EMAP_2012_Report_6_1.pdf (7.3 MB) - The Heritage Council

also been found within or close to enclosures at Balriggan, Raystown, Gortnahown 2 and
Castlefarm all of which produced varying quantities of metallurgical residues (Delaney 2011,
Seaver 2009, Young 2009b, O’Connell and Clark 2009).
2.3.3: Furnaces and hearths
The charcoal could be used in the multiple stages required to covert the ore into iron bloom,
subsequently into material suitable for smithing and finally used to make and repair objects.
All these metallurgical processes require intense heat and the control of temperature through
restricting or introducing oxygen. There is a large group of archaeological features which
suggest ferrous metallurgy. From the EMAP 2012 gazetteer, 57/317 sites were described as
having furnaces or hearths with metallurgical residues with some sites having multiple
examples. The difficulty arises in determining which stage of the process is represented by
each feature. The dimensions and character of features can help in determining whether they
were hearths or furnaces. More precise indication of their purpose can usually only be
determined by examining the feature itself along with the metallurgical remains and even
then there can be considerable ambiguity.
2.3.3.1: Smelting Furnaces
The primary stage of iron-working is to convert the ore into a useable form which involves
smelting in a furnace. It is difficult to reconstruct the original form and superstructure of early
medieval smelting furnaces because these rarely survived above ground as they were
dismantled to remove the iron bloom. It was thought that the simple bowl furnace was the
only type used in Ireland during this period (Scott 1991, 159) and these have been identified
as shallow hemispherical burnt depressions in the ground (Edwards 1990, 87). These ‘bowl
furnaces’ may have contained a low clay domed superstructure, which would result in the
discovery of metallurgical ceramic material from the fired lining within the residues.
Both Mytum (1992, 231) and Young (2003, 1-4) have suggested that smelting in early
medieval Ireland occurred within more efficient non-slag-tapping shaft furnaces, known
interchangeably as slag-pit furnaces or low-shaft furnaces. Dowd and Fairburn in their
analysis of a later medieval slag tapping furnace at Farranstack suggested that evidence for
shaft furnaces may extend their use into the early medieval period (Dowd & Fairburn 2005,
115-21). This argument was followed by Carlin in his analysis of the M4 metallurgical features
(2008, 92). These non-tapping slag furnaces comprised a low cylindrical clay shaft 1-2m in
height, with walls 0.2m thick built over a basal pit which preserved the hollow hemispherical
bowl in the ground (Carlin 2008, 92, Wallace & Anguilano 2010b, 70). The shaft is defined as
a ratio of 2:1 of the height of the furnace to the width of the furnace (Photos-Jones 2011,
clxxxiv). The clay material used in these shafts are unlikely to survive as they were distant
from the heat and therefore did not become vitrified and were susceptible to erosion by the
rain (Young 2003, 1). The sides of the chimney may have contained clay blocks with tubular
openings or tuyères to allow blasts of air into the furnace using a bellows to reach the high
temperatures necessary for smelting. Tuyères were also used in smithing hearths and nonferrous
metal-working, thus sometimes complicating the interpretation of iron-working debris
(Scott 1991, 162-63; Carlin 2008, 93).
The furnace was charged with fuel and preheated. When it was hot, mixtures of combustible
organic material such as charcoal and iron ore were fed into the shaft and blasts of air were
pumped in using the bellows. Initial reduction of ore took place at 800°C high up in the
furnace to slag liquidation at over 1,000°C near the base (Wallace & Anguilano 2010b, 70).
During this process, the iron ore was reduced to form an iron bloom (a spongy mass of
metallic iron mixed with slag impurities) and liquid waste slag. The latter ran into the basal pit
to form distinctive bowl-shaped blocks of slag, known as ‘furnace-bottoms’. The raw ‘bloom’
remained within the shaft above ground level near the blow-hole of the bellows and required
further refinement, reheating and hammering in a smithing hearth to remove excess slag and
impurities. The bloom was removed through either the top of the shaft or the breaking of its
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