3.3 Iron in bars
Wrought-iron bars make up a large part of the cargo. Although oxidation is limited, the bars are cemented together in a continuous concretion (fig. 19). The bulk is 17.0 m long and varies between 6.30 and 7.50 m in width. At the southeastern end it has a central extension of 3.35 m, which is only 1.70 to 2.60 m wide. The pack is 30 to 60 cm thick along its sides. Several fissures occur in the concretion, notably a large lengthwise crack and a crack at right angles midways. Although the cracks may reflect some discontinuities in packing, they were specifically inspected for evidence of bundling in batches. As no such bundling has been observed, the breaks apparently occurred during the formation of the wreck site. No bars are folded as in the cargo of the Gresham Ship of half a century earlier (Auer & Firth 2007). The bars are tightly packed. The total weight of the shipment is estimated at more than 500 tons.
Most bars are rectangular in cross-section, 6 cm wide and 1.5-2 cm thick. Some are square with dimensions of 3.5 x 3.5 cm. Individual bars are at least 2.5 to 3.5 m long. The excavation did not interfere with the consolidated and concreted mass of this part of the cargo. Only eleven loose-lying bars were recovered. Samples show that the iron is in excellent condition (fig. 20). No marks were found on them.
Figure 20 A section of an iron bar shows the metal to be of excellent quality and preservation (photo: A. Overmeer (RCE)).
In order to define production and determine or exclude provenance, a sample was taken of iron bar AM-1999-73 for metallurgical analysis by Joosten & Nienhuis (2012hyperlinkJ&N). Iron and slag inclusions were analysed by energy-dispersive X-ray analysis (EDX, ThermoScientific, NSS) in a scanning electron microscope (SEM, JSM5910LV). Carbon content is 0.7% of the weight, which is appropriate for wrought-iron. The iron contains around 1% of small slag inclusions that are aligned as a result of folding during forging. The core of the sample is ferrite; the cover is steel. This might indicate that the wrought-iron is carbon-enriched in a specialised furnace (Tylecote 1992). Most of the inclusions consist of one phase, fayalite, a glass low in iron and high in calcium, wüstite or quartz. Some inclusions consist of two phases, mainly fayalite and glass. The ratio between SiO2/Al2O3 indicates that some inclusions derive from additions, i.e. sand, during the post-smelting phase. In plotting the major element composition of the rest of the inclusions in a diagram distinguishing the direct from the indirect processes (Dillmann et al. 2007), it is shown that they most probably derive from the direct process. The low manganese and phosphorus content of the inclusions excludes production from high manganese and phosphorus ores.
Iron was produced in the Low Countries, but not on a scale to assume wholesale export; rather, it was imports from Sweden that satisfied the demand (Kuiper 2006, 65; Gawronski 1996, 281). The provenance of the wrought-iron bars might effectively be Sweden, which is known for exporting low-phosphorus steel in the period (Pleiner 2000). Witsen (1671, 119) discusses iron bars, their markings and relative qualities and refers to ‘steel’ from Nuremberg as being 10% more valuable than ‘Swedish steel’. Present research does not permit a final characterisation. No markings have been identified and the relative manganese and phosphor content of ores exploited in the 17th century, including those from Sweden and Bavaria, has not been studied comprehensively. Archaeological parallels are few. The bars in the ‘Gresham ship’ of the outgoing 16th century (Auer & Firth 2007) derive – at least partly – from a manganese-rich and therefore different source (Birch 2010; Birch & Martinón-Torres forthcoming). An adequate comparison for the 17th century is BZN2/BZN15 (Vos 2012), whereas the Hollandia and the Sophia Albertina provide similar material for the 18th century (Gawronski 1996; Overmeer 2012).