METAL LA
International Conference
Archaeometallurgy in Europe III
Abstracts
Andreas Hauptmann, Diana Modarressi-Tehrani and Michael Prange (eds.)
Deutsches Bergbau-Museum Bochum, Germany June 29th -July 1st 2011
I
Finding the invisible sm elt:
using n e w attributes to find the furnace
Birch T.1, Cech B.2, Scholger R.3, Walach G.3 and Stremke F .4
1 D epartm ent o f Archaeology, School o f Geosciences, King's College, U n ive rsity o f Aberdeen, Scotland.
t.birch@ abdn.ac.uk
2 In stitu t fü r Ur- und Frühgeschichte der U niversität Wien, Franz-Kleingasse 1, 1190 Wien, A ustria 3 A ngew andte Geowissenschaften und Geophysik, M on tan u niversitä t Leoben, A ustria
4 Bremen, Germ any
D uring the fie ld w o rk season of 2010, the Ferricum Noricum Project at Hüttenberg (Austria) undertook a short cam paign o f experim ental iron sm elting. Dur
ing the last sm elt, the furnace had to be breached to retrieve the bloom . A fterw ards, the landow ners requested that the site we had been using be cleared and returned to its original state.This cleaning event presented an invaluable opportunity.
The e x p e rim e n ta l s m e ltin g area w as tr a n s fo rm e d fro m s o m e th in g re c o g n iz a b le , to s o m e th in g e n tire ly u n id e n tifia b le . A n y s tra n g e r to the site w o u ld have extrem e d iffic u lty in ju d g in g w h a t events had p re v io u s ly taken place. It became our aim to record the rem ains. H ow v is ib le w ere the s m e ltin g e pisod es a fte r s ig n ific a n t p o s t-d e p o s i- tio n a l disturbances?
The fie ld w o rk m e th o d o lo g y e m p lo ye d w ere a com bination o f standard fie ld w ork techniques com bined w ith techniques dedicated to the recovery of a rch ae om etallurg ica l residues (Bayley et al 2001;
Bayley et al 2008). A to po gra ph ic fie ld w o rk survey was carried o u t along w ith a g eo m a gn etic survey (Figure 1). A tte m p ts w ere m ade to co lle ct surface debris via a m agnetic survey, but the m agnetic na
ture of the secondary diposed gravels rendered this technique ineffective. Features were fin a lly recorded and excavated.
Geophysical prospection by means of geom ag
netic m apping is based on contrasts in the magnetic susceptibility of the archaeological objects (e.g. fur
naces) and the surrounding natural soils and rocks.
These variations cause anom alies in the earth m ag
n etic fie ld w hich can be m easured w ith a p ro to n precession m agnetom eter. Shape and in te n s ity of the m agnetic anomalies give inform ation about the dim ensions and the type o f the soil m onum ent. In the w orking area of the sm elting experim ent in Knap
penberg a geom agnetic survey has been conducted.
346 measuring stations w ith a grid size of 0.5 x 0.5m covered an area o f 10m by 9m (m a xim u m ).T h e in tensity of the total m agnetic field (TMI), the vertical gradient (VG, sensor level 0.5m and 2m) as w ell as the magnetic susceptibility K on the ground level has been observed.
Figure 1 shows the result of the m agnetic survey w ith archaeological features on top. Positive anom a
lies of th eT M l are shown by grey shaded areas w ith s o lid lines w h ile n eg ative areas are g re y shaded w ith dashed lines.The m axim um intensity of theTM l (+300nT) can be id e n tifie d in a zone betw een the sm ithing area in the southeast and the ore roasting area in the center which fits w ell w ith the data of the susceptibility m easurement. The area o f the furnace where high values of K have been observed, is char
acterized by a negative anom aly of theTM l (-150nT).
The hearth and the northern s m ith in g area (low er values o f K) appear as negative anom alies w ith a m in im u m in te n sity of-250nT. The m agnetic d ipole at the western border of the measuring area is likely caused by a shallow buried m etallic body.
The geom agnetic m apping and the archaeologi
cal evidence confirm that heating episodes had taken place.This inform ation alone, however, w ould not be able to specify the nature o f these heating events.
It quickly became apparent that, despite the ado p tio n o f dedicated fie ld w o rk te chn iqu es com b in e d w ith standard m ethods, the sm elting activities that had taken place w ere v irtu a lly invisible. W h ils t the
Fig. 1: Results o f the magnetic survey.
Metaila (Bochum) Sonderheft 4, 2011 1-294
exercise provided a critical evaluation of fie ld w o rk methods, it also yielded further inform ation concern
ing the discovery of 'the smelts'.
The co nstructio n and repair of the furnace had produced a fine lens around the structure, consisting o f a m ixture of ash, fired clay, raw clay and charcoal fragm ents.These attributes clearly distinguished the furnace v ic in ity fro m o the r areas o f b urnin g, such as the s m ith in g hearth and ore roasting area. The ore roasting had left a coarse residue. Despite the ephem eral nature o f our short cam paign o f s m e lt
ing, these lenses provide an invaluable insight into the events th a t occurred.These fin e lenses, which w e te rm 'a ttrib u te s ', m ay im p ly the occurrence of sm elting. It is these attributes that this paper wishes to emphasize, and these a ttrib utes th a t should be considered in fu tu re archaeological investigations, w here sm elting may be implied.
The a tte n tio n m ade to the lenses helped shed new lig h t on the archaeological evidence recorded fro m the Roman sm e ltin g co m p le x o f H üttenberg (see Cech 2008). A ll furnaces recorded so far have yielded a w ealth o f inform a tion concerning aspects o f Roman sm elting. Much o f the m aterial associated w ith sm elting, such as furnace debris, slag and rub
ble, have often been used to infill areas.This year, we looked upon such debris m ore suspiciously. In light of the excavated remains of the experim ental sm elt, layers norm ally interpreted as infill may also contain heavily disturbed traces of sm elting activities. During excavation, lumps of raw and tempered clay no long
er were regarded as infill products. Sim ilarly, coarse textured purplish-red lenses looked surprisingly sim i
lar to the residual deposits generated through experi
m ental ore roasting.These archaeological features could be fu rthe r be understood in term s o f furnace production and ore processing respectively. Associ
ated w ith contexts containing m ixed layers of burnt m ate ria l, raw and fire d clay, these a ttrib u te s may provide the negative im p rin t of heavily disturbed/
removed furnaces, which can be distinguished from other heating activities.
The m in o r traces left beh in d a fte r th e e x p e ri
m ental sm elt, the 'attributes', may act as a referent and be applied to archaeological contexts in order to reconsider and re-interpret those same attributes discovered archaeologically.
Alone, these individual attributes may not seem that significant. W hen considered together, they can prove fundam ental in identifying the invisible smelt.
m aterials fo r conducting the season of experim ental sm elting . We w o u ld like to th an k the project team m e m b e rs w h o p a rtic ip a te d in th e e x p e rim e n ta l sm elting and post-sm elting fieldw ork.
Thanks also to the A ustrian Science Fund fo r f i
nancing the interdisciplinary project on Ferrum N ori
cum in the course of which the experim ental smelts and subsequent experim ental fie ld w o rk have been carried out.
References
Bayley, J., D. Crossley & M. Ponting (2008). M etals and M etalw orking: A research fram ew ork for a rc h a e o m e ta llu rg y . L o n d o n :T h e H istorical M etallurgy Society.
Bayley, J., D. Dungworth & S. Paynter (2001). Archae
om etallurgy. Centre fo r A rchaeology G uide
lines. London: English Heritage.
Cech, B. (ed.) (2008). Die Produktion von Ferrum N o
ricum am H üttenberger Erzberg - Die Ergeb
nisse der interdisziplinären Forschungen a u f der Fundstelle Sem lach/Eisner in den Jahren 2003 - 2005 (The pro du ction o f Ferrum N o ri
cum a t the H üttenberger Erzberg: the results o f interdisciplinary research at Semlach/Eisner between 2003 - 2005).\N\en: Austria Antiqua 2.
Acknowledgements
M any thanks are due to Horst Peter Polzer and Ger
hild Polzer M usenbichler fo r th eir support, enthusi
asm and fo r p ro v id in g us w ith land, fa c ilitie s and
Metalla (Bochum) Sonderheft 4, 2011 1-294
