Ilon Gábor szerk.: Pápai Múzeumi Értesítő 6. (Pápa, 1996)

Bronzkor a Nyugat-Dunántúlon - P. I. Maclaen - J. G. Mcdonell: New archaeo metrical investigations of the bronzes in the Carpathian Basin. Újabb természettudományos adatok a Kárpát-medencei bronzok összetételéről

Metallographie analysis can provide a wealth of information on the manufacturing history of a metallic artefact. In the case of most non-ferrous metallurgy this can include details as to whether the object is „as-cast" i.e. subject to no further alteration after casting, if it has been worked cold, annealed, hot-worked or quenched. Where samples have been taken from material before formal conservation, it is sometimes also possible to detect evidence of chemically altered surfaces (patination natural or induced) and additional information on the nature of any associated corrosion. In regard to chemical analysis, antimony bronzes may only be positively identified by chemical means. Previous analytical studies by workers such as Otto Helm at the turn of the century using wet chemical techniques (Helm 1900) and the S.A.M. analytical programme from the Stuttgart group during the 1960's and 1970's (using Optical Emission Spectroscopy) have provided useful information regarding the chemical variability of these alloys and acted as an indicator for directed sampling for metallographic study. The current project makes use of two methods of chemical analysis, X-Ray Fluorescence (XRF) and Scanning Electron Microscopy with an Energy Dispersive X-ray Analysis facility (SEM with EDAX). The qualitative use of XRF on the metallographic samples allows rapid pre-screening for the presence of antimony on the basis of the Kçi peak in the X-ray spectra at 26.27 KeV which is distinguishable from the neighbouring tin K(x peak at located at 25.19 KeV. Samples showing evidence of antimony may then be prioritised for analysis with the SEM. Laboratory alloys As part of die research programme, analyses of archaeological samples are carried out in conjunction with the preparation and examination of a series of laboratory alloys created from high purity metal stock (typically 99.99+% pure). The initial alloy series comprises of Cu-Sb and CuSb-Sn alloys encompassing compositions known from the archaeological record (to date up to c.30% Sb and 20% Sn). These alloys act as a control from which to investigate aspects of the binary and ternary systems. In some archaeological contexts outside die Carpathian Basin these alloys provide a good comparison, however within the CarpaÜiian Basin the high antimony alloys tend to be more complex, often witíi the inclusion of several other elements such as arsenic, lead, sulphur, nickel, cobalt. The effect of these additional elements on the properties and microstructure of these alloys is the subject of continuing study. The role of antimony bronze alloys One of the main aims of the current research is to discuss the possible reasons for the existence of antimony bronze alloys, wheüicr their production was deliberate or accidental. It is possible that antimony may enter die alloy composition of an artefact tluough its pre-inclusion in the source of another desired element such as that of copper. The use of the tennantite-tetrahedrite ('fahlerz') mineral series (CU3ASS3..4 to Cu3SbS3_4) as one possible ore source is likely, given the range of composition and complexity of the more polymetallic alloys; however this does not preclude the deliberate selection of such minerals because of Ulis particular quality. The elevated levels of antimony found in these objects and die selectiveness widi which they appear to occur would suggest tiiat for the majority of antimony bronze material its production may have been the result of deliberate selection. On such a basis a number of possibilities for the presence of antimony present themselves: 1. Physical properties - the inclusion of up to c. 2% Sb in copper may produce a desirable hardening effect, however in quantities in excess of tliis tiie alloy will become increasingly brittle. Laboratoiy experiments have shown that in an „as-cast" Cu-5%Sb alloy cracking will occur after a 15% reduction in Üiickness, in a Cu-10% Sb alloy cracking will occur after only a 4% reduction (Figure 2.). It may be possible to Jiot-work" such alloys up to 8% Sb (Charles 1980, 170-172) however there appears to be little evidence for this in the archaeological record (see below). Antimony is one of tiie few elements (including bismuth) tíiat expands on solidification, a desirable property where surface detail is required from a casting. A modem example is the use of