The Comparative Study of Tool Marks on Non-Ferrous Metalwork from Archaeological Contexts: Methodological Considerations, Case Studies and Experimental Archaeology

The aim of this paper is to discuss methodological issues of comparative tool mark studies (with special attention to decorative punched motifs), using a case study on sheet-gold metalwork from the Middle and Late Avar Period (7th–8th centuries AD). The historical interpretation of tool marks on nonferrous metalwork1 can take place at different levels of complexity. In some cases, the identification of products from the same craftsperson is possible, via the identification of specific tools. However, when interpreting the results of comparative tool mark studies, many aspects are relevant, including individual variations in tool usage. In this context, experimental archaeology can facilitate the interpretation of tool marks, as is demonstrated in a preliminary study on two types of punches, which are common on high-quality metalwork from the Avar Period. 1 The term “non-ferrous metals” encompasses all the metals and their alloys, which do not contain iron. Hence, this denomination includes precious metals (e.g. gold and silver) and non-precious metals, such as lead, tin, zinc, copper and its alloys (e.g. bronze, brass, etc.), to mention the most relevant concerning tool mark analyses on archaeological metal artefacts. IANSA 2021 ● XII/1 ● Online First Birgit Bühler, Dávid Zsolt Schwarcz: The Comparative Study of Tool Marks on Non-Ferrous Metalwork from Archaeological Contexts: Methodological Considerations, Case Studies and Experimental Archaeology Online First 2. Tool mark studies on gold and silver metalwork from archaeological contexts 2.1 Methodological considerations The purpose of “basic tool mark studies” on precious metalwork from archaeological contexts is to identify the techniques used in manufacturing and decorating a particular artefact and to reconstruct its production process. In contrast, the aim of “comparative tool mark studies” on precious metalwork from archaeological contexts is the historical interpretation of the technological evidence. Hence, “basic tool mark studies” constitute a pre-requisite for further, historical interpretation of the evidence, which also requires the inclusion of additional criteria, derived from other, related metalwork and the archaeological context (“comparative tool mark studies”). As will be illustrated below, in a case study, the process of historical interpretation of tool marks can take place at different levels. Starting at the lowest, most specific level, this process can involve the identification of individual tools, followed by the identification of specific types of tool, by technical preferences and idiosyncrasies of individual metalworkers and progressing to the higher-level aspects, such as the identification of metalworking traditions characteristic for individual workshops, as well as to larger-scale, local or regional metalworking traditions. Therefore, comparative tool mark studies may permit researchers to identify artefacts produced by the same Figure 1. Gold belt-buckle from the “Brestovac Hoard” (Croatia; end of eighth – early ninth century AD). Kunsthistorisches Museum Wien, Antikensammlung, inv. no. VII B 70. Detail of sheet gold fitting: Highly stylised plant motif (“Stäbchenranke”) characteristic for the Late Avar Period III in repoussé (“Treibziselieren”), worked predominantly from the front; surface of leaves decorated with “stippling punch” (Photo: B. Bühler). Figure 2. Gold belt-buckle from the “Brestovac Hoard” (Croatia; end of eighth – early ninth century AD). Kunsthistorisches Museum Wien, Antikensammlung, inv. no. VII B 70. SEM-Detail of sheet gold fitting: Repoussé (“Treibziselieren”), worked predominantly from the front; surface of leaf decorated with “stippling punch” (Photo: M. Mehofer, VIAS). IANSA 2021 ● XII/1 ● Online First Birgit Bühler, Dávid Zsolt Schwarcz: The Comparative Study of Tool Marks on Non-Ferrous Metalwork from Archaeological Contexts: Methodological Considerations, Case Studies and Experimental Archaeology Online First craftsperson: The best way of verifying such an assumption is to demonstrate the use of identical tools in the production process of two or more particular metal products. The prerequisite for this, however, is the identification of individual tools on the metal objects in question. Characteristic irregularities on the surface of a tool’s working-edge (see, for example, Figure 4, below) can facilitate the identification of specific, individual metalworking tools. In the absence of such typical faults, exact measurements of the dimensions of the tool-marks may assist in identifying a specific tool. Ideally, a Scanning Electron Microscope (SEM) is available for precise measurements of tool mark dimensions, although a variety of other measurement options exists (see other comparative tool mark studies focusing on punching especially as a decorative technique, Larsen, 1987, Mortimer, Stoney, 1997 and Dal, 2017, pp.131–144). However, when interpreting the results of comparative tool mark studies, several aspects are relevant: Firstly, the surface of a tool’s working-edge may change with time. This could either be due to the natural process of use-wear or to intentional re-shaping by a craftsperson. Secondly, tool marks originating from the same tool may vary in size and appearance (for examples, see chapter 2.2), due to variations in the orientation of the tool to the metal surface and because of variations in pressure when using the tool. As will be demonstrated in chapter 3, experimental archaeology can assist in the interpretation of tool marks on non-ferrous metalwork from archaeological contexts. Thirdly, traces of Figure 3. Gold bowl number 13 from the “Nagyszentmiklós (Sânnicolau Mare) Hoard” (Romania; end of eighth – early ninth century AD). Kunsthistorisches Museum Wien, Antikensammlung, inv. no. VII B 10. Detail of stylised plant ornament (“Stäbchenranke”) in repoussé, including leaves with “stippled surface” and background filled with ring-shaped punchmarks (Photo: Kunsthistorisches Museum Wien). Figure 4. Gold bowl number 13 from the “Nagyszentmiklós (Sânnicolau Mare) Hoard” (Romania; end of eighth – early ninth century AD). Kunsthistorisches Museum Wien, Antikensammlung, inv. no. VII B 10. SEM-Detail: Tool marks of “stippling punch” with characteristic irregularity (Photo: M. Mehofer, VIAS). IANSA 2021 ● XII/1 ● Online First Birgit Bühler, Dávid Zsolt Schwarcz: The Comparative Study of Tool Marks on Non-Ferrous Metalwork from Archaeological Contexts: Methodological Considerations, Case Studies and Experimental Archaeology Online First wear on the surface of a metal object may alter the original shape and size of tool marks. 2.2 Case study: Interpreting decorative techniques on sheet-gold metalwork from the Middle and Late Avar Period (7th–8th centuries AD) Interdisciplinary research on non-ferrous metalwork can be helpful in identifying cultural traditions in archaeological contexts. For example, there is ample evidence for a close connection between “Byzantine” and “Avar” culture in the 7th and 8th centuries AD. However, the mechanisms of cultural and technological transfer, as well as the regions where such transfer processes may have taken place, are still subject to debate. Comparative tool mark studies on precious metalwork could contribute to resolving these questions, in particular in conjunction with other criteria derived from archaeological research. In this context, specific variations in decorative techniques for working precious metals are relevant: True repoussé (= three-dimensional modelling of sheet metal using individual tools, such as several different types of punches; “Treibziseliertechnik”) was an uncommon technique in the Avar realm. However, it occurs within Avar territory on a number of high-quality items of metalwork (mainly gold objects; see Figures 1–5), which may be associated – for typological, stylistic and/ or technological reasons – with “Byzantine” culture. In the Early and Middle Avar Period (7th century), the standard production method for gold-, silverand copper-alloy jewellery and dress ornaments was pressing pieces of sheet metal over positive models (“formers”). In the Late Avar Period (8th century), on the other hand, the dominant technique for manufacturing non-ferrous jewellery and dress ornaments was the casting process. However, in the Mediterranean region, true repoussé (“Treibziseliertechnik”) was a common technique for producing three-dimensional decoration on highquality sheet metal (gold, silver and copper alloy) objects. Although the import of top-quality gold metalwork with repoussé ornament from the Byzantine Empire is a feasible interpretation, an alternative hypothesis is that they are in fact products of “Byzantine” type, produced in workshops on the periphery of the Byzantine Empire with technical expertise from Byzantium. In any case, it seems feasible to associate the occurrence of these specific decorative techniques on metalwork found on Avar territory with “Byzantine metalworking traditions”, although the specific workshops cannot yet be located (Bühler, 2014). This case study summarizes the results of comparative tool mark studies on high-quality sheet gold metalwork from the Middle and Late Avar Period (7th–8th centuries AD), by focusing on two key finds from this period, both of which are in the collection of the Kunsthistorisches Museum Wien, Austria (Antikensammlung). Firstly, there are the 23 gold vessels from the “Nagyszentmiklós (Sânnicolau Mare) Treasure” (for an overview of archaeological research, see Bálint, 2010; for the results of archaeometallurgical research, see: Bühler, Freiberger, 2018), found in presentday Romania in 1799. Secondly, there are four gold belt ornaments from the so-called “Brestovac Hoard”, found in present-day Croatia in 1821. The plant ornament on the youngest group of gold vessels (seven bowls with “Stäbchenrankenzier” = Nos. 13–16 and 19–21; late 8th century AD) from the “Nagyszentmikl