EMAP_2012_Report_6_1.pdf (7.3 MB) - The Heritage Council

clay superstructure (Carlin 2008, 93). These non-slag tapping shaft furnaces were superior to
bowl furnaces as they increase the carbon content of the iron and produce greater amounts
of it (Photos-Jones 2008, 233).
Evidence for the unrefined ‘bloom’ produced in smelting furnaces rarely survive though
examples have been identified at Hardwood 3, Co. Meath (Carlin 2008, 101), Lough Faughan
crannóg (Collins 1955, 71) and Borris (Wallace & Anguilano 2010b, 80-82). The most
common evidence for iron-working comprises the waste slag, produced in the smelting,
bloom-smithing and forging processes (Scott 1991, 151). Microscopic analysis of the slag can
inform about the iron-working process and whether smelting or smithing occurred in a
particular context. It is, theoretically, possible to differentiate between the slag mainly
created in ‘bowl’ furnaces or slag-pit furnaces and the tapped shaft furnaces. The tapped slag
from shaft furnaces has a ‘characteristic drop like surface texture’ (Photos-Jones 2008a, 193)
while the non-tapped slag, characteristic of the slag-pit bowl furnace, tends to form into
rounded ‘furnace bottoms’.
The liquid slag that sunk to the base of the basal pits formed distinctive bowl-shaped ‘furnace
bottoms’ when they solidified. These ‘bowl’ furnace bottoms can easily be confused with the
‘smithing hearth bottoms’. In general these are differentiated on the basis of size, with the
larger being from the smelting process (Scott 1991, 155–60). On this basis Scott re-identified
the furnace bottoms from Ballyvourney as representing smithing rather than smelting activity,
and thought that the same applied to the material from Garranes (ibid. 161–2). He also cast
doubt on the identification of ‘furnace bottoms’ on several other sites. ‘Furnace bottoms’ are a
very common find and were frequently broken up when the furnace was dismantled. If nontapping
shaft furnaces are used they do not produce large furnace bottoms (Young 2011b,
10). Furnace bottoms are defined as being large plano-covex cakes of slag larger than 0.15m
in diameter and weighing more than 4-5kg (Photos-Jones 2010, lxvii).
Site reports using the term ‘furnace bottoms’ have been recorded at 35 sites within the EMAP
2012 gazetteer in varying quantities with Garryduff 1 and St Gobnetts, Ballyvourney
producing by far the greatest number (O’Kelly 1962-4 and O’Kelly 1951-2).
2.3.3.2: Archaeological evidence for smelting furnaces
There was archaeological evidence for early medieval Irish smelting furnaces at 11 older sites
reviewed by Comber (Comber 2008, 115–7). This included sites with multiple furnaces such
as Garranes and Altanagh as well as an example at Reask (Ó Ríordáin 1942 and Williams
1986). A range of thirteen more recently excavated sites within the EMAP 2012 gazetteer,
where specialist reports were available, have produced at least 40 features described as
smelting furnaces with accompanying assemblages of metallurgical residues (Appendix 1.1).
Recently excavated furnaces show a range of a relatively restricted range of dimensions
generally between 0.4-0.7m with a depth of 0.1-0.2m (Table 2.1). Measuring them accurately
can be difficult as the red scorched area or reduced natural clay around the bowl can be
interpreted as a clay lining and removed by the excavator thus enlarging the feature (Young
2009a, 6). Most of the sites in Table 2.1 and those discussed by Comber (2008, 115-124) had
evidence for other metalworking processes such as bloom refinement and other forms of
smithing. Many were also within or on the periphery of enclosures which contained other
habitation evidence. More isolated furnaces are also found for example Hardwood 2 & 3,
Towlaght 1 and Newcastle 2 found along the route of the M4 in counties Meath and Kildare
(Carlin 2008, 94). The remains of vitrified clay fragments were found in several furnaces at
Johnstown 1, Killickaweeny 1, Newcastle 2 and Rossan 3 and were interpreted as the
probable walls of these clay shafts which were broken to remove the bloom (ibid.). However,
it should also be noted that simple ‘bowl’ furnaces may also have contained low clay domed
covers which could have greatly increased their efficiency (Comber 2008, 116–7). It is also
likely that fragments of clay superstructures have been found though have not been
identified as such and items described as ‘furnace linings’ may have as easily formed part of a
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