Achaeometrical Research in Hungary II., 1988

ANALYSES - METALS - Géza SZABÓ: Evaluation of late Bronze Age Carpathian tinbronzes based on the alloying content

At the articles which were annealed after working, the strain annealing process is indicated by the split twinned crystallites appearing along straight lines and is observable on the cut surfaces. (SZABÓ: 1993 No. 63.82.89.92.112.124.130.149) Efficiency of the homogenisation. as it can be concluded observing the cuts, is rather various. Due to the rather low temperature and its short duration, a soft half- product was produced suitable to further processing. It was very important in the case of bronze pins. The texture characteristic for bronze pins can fairly be traced on a polished surface of a wire spiral fragment.(SZABĈ): 1993 No. 130; Figure 3.1). At a magnification of 400 the twinned crystallites along straight lines, and at the edge a yellowish layer of different structure abundant with tin can be seen clearly (Figure 3.2) The twinned crystallites with sizes changing between 40 to 100 micron indicate a relatively soft material suitable to further processing. In the case of a melted fibula the fact that the boundary of twinned crystallites remained slightly wavy in spite of intense later thermal impact indicates that the recrystallisation was not complete everywhere. This can be attributed presumably to the short time, high temperature annealing folbwed by fast cooling. This "annealing" was not enough to melt the remainder of the alpha+delta residue between the alpha crystals and to fully soften the material of bronze pin. This process can be observed at other objects, too. In the structure of a cast bronze ring (SZABÓ: 1993 No. 90). beside the small polygonal crystallites indicating fast cooling of the mould, several twinned crystallites can also be observed. The pattern of the cut surface shows that the casting mould was not warmed up or a quickly cooling form was used.(MEEKS: 1994 262-63). Micro-spectrum analyses of the Regöly-Veravár treasure find performed with X-rayfluorescence (XRF) method support the conclusion about the low tin content drawn from .he microscopic patterns too. In every case tin content is much below the saturation i.e. 14 %. vt'mzb. means at the same time, that the objects were made of alloys suitable to further processing. According to the results of XRF analyses for most objects, particularly for simpler ones, the tin is present in an order of magnitude of per cent, but it can also be seen on the curve that the peak is quite small, mostly below the limit value of 3 to 4 per cent.(SZABÓ: 1993 No. 47.77) This observation is supported also by the data of SEM analyses (Figure 4). Somewhat higher values can be observed for the tin content, at about 7 per cent in the case of the simple cast ring (SZABÓ: IS 93 No. 90) and generaly at the objects manufactured from worked, annealed bronze pins, too.

(SZABÓ: 1993 No. 112.130.87) Similar problems of relation between the raw material used to manufacturing the bronze pins and the ring-shaped casts are common in the Late Bonze Age. For the annealed bronzes textures of different structures can be observed in the interior and the exterior of the bronze and near to the surface (Figure 3.1). Using a microscope a tin-rich net can be observe covering the whole surface (Figure 1-2). For a more accurate evaluation of the phenomenon the percentage of alloying material was measured in the bulk phase and on the surface. Cuts of a needle (SZABÓ: 1993 No. 77) and a wire spiral (SZABÓ: 1993 No. 130) fragments were intentionally prepared for the SEM studyso 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 was any instrumentally measurable material composition difference behind the phenomena appearing under the microscope whe same article is studied using the two basic methods simultaneously. In the practice of archaeology the most frequently used method are the non-destructive surface analytical tests. In the metallurgical practice the preferred methods are the destructive tests 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 the bulk 160