3.3 Production
When looking at macroscopic aspects of the two axes from Museum Het Valkhof (AC20) and the Gallo-Romeins Museum (BH76), it is evident that their size and socketed shape are almost the same. Casting seams are also present on both. These features suggest a casting technique which is the same for both axes and for which most probably a bi-valve mould has been used. The microstructural research presented in this paper provides valuable additional information.
A decisive indication for the casting process is the porous dendritic microstructure with different types of inclusions. It indicates that the bronze has been molten and relatively rapidly solidified. The average cooling rate is approximately 30 °C/s for both axes, which suggests that they have been cooled using the same method. This rapid cooling can be attained by quenching with water (Frame & Vandiver 2008).
Several materials can be used as mould material, of which clay (cf. Van Impe 1995/96, 20) and bronze are frequently used for bi-valve moulds (Kuijpers 2008). Clay has a very low thermal conductivity, which means that several steps need to be undertaken to cast a sound and strong bronze object. After casting the bronze in the pre-heated clay mould, the ensemble needs to be cooled with water for a few minutes. As soon as the bronze is solidified, the clay mould should be removed to allow a faster cooling of the object by submerging it in (cold) water again. This is a practice that calls for considerable skill and experience since the more rapid cooling should be applied during the formation of the microstructure, i.e. during the solidification process. Using a bronze mould (cf. Butler 1973, 322; 338; Butler & Steegstra 2005/06, 209) will eliminate the extra step of removing the mould and is therefore more likely to be applied. In addition, the identical shape of the various Geistingen axes supports the use of a re-usable bronze mould.
There are two options for the type of raw material used; either ore is smelted, or bronze scrap or exchanged ingots are re-melted. One indication for smelting lies in the presence of matte particles in the microstructure, which are remains of the roasting step in the smelting process. Since other impurities like titanium, silver, cobalt and zinc are present as well, no further refining steps have then been undertaken (Lindgren 1933, Craddock 1995, Figueiredo et al. 2009). The composition of the two bronzes indicates it is a possibility that both axes are smelted using the same type of ore with different ratios and additions of tin and/or nickel. Fahlerz is a sulphidic ore known to have been used during the Copper Age and the Early Bronze Age (Biek 1957, Tylecote et al. 1977) and contains elements like copper, arsenic and antimony (Gainov et al. 2008). If ore from the same source is used for both Geistingen axes, the Sb/As ratio should be the same in both cases. However, the Sb/As ratio is different for both objects (2.0 for AC20 and 1.5 for BH76). If nickel would have been added, AC20 and BH76 should contain at least 4 times the arsenic amount than they do now since nickel is usually associated with arsenic (Lindgren 1933). These two features indicate that it is unlikely that the same type of ore from the same source has been used for both axes. So either different types of ore have been used during smelting or (different pieces of) bronze scrap formed the raw material of these Geistingen axes. The lack of data in the literature on microstructures of re-melted bronze makes it difficult to reach a firm conclusion on the raw materials used. However, some information on the change in composition during re-melting is available. Two cycles of re-melting and hot-working in air can result in the loss of antimony and arsenic content until less than one per cent is left (Junk 2003, 26; 29; Tylecote 1977, 329). Since these two Geistingen axes still contain relatively high amounts of both elements (1-4 at%) it is assumed that extensive re-melting and hot-working have not taken place. This assumption is supported by the relatively large amount of copper sulphide particles in the microstructure.
From an archaeological perspective, it is very plausible that ingots or, even more likely, scrap was transported to areas themselves lacking metal ores. In the Netherlands, the famous Drouwen hoard (Butler 1986) found in Drenthe yielded 1.1 kg of bronze scrap of non-local (southern Scandinavian and central European) origin (Butler 1986, 138). The equally well-known Voorhout hoard (traditionally interpreted as a trader’s stock), was recently convincingly shown to have contained not pristine axes (as is to be expected for a ‘merchant’s hoard’) but old, worn and no longer functional axes (i.e. scrap) of French and English origin instead (Fontijn 2008, cf. Kuijpers 2008, 43; 74). Finds like Drouwen and Voorhout, together with the unlikelihood of transporting large quantities of ore, suggest that the Geistingen axes have probably been re-melted from different scrap resources.