VSF 2010 Report - Nabo

analysed, the inclusions are awaiting formal identification to distinguish between the different
geological types. Many were confirmed as angular grains of silica. This semi-fused structure
may provide information on the make-up and input materials of the original smelting
environment. Figures 17 and 18 show obvious cellular structures of vegetatious matter that
have been incorporated into the semi-fused structure. Figure 18 shows a negative impression
of charcoal, whilst figure 17 has preliminarlity been identified as conifer (pers. comm. Dawn
Mooney). There is also the possibility that this semi-fused material derives from another
pyrotechnical activity or event.
Flowed slag
The flowed slag exhibited a similar microstructure to the undiagnostic slag sample analysed,
revealing a microstructure consisting of an even distribution of globular wüstite (some
developing into dendrites), in a matrix of blocky fayalite and glass. Prills of iron were also
confirmed on analysis in the slag (Figure 19).
Composition
The bulk composition of the slag samples analysed show a strong degree of similarity. This
would indicate that they derive from the same, or similar, smelting systems As per most
ferrous slags, the main components of the composition are iron oxides (FeO at around 55-60
wt%), silica (SiO 2 19-23% wt%) and alumina (Al 2 O 3 5-7 wt%). The slag contains less than 1
wt% soda (Na 2 O), which we cannot quantify accurately due to the quantification error, often
associated with sodium and SEM analyses. They all contain around 1% phosphorous (P 2 O 5 )
and less than 0.5 wt% sulphur. The alkali elements present, such as potash (K 2 O) and calcium
oxide (CaO) are quantified to around 1 wt% and 3-4 wt% respectively. What is interesting to
observe are the amounts of manganese oxide (MnO), which are consistently high in the
production slags at 5-7 wt%, which could be associated with a manganese-rich bog ore. Small
amounts of titania (TiO 2 ) and barium (BaO) were also detected.
Differences between the slag compositions are worthy to note. The undiagnostic slag
is richer in iron oxides than the corpus of slags analysed, by about 10 wt%, which also marks
the lesser quantity of silica at around 16 wt%. Both the undiagnostic slag and the hearth
bottom fragment analysed contain considerably less manganese than the flowed and smelting
slags analysed. This may indicate some transformation in composition as a result of smithing,
or that the slags themselves derive from a different type of ore.
The smelting slags contain less iron oxides, allowing them to flow more easily than
the other types of slag that are slightly richer (by 5 wt%) in iron oxides. The undiagnostic
residues and smithing hearth bottom may show less fluidity in their appearance which may be
explained by their higher levels of iron oxides. Tap slag is characterised as containing the
least silica, at around 12%, which seems to be substituted by an increase in alkali oxides,
magnesia and iron oxides.
The chemical composition of the slags analysed confirms that the archaeometallurgical
residues result from ferrous metallurgy. When the three main chemical components are
normalised (see Table 6) and the composition plotted onto a FeO-SiO 2 -Al 2 O 3 ternary phase
diagram, information on the temperatures of slag formation can be deduced (see Figure 21).
Discussion and results
The archaeometallurgical assessment of the residues from Vatnsfjörður clearly demonstrate
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