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11. Badania składu chemicznego oraz technologia wykonania wybranych zabytków ze stopu miedzi

DOI: 10.33547/ODA-SAH.10.ZN.11

Badania składu chemicznego oraz technologia wykonania wybranych zabytków ze stopu miedzi (Analysis of chemical composition and technology of manufacture of selected copper alloy artefacts)

by Marcin J. Biborski 1, Mateusz R. Biborski 1

1 – Laboratorium Archeometalurgii i Konserwacji Zabytków Instytut Archeologii Uniwersytetu Jagiellońskiego, ul. Gołębia 11; 31-007 Kraków

In:  M. Cieślak-Kopyt, D. Pogodzińska 2020. Żelazna Nowa, stanowisko 2. Cmentarzysko kultury przeworskiej z Zapilcza na południowym Mazowszu, Ocalone Dziedzictwo Archeologiczne 10, Radom – Pękowice: Muzeum im. J. Malczewskiego w Radomiu, Wydawnictwo Profil-Archeo, p. 121-134.

Summary: 41 artefacts recovered from the Przeworsk culture cemetery in Żelazna Nowa were originally selected for archaeometric examination. Of that number, 24 brooches and 13 various objects made from copper alloy were analysed. The choice was dictated by their good state of preservation, allowing for reliable results. The method applied was non-destructive energy dispersive X-ray fluorescence analysis (ED-XRF), using an X-ray spectrometer operating at 45kV. In addition, selected artefacts were examined using a scanning electron microscope. The results of chemical analyses are presented in Tables 11.1 and 11.2, with weight percent values given, while the raw material and technical data are shown in Tables 11.3 and 11.4. The results indicate that among the 24 brooches and their fragments analysed, 16 were made of brass, including eight made of multi-component tin brass, one made of tinlead brass, and one made of tin-silver brass. Among the remaining eight brooches one was made of iron and seven were bronze pieces, including two made of multi-component zinc bronze, one of lead bronze, and one of silver bronze (inv. no. CCXXIII/15). The addition of lead had significant technological impact on the alloys used for creating the brooches. Even in small quantities, even far below 1%, this element improves the castability and workability of the metal. Zinc, in turn, is added to bronzes as a reducing agent, making the alloy self-reducible. This also makes it possible to reduce the contents of phosphorus. Moreover, the addition of zinc to bronze significantly lowers its melting temperature. The remaining alloying additions (apart from elements recorded in trace amounts, like Ag, Sb, and Fe, which naturally occurred in copper alloys) have had no significant impact on the quality of the raw material used for manufacturing the artefacts under discussion. The presence of silver (a few percent)
recorded in two brooches (inv. nos XXXIII/15 and CCXXIII/15) indicates they may have originally been silver-plated. Two brooches (inv. nos XCIX/15 and 21/08) were inlaid with silver wire. The first one was cast from tin bronze and adorned with a filigree wire (Fig. 11.1) in the type of a simple braid, made of highquality (92.99%) silver (Fig. 11.2). The silver wire with a diameter of merely 338.38 μm (Fig. 11.3) was produced by drawing through a die, and it was later braided and hammered into grooves previously prepared on the two crests of the brooch. The second brooch, made of iron (inv. no. 21/08), was adorned with silver of a similarly high quality (91.85%). In this case, however, the exposure to fire resulted in complete destruction of the decoration: its traces only remained in a few places in the form of tiny melt-down droplets. It is worth noting the similar standard of silver used as inlays in both brooches. This seems to corroborate the assumption that the
standards of silver used in trade were generally similar over larger areas. It is worth noting two brooches which, despite representing different types, are made of chemically very similar alloys (Tables 11.1–3). One (inv. no. CCIX/15) belongs to type A.III.58, while another represents group A.IV (inv. no. CIX/17). This may indicate that both were manufactured in the same workshop. Except for one brooch forged from iron, all the remaining brooches were cast using the lost wax technique (Table 11.3). Cast pieces also prevail among other artefacts, especially those having profiled surfaces (Table 11.4). Examples include a fragment of a J.7 shield grip (inv. no. XVI/15), a belt finial of type “O” (inv. no. CCXLIX/15), and fragments of bracelets with round terminals (inv. nos XIII/15 and LXXXVI/17). The performed raw material analysis of copper alloy artefacts from Żelazna Nowa produced interesting findings. The results confirm that during the Older Roman Period brass artefacts prevailed among small metal dress items in the Przeworsk culture. Among the 37 copper alloy artefacts analysed, as many as 27 (75%) are brass objects. A similar picture was observed in other sites, with a prevalence of brass artefacts recorded in cemeteries such as those in Karczyn/Witowy, Sadłów, and Szarbia.