VSF 2010 Report - Nabo

Microscopy
The results from the optical microscopy and the SEM-EDS analysis for each sample are
presented in Table 5. The results from the chemical analyses have been normalised, with the
averages given (n= number of analyses), to assist comparability between analyses. All
elements measured and quantified are expressed in oxides (stoichiometrically) in weight
percent (wt%) as normalised values, along with the average value for the raw analytical totals.
All phases identified through optical microscopy were confirmed through chemical analysis.
Tap slag
Figure 10 shows a BSE image of the tap slag analysed. The microstructure consisted of a
largely even distribution of angular and skeletal magnetite in a very fine matrix of fayalite and
glass. The magnetite is quite densely dispersed throughout the sample, except towards the
upper cooling surface which is characterised by large vesicles and a glassy matrix with a
decreased concentration of iron oxides. The high degree of homogeneity within the
microstructure of the sample indicates that the tap slag formed at the same temperature. The
oxidising conditions of the slag formation may be inferred by the high concentration of
densely packed magnetite.
Smelting slag
Two fragments of smelting slag were analysed. The slag contained multiple phases. Wūstite
and fayalite were identified in different crystalline forms along with a glassy phase. A single
iron prill can be seen in Figure 11, surrounded by fine branching wūstite dendrites embedded
in a matrix of acicular fayalite and glass. Many iron prills were discovered in the slag matrix,
demonstrating that the slag was formed from ferrous metallurgy. Different areas within the
slag were characterised by changes in the microstructure, as well as distinctive boundaries and
cracks between the different zones of interest. Transgranular cracks were common throughout
the slag (Figures 13 and 14), which often separated different zones of interest. The zones
differ in their microstructure. This feature was also observed in areas where transgranular
cracks had not formed. The boundary between these different zones is very distinctive,
separating one area densely packed with wüstite in a matrix of large grains of fayalite, from
another area characterised by a much finer grain structure of acicular fayalite and fewer
dendrites of wüstite (Figure 12). The heterogeneity of the crystalline forms reflects changes in
the temperature conditions in which the slag was produced. The contrast between the high
concentration and low concentration regions of wüstite reflect the variability in oxidation.
Smithing hearth bottom
One fragment of smithing hearth bottom slag was analysed. This slag exhibited a
microstructure consisting of iron oxide grains in a matrix of fayalite and glass.
Undiagnostic slag
Two fragments of undiagnostic slag were analysed. Figure 15 shows the microstructure of one
fragment consisting of globular wūstite with few dendritic structures embedded in a matrix of
polycrystalline fayalite and glass. What is interesting is the second fragment of undiagnostic
slag, which revealed semi-fused structure showing mineralic inclusions (Figure 16 and 17).
Although a representative proportion of the mineralic inclusions have been chemically
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