VSF 2010 Report - Nabo

Iceland, the previous two studies have been fundamental in making careful deductions about
furnace morphology. They outline a general furnace model based on the archaeological
referents, deducing that it consists of a subterranean stone-lined base with a turf walled
superstructure. Not only is this implied by the evidence in the archaeological record, but it
corresponds neatly with the apparent absence of technical ceramics associated with iron
production. This also reflects the scarcity of good clay deposits from which to construct a
furnace. The parent material of Iceland’s young geology consists mostly of basic extrusive
igneous rock (mainly basaltic) that has yet to produce mature clay sediments through
chemical weathering.
Relatively little work in the way of archaeometallurgical studies exist for Iceland when
compared to the wider context of North-Western Europe. It is during the last decade that
analytical data on slag compositions has been published from archaeological sites. Tap slag
from two iron production sites, Belgsá and Lundur (Fjnóskadalur), along with a few slag
inclusions from an anvil found in Skógar (southern coast), were analysed for their
composition and microstructure by Buchwald (2005: 332-333). Production slags, metallic
fragments and bog ores were also examined and analysed by Espelund (2003: 158-161; 2007:
65-67) from Viðivellir and Sandartunga in addition to the latter two sites mentioned.
Sigurđardóttir’s (1999; 2004) investigation into the provenance of iron in Iceland examined
smelting and smithing slags from a number of sites, as well as undertaking extensive
programme of analyses of slag inclusions entrapped in iron artefacts. The most recent and
comprehensive analysis of iron production and iron working residues comes from McDonnell
and Maclean’s (2010) slag report for Hofstaðir.
The difference in the format of the results and in aspects of the methodology of slag
analyses between publications prevents an effective comparison of their compositions.
Espelund’s results only report an incomplete selection of oxides present, making it difficult to
judge how representative the data is of the overall composition. Only individual slag phases
were analysed in Sigurđardóttir’s study, and so the results are not fully representative of the
bulk average slag composition. McDonnell and Maclean’s reported results, as well as
Buchwald’s few analyses, list the full range of oxides detected, which are representative of
the average slag composition. Future slag analyses and report of results should promote
comparability with previous studies in order to gain a better understanding of iron production
and iron working in Iceland.
The ’bloomery/direct’ process
Iron may have be recovered from the earth’s surface in the form of meteoric iron, however,
the most common way of obtaining iron was to extract it from its ore. During the Viking Age
and Early Medieval Period, the main method was to smelt iron ores by the ‘direct’ process, or
‘bloomery’ method. Bog iron ore deposits were widespread in Iceland due to the leaching of
iron compounds from the basaltic bedrock that precipitate to form accumulations or
concretions of ‘bog ore’ (hydrated iron oxides). The unwanted non-metallic mineral
component of any ore is known as ‘gangue’, and can often be separated from the iron bearing
minerals through a process often termed ‘ore dressing’ or ‘beneficiation’. Crushing, sorting
and roasting may contribute towards benefiting the ore, as well as removing sulphur from
sulphidic ores.
The direct process is a solid state reaction in which iron ore is processed in a furnace
with fuel (charcoal) at below 1500°C, usually at around 1200°C. In this reaction iron oxides
are reduced to metallic iron, which coalesces and sinters to form a large lump of slag-rich iron
known as the ‘bloom’. The bloom forms around the blow hole, or tuyère, on the inside of the
furnace. The main by-product of this smelting process is known as ‘slag’. A great deal in
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