VSF 2010 Report - Nabo

depth has been written about the direct process in iron smelting technology, of which only a
brief description has been given here (McDonnell 1989: 474; Rostoker and Bronson 1990: 25-
32, 89-91; Scott 1990: 158-70; Tylecote 1980: 209-20).
Slag has been generalized as the “waste product of smelting processes”, however,
“slag is a rather general term, much like ‘ceramic’, and any given smelting site often
comprises a multitude of subtypes of slag” (Pleiner 2000: 251; Rehren et al. 2007: 215). Slag
is formed during the bloomery process from unreduced iron oxides, silica and alumina. The
silica and alumina may come from the furnace clay lining, any additional fluxes added, or
may be present in the ore being processed (as ‘gangue’). Slag liquates at around 1000°C, and
is often described as being liquid or ‘semi-liquid’ during the smelting process. Slag may also
be made from the technical ceramics (such as the ‘tuyeres’ or furnace lining), the fuel ash and
other impurities in the gangue component of the ore.
There are, generally, two furnace construction types; those that tap the slag from the
furnace, and those that do not tap the slag from the furnace. Smelting slag morphologies have
been used to identify different slags produced due to the variation in the furnace structure,
conditions of the smelt, and the raw materials used. Tapping furnaces allow the slag to run out
of the furnace as a slag flow, whilst non-tapping furnaces retain the slag which solidifies into
a solid block at the bottom of the furnace.
Non-tapping furnace constructions usually collect the slag at the bottom of the
furnace. Non-tapping furnace slags are often referred to as ‘slag blocks’, ‘slag cakes’ or
‘furnace bottoms’ (Paynter 2007: 205). One must be wary that there is a distinction between
‘slag blocks’ formed in a pit underneath the furnace, and those formed at the bottom of the
furnace referred to as ‘furnace bottoms’ (Joosten 2004: 16). Slag-pit furnaces utilize pits
which are completely filled with slag to form heavy slag blocks, and these can be recognised
by their characteristic cauldron or ‘plano-convex’ shape due to their cast impression from the
subterranean hearths of slag pit furnaces (Pleiner 2000: 257-9). ‘Furnace bottom’ slag is often
a direct impression of the bottom of the furnace. As a result, iron slag can provide us with
specific information about the furnace design, its shape and dimensions (Pleiner 2000: 251-9).
‘Tap slag’, is formed in tapping furnaces where the structure taps away the slag from
the smelt, often resulting in flows of slag (or ‘runnels’) similar in texture to lava with runs on
its uppermost surface, sometimes referred to as ‘ropey’ (Joosten 2004: 17; Pleiner 2000: 262).
There is also ‘furnace slag’ which is formed in the shaft of the structure, often containing
cinder and charcoal. Overall, iron slag can inform us of the mode of operation; how the slag
was removed from the furnace, providing us with a fundamental insight into the smelting
operations of the furnace used. The composition of slag can lead us to further conclusions on
the temperature of the smelting operations. As already mentioned, it can provide us with
information about the furnace being used. Slag morphology has helped distinguish between
tapped furnaces and non-tapping furnaces; however, the study of slag morphology is not free
from limitations.
Slag can often be inconclusive as to the processes by which it was derived (Joosten
2004: 18), as often the term ‘undiagnostic slag’ is used, as well as the fact that there can be a
great deal of similarity between slags, which offer “little representation of the variability in
furnace designs” (Rehren et al. 2007: 212). Tap slag may be present in archaeological
assemblages associated with non-tapping furnaces (Paynter 2007: 206). Similarly, ‘reheating
slags’ can often be mistaken for furnace bottoms (Joosten 2004: 18). Slag that is tapped into a
pit away from the furnace, in a similar fashion to a slag-pit furnace, can be mistaken for slag
blocks from a non-tapping furnace construction, therefore it is imperative that the contextual
evidence is considered when studying slag (Pleiner 2000: 62-63, 257). In summary, we must
accept that the visual analysis of slag may not always be a conclusive method for
distinguishing between mode of operation in bloomery smelting, or furnace designs. A lot of
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