the Bukowina and even Sicily contain succinic acid in varying amounts which may be as high as those in Baltic succinite. He dealt with these difficulties in a manner which suggests that his theory was too dear to him to permit evidence to weaken it. Dahms who himself compiled extensive comparative data of European fossil resins14 has said that both the method and the results of the succinic acid analyses were inadequate to support all of Helms conclusions.~5 </p><p>After Helms death, Olshausen carried out additional analyses of amber,16 and collected all the earlier work on geological and archaeological amber samples in a table which he did not live to publish, but which was appended to a critical review of the problem by LaBaume in 1935.17 </p><p>The table lists 49 analyses of geological samples, of which 29 are of Italian or Sicilian origin. Among the latter, three contain from 2 to 5.7% succinic acid. Olshausen also found more than five per cent of succinic acid in raw amber from Figueira, Portugal, the Isle dAix on the West Coast of France, and Lemberg in Galicia (now Lvov, USSR). These notable exceptions throw rather more than reasonable doubt on the conclusions which have been drawn from the succinic acid content of 59 artifacts which Olshausen culled from the Iiterature or made himself. </p><p>Even this quite summary review of earlier chemical work on amber in the service of archaeology shows that for all their merit the methods employed and the results obtained fail to settle the problem of the provenance of amber artifacts conclusively. It is clear that future work could profit from past experience in several ways: </p><p>(1) The number of analyses of geological samples (i.e. of samples of known origin) must be sufficiently large to give a broad statistical basis to the interpretation of results from archaeological samples. </p><p>To make this possible, the method of analysis should require little time and skill, so that a great many analyses may be carried out by workers who have not been trained in highly specialized analytical techniques. </p><p>(3) As in all problems confronting the archaeometrist, the analysis should be either non-destructive or else use so little material that sampling will not impair the scientific and aesthetic value of the artifact. </p><p>(4) If possible, the analytical method should give not one but several parameters which have potential diagnostic value for distinguishing the bewildering number of varieties of amber and related fossil resins. The most evident disadvan- tage of all earlier work lies in that every single property of amber has proved to fall on a scale on which there is overlap among species. </p><p>These considerations led us to investigate the infrared spectra of whole amber in the solid state. The sample is prepared by grinding two milligrams of amber with a hundredfold excess of pure potassium bromide in a dental amalgamator and pressing the powder into a clear pellet in an evacuated die at 80,000 psi. After trials with infrared spectrophotometers of high resolving power (Perkin-Elmer Models 21 and 221), we found the capabilities of the less elaborate Perkin-Elmer Model 237 grating instrument with a range of 4,000 to 625 cm-1 (2.5 to 16 p ) equal to the task. The large number of absorption bands which amber shows in this region </p><p>(2) </p></li><li><p>98 A R C H A E O M E T R Y </p><p>offers a multiplicity of parameters from a single experiment which can be per- formed in 20 minutes, or in very much less time if several samples are prepared simultaneously. </p><p>We have briefly reported before on the result of the first 120 spectra.18 Since then Savkevich and Shaksig have published a very detailed discussion of the infra- red spectra of ten samples of Baltic amber. </p><p>The interpretation of the infrared spectra of fossil resins calls for restraint. The complexity and the heterogeneity of the material makes it difficult to assign bands to highly specific functional groups or structural types. Few bands are sharp, and even sharp bands are rarely reproducible to within better than 20 to 50 cm-1 on samples taken from the same specimen. Still greater shifts can be found when com- paring spectra of samples from different, though mineralogically and geographically identical, specimens. Shoulders abound, indicating the presence of many closely related compounds or structura: features. The intensities of absorption bands show even greater variation than do their wavenumbers. The superposition of 24 spectra of samples from a single specimen of Baltic amber which appears quite homo- geneous under low-power magnification will illustrale this point (Figure 1). </p><p>73- 75; </p><p>77-81; </p><p>85 -100. </p><p>8 9 10 1 1 1 2 CI </p><p>Geologigches Staatsinstitut Hamburg </p><p>FIG. 1. Comparison of 24 Spectra of Baltic Amber Sample B-1. </p></li><li><p>A R C H A E O M E T R Y 99 </p><p>Our assignments, and our criticism of assignments by others,'g are based on the evaluation of nearly 600 spectra and take into account the limits of reproducibility we have observed. A typical spectrum of Baltic amber over ,the entire range is shown in Figure 4, Spectrum 95. </p><p>OXYGEN-HYDROGEN BONDS The oxygen-hydrogen deformation vibrations are not readily distinguishable </p><p>from carbon-oxygen stretching bands even in simple alcohols.20 but since we have found no diagnostic use for them, these uncertainties are happily irrelevant to our purpose. </p><p>The oxygen-hydrogen stretching band is always broad and extends from 3100 to 3700 cm-I (2.7 to 3.2 p ) , i.e. over the entire range of free and associated hydroxyl groups. In our spectra it is usually one of the two strongest bands with about the same intensity as the carbon-hydrogen stretching absorption. We have however found that the intensity of the oxygen-hydrogen stretching band and also of the deformation band at 1640 cm-1 (6.1 p ) increases dramatically with grinding time (Figure 2). This clearly shows that new oxygen-hydrogen bonds are formed as new surfaces of resin are exposed in the grinding process, either by thorough adsorption of atmospheric moisture or through oxidation by atmospheric oxygen. There are no significant changes which would indicate epoxides (865 cm-1. 1165 cm-'. and 1265 cm-') or peroxides (830-890 cm-'), but the latter band is weak and difficult to identify, especially in large molecules where skeletal frequencies </p><p>3.0 3.5 </p><p>I l l l l l l t l l Spectrum 78 </p><p>ground 10 mir F </p><p>i 3; 5 </p><p>1111111111 Spectrum 7 9 </p><p>around 15 min </p><p>i 3;s </p><p>- L L d u u h Spectrum 8 0 </p><p>round 2 0 rnin. </p><p>3:O 3; 5 </p><p>J . . W Spectrum 81 </p><p>B a l t i c Amber Sample 8-1; Geologischer Staats inr t i tu l Homburg </p><p>FIG. 2. OH and CH Stretching in Amber. </p></li><li><p>100 A R C H A E O M E T R Y </p><p>intervene or in alcohols which may themselves absorb in that range. On the evidence of the parallel changes of the band at 1640 cm-' (6.1 p; see below), the increase in the oxygen-hydrogen stretching band may be due to the adsorption of water, but if so, it is held tenaciously, for we have not been able to remove it by drying for several days, with or without phosphorus pentoxide. Savkevich and Shaks have found that the oxygen-hydrogen siretching band increases in their pellets even after storage in a desicator for several days.19 </p><p>Since a grinding time of more than ten minutes is necessary to produce the uniformly small particle size required for reproducible spectra by the potassium bromide technique, we have sought to standardize rather than to eliminate the effect: all our samples are ground for exactly 15 minutes. Large differences in the oxygen-hydrogen stretching absorption could then be used to distinguish resins by their relative avidity for adsorbing water if not by their original composition. Thus far we have not found such differences which can be related to sample type. </p><p>CARBON-HYDROGEN BONDS The strongest absorption in the carbon-hydrogen stretching range is clearly due </p><p>to the methylene doublet at 2926 cm-1 and 2853 t 10 cm-1 (3.42 and 3.50, p) and the methyl doublet a: 2962 cm-1 and 2872 f. 10 cm-1 (3.38 and 3.48 p), of which the former seems to predominate and the latter appears as more or less obvious shoulders. Only in a small group of North American fossil resins which are characterised by a large proportion of aromatic constituents are the carbon- hydrogen stretching frequencies sharply separated. </p><p>The presence of a large number of methyl groups in Baltic amber also seems to be indicated by the fact that the carbon-hydrogen deformation bands at 1450 1 20 cm-1 (6.9 p; due to methylene and symmetrical methyl bending) and at 1375 f 5 cm-l (7.25 p; due to asymmetrical bending) have approximately equal intensities. </p><p>Tertiary hydrogens absorb Loo weakly to be identifiable in the carbon-hydrogen stretching region in most of our spectra, but a commonly occurring shoulder near 1340 cm-1 (7.46 p) may be due to their deformation mode. </p><p>All these bands are in keeping with the expectations for terpenoid compounds and afford no means for distinction in our spectra of whole amber. On these same expectations the consistent though broad band between 1050 and 950 cm-1 (9.5 and 10.5 p) could be assigned tentatively to the cyclohexane frequencies identified by Marrison21 in this region. </p><p>More critical information can be derived from absorption frequencies associated with hydrogen atoms attached to carbon-carbon double bonds. Savkevich and Shakslg have claimed to recognize olefinic unsaturation of the type CR, R, = CH,, CR, R, = CHR,. and RCH = CH,, as well as aromatic stretching frequencies, but the evidence decidedly does not warrant such detailed assignments. The small but distinct absorption at 3095 cm-1 (3.23 p) is the only olefinic carbon-hydrogen stretch- ing frequency which can be identified unmistakably because it is well out of the range of all other strong bands in this region. It may equally well arise from the groupings RCH = CH, or CR, R, = CH,, but we cannot agree with the Russian authors that both are separately recognizable. While the associated carbon-carbon stretching frequency near 1650 cm-1 (6.05 p) is obscured by water absorption, a sharp band at 885 cm-1 (11.3 p) is almost certainly due to the out-of-plane carbon- hydrogen bending of the terminal olefin. Its position favours the structure </p></li><li><p>A R C H A E O M E T R Y 101 </p><p>CR, R, = CH, over the alternative RCH = CH,. In light of the fact that Schmid22 and co-workers have isolated agathaline (1, 2, 5-trimethylnaphthalene) as a dehydro- genation product from Baltic amber, this suggests a diterpene of the agathic acid type with an exocyclic double bond. </p><p>The intensity of the 11.3 band is variable, probably as a result of oxidative changes which will be discussed below, but it survives at least as a distinct shoulder in all the samples of Baltic amber which we have examined. The band is of further interest because it allows distinctions to be drawn among other fossil resins. Of the Baltic non-succinite varieties, only gedanite shows it, lbut glessite, krantzite, and beckerite do not. Of other European fossil resins neudorfite, euosmite, and rumanite show it, but duxite, ajkaite, walchowite, schraufite, and the French pseudo- succinite do not. Sicilian amber sometimes, but not always, absorbs near 11.3 p (Figure 3). Several resins classed as copalites, fro...</p></li></ul>