1 Introduction
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Fig. 1 Location of Geistingen.
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Bronze axes of the Geistingen type (fig. 2) are dated to the Late Bronze Age, approximately 925-800 BC (Butler & Steegstra 2002/03, 309). They are named after a hoard of 26 or 28[2] axes found in 1935 in Geistingen, Belgium (Wielockx 1986; Butler & Steegstra 2002/03, 303). To date there are 29 additional examples with a relatively certain origin (Fontijn 2003, 325; Butler & Steegstra 2002/03, 304). Their distribution appears to be confined to the western half of Nordrhein-Westfalen and the Dutch-Belgium Meuse river valley (Butler & Steegstra 2002/03, 304 map 6). Such a confined distribution is atypical of Late Bronze Age socketed axes (cf. Butler & Steegstra 2003/04, 204 map 11; 248 map 14) and suggests a single workshop from which these axes could have originated (Butler 1973, 341; Kibbert 1984, 168; Butler & Steegstra 2002/03, 304).
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Fig. 2 Example of an axe of the Geistingen type. Length: 14.3 cm, width 4.1 cm, wall thickness 1-2 mm. (cat. no. 551, from: Butler & Steegstra 2002/03, 306 fig. 33a) . |
Their confined distribution is not the only aspect that sets these axes apart from other contemporary axe(-type)s. First, the few known Geistingen hoards are situated on drier plateaus instead of the typical wetland settings (rivers, streams, bogs) that characterise other axe hoards (Fontijn 2003, 160). Second, contrary to expectations of use as tools prior to deposition, the examples presently known rarely show use-wear (yet are frequently sharpened; Fontijn 2003, 325. Only two axes show a ‘battered’ cutting edge: Butler & Steegstra 2002/03, 304). Third, the walls of these axes are extraordinarily thin (1-2 mm), which renders a functional use as a cutting tool improbable (Butler 1973, 339-340; Kibbert 1984, 166; Butler & Steegstra 2002/03, 304). Moreover, the Geistingen axes from Ool and Nijmegen display casting flaws (bronze projecting into the socket) that would have prevented these axes from ever having been hafted (Butler & Steegstra 2002/03, 309, cf. Guilbert 1996, 13).
These peculiar characteristics gave rise to hypotheses that Geistingen axes were perhaps never intended to be used as tools, but served solely as beautified votive copies of functional types (perhaps of type Wesseling axes) or as standardized metal (currency?) units or ‘axe money’ (Beilgeld; Kibbert 1984, 167; Rivallain 2001). Indeed, one may argue that the role of the axe as an exchangeable, convertible ingot was first materialized through a specialised ingot form during the Late Bronze Age, of which the production of Geistingen axes is an example (Fontijn & Fokkens 2007, 364). Such a usage of axes as ingot and/or votive units is also documented for the contemporary – yet better known – Armorican axe depositions (Briard 1965, 241-282; 1979, 207; 1991, esp. 23). Consequently, a study of the metallurgy and production processes of Geistingen axes may help to determine whether these axes were indeed never intended to be put to use.
However, in order to do so, the information obtained for Geistingen axes should be compared with data for what were contemporary, and explicitly functional, socketed axes from the same region and period. Such axes have been inventoried by Butler and Steegstra (2003/03) and comprise several axe-types such as the Niedermaas type (c. 44 find spots known in the Netherlands and adjacent Germany and Belgium; Butler & Steegstra 2003/03, 268 map 3; 269-271) and the Plainseau type (c. 51 find spots known from the same areas; Butler & Steegstra 2003/03, 279-281 map 4). In particular the latter, mostly found in the Paris basin and Picardy during Bronze Final IIIb (Blanchet 1984, 368-373; Van Impe 1994, 16 fig. 4), can be assessed for pre-deposition functionality through documented use-wear (e.g. Van Impe 1995/96, 10; 31; Butler & Steegstra 2003/03, 267; 283 no. 508; 289 no. 529) and compositional analyses (e.g. Wouters 1990; Van Impe 1994). These thus provide an excellent basis for comparison.
In general, bronze objects can be produced either from ore by smelting or from scrap by re-melting. Smelting copper oxide ore involves heating to about 1250 °C, at which temperature two liquid layers will form according to their density and immiscibility: copper forms the bulk of the melt and a slag, rich in iron and silicon, floats on top (Caley & Easby 1959, 60; Lechtman & Klein 1999, 499; Davis 2001, 10; Bassiakos & Catapotis 2006, 348). The molten copper can be tapped from the furnace and further processed. When sulphide-rich ore is used, an extra step prior to smelting is required: roasting. Roasting takes place at about 350 C in an oxidising atmosphere and the resulting ore will be partially oxidised and partially desulphurised (Caley & Easby 1959, 61; Lechtman & Klein 1999, 498). When this ore is subsequently smelted, three liquid layers will form: copper at the bottom, matte in the middle and slag on top (Caley & Easby 1959, 61). Matte contains copper, sulphur and impurities like iron and nickel (Lechtman & Klein 1999, 499; Bassiakos & Catapotis 2006, 342). The standard practice applied during the Bronze Age did not adequately remove all the sulphur, which means that sulphur in the form of matte inclusions and other impurities may remain in the copper (Caley & Easby 1959, 62; Craddock 1995, 153). The alloying of copper with other elements to produce bronze can take place by either natural co-smelting or intentional co-smelting (Lechtman & Klein 1999, 498-499). After smelting, an object is formed by casting the liquid bronze into a mould.
Re-melting of scrap or ingots and subsequent casting can also produce bronze artefacts. This method is mostly practised in regions where no sources of ore are present, like in The Netherlands (Kuijpers 2008, 20). A lower temperature than in smelting (about 1000°C, depending on the alloy) can be applied to melt the metal (Davis 2001). The concentration of impurities and inclusions in the resulting metal will be lower in re-melted bronze than in bronze produced by smelting (Wheeler et al. 1975, 38; Craddock 1995, 152; Figueiredo et al. 2009, 953). Working of the finished product involves plastic deformation of the metal which results in features in the microstructure, like slip bands, recrystallised grains, deformed phases or broken dendrites (Gordon & Knopf 2006).
This study applies microstructural research on small samples from two broken axes, enabling us to better understand the production and former function(s) of Geistingen axes. The average composition of the bronze has been determined using several analytical techniques, including X-Ray Fluorescence and Electron Probe Micro Analysis. The microstructure of the metal was characterised by means of Scanning Electron Microscopy and contained a fingerprint of the solidification process of the bronze. Several specific features of the microstructures in the two axes will be presented and discussed. This process enabled us to form a hypothesis on the production and potential usage of these Geistingen axes.