Dani János - Hajdú Zsigmond - Nagy Emese Gyöngyvér szerk.: MÓMOSZ I. (Debrecen, 2001)

Szabó Géza: Újabb eredmények és módszerek a Kárpát-medence késő bronzkori tárgyainak archaeometallurgiai vizsgálataiban

/Fig.4.4., 5.2./ Using a microscope, one can often observe a tin-rich net covering the whole surface. For a more accurate evaluation of the phenomenon, the percentage of alloying material has been measured in the inside and on the surface. Cuts of a needle (No. 77) and a wire spiral (No. 130) fragments were intentionally prepared for the SEM study so, that the samples from the surface and from the cross section could be studied under the same conditions. Our objective was to see if there is any instrumentally measurable material composition difference behind the phenomena appearing under the microscope, if one studies the same article using the two basic method simultaneously. In the practice of archaeology the most frequently used method is the non-destructive study limited mainly to the surface. In the metallurgical practice the preferred method is the destructive study of the samples taken from the articles. In the process of sample preparation particular care was taken to keep both samples, embedded in plastic casing, accurately at the same height facilitating the study of internal aparts at perfectly identical parameters, in the same space, with the same focal length. When the surface was studied, the fragment of needle manufactured of a bronze pin with about 3 per cent of tin content, showed nearly 20 per cent i.e. about six times higher than the actual value of tin content. In the case of wire coil fragment, the surface segregation of tin was lower than with the needle, but the measured values indicated many times higher tin content than the actual value. So the studies unambiguously justified the microscope observations concerning the tin-rich surface. At the same time it means, that the results of mikrospektrum analyses aimed at the surface or at near-surface layers of archaeological finds, particularly of annealed objects, should be taken with reservation. The impurities observed in the studied archaeological finds, Fe, Zn, As, Pb, Sb, Ni in quantities of several tenth per cent and the results of analyses of plate-backed fibula (No. 87) show unambiguously the sulphide origin of the basic material used. Sb occurred in a per cent

7 order of magnitude in the case of handle fixtured sickle (No. 47) only and this is not characteristic for the other appliances (neither is the unusual high, 1,23 per cent nickel content) therefore it can be rather regarded as casual.

8 Presumably, a fragment recycled for the casting of sickle had a higher antimony content. Lead is mostly present also in an order of magnitude of tenth per cents, but as it can be seen also in microscopic pictures, it appears in form of small bluish-grey stains separated of the texture of the alloy. Data measured inside a twinned crystallite of a wire spiral fragment clearly show, that there is no solved lead in the alpha-phase grains. In case of annealed objects the small quantity of unsolved lead migrates, sometimes completely, up to the surface, as e.g. in the case of plate-backed fibula, where it can be observed in a proportion of 25 per cent.

9 In other cases enrichment of lead show a factor of 15 (No. 77)

10 or even nearly 100.

11 At the same time this means, that because of lead segregation on the surface, particularly in the case of annealed articles, the data from mikrospektrum analyses aimed at the archaeological study of surface and near-surface lazers- similarly to the tin segregation - can be taken with reservation only. In investigated objects the iron is present mostly in an order of magnitude of tenth of per cent, too, but one can observe some enrichment of the iron on the surface of annealed bronzes. Major segregation of iron, about 6 per cent, can be measured only in the single case of plate-backed fibula (No. 87). In the analyses of texture the plano-convex ingots formed a particular group with their higher iron content of per cent magnitude. On the XRF patterns of these articles one can clearly see the distinct iron peaks showing an iron content of about 4 per cent, and the curve, completely smooth in other regions, clearly show the extraordinary low tin content present in not higher quantities than tenths of per cent. The alloying metal content of analysed objects, in accordance wit the microscopic picture of polished surfaces, showed a low tin content in every case, along with other components present in a ratio of tenths of percent, and so regarded as impurities. Only the ingots show a different composition: their iron content is higher, about 4 per cent, while their tin content is quite low. The experiences reported above show, that the study of archaeological bronze finds revealed significant differences in the material compositional, depending on processing, on the method of study used and also on the place of taking particular samples. EVALUATION OF STUDIED OBJECTS BASED ON THE MEASUREMENTS OF HARDNESS The measured values covered an extremely wide range beginning from the very low value of 31,9, 32,5 Hv (No. 89., 168.)

12 up to the multiple ofthat, 303 Hv(No. 17).