LATE BRONZE AGE COPPER SMELTING SLAGS FROM LUSERNA (TRENTINO, ITALY) : INTERPRETATION OF THE...

LATE BRONZE AGE COPPER SMELTING SLAGS FROMLUSERNA (TRENTINO , ITALY ) : INTERPRETATION OF THE

METALLURGICAL PROCESS*

A. ADDIS,† I. ANGELINI, P. NIMIS and G. ARTIOLI

Dipartimento di Geoscienze, Università di Padova, via Gradenigo 6, 35131 Padova, Italy;

In the Late Bronze Age, the extractive metallurgy of copper in north-eastern Italy achieved apeak of technological efficiency and mass production, as evidenced by the substantial numberof metallurgical sites and the large volume of slags resulting from smelting activities. In orderto define the technological features of the Late Bronze Age metallurgical process, more than20 slags from the smelting site of Luserna (Trentino, Italy) were fully analysed by means ofoptical microscopy, X-ray powder diffraction, X-ray fluorescence spectrometry and scanningelectron microscopy. Three different slag types were identified based on mineralogical andchemico-physical parameters, each being interpreted as the product of distinct metallurgicalsteps. A Cu-smelting model is proposed accordingly.

KEYWORDS: COPPER SMELTING, SLAGS, LATE BRONZE AGE, ITALY, METALLURGICALPROCESS, TRENTINO

INTRODUCTION

In the Late Bronze Age, intensive metallurgical activities exploited the copper-rich mines innorth-eastern Italy (Marzatico 1997, 2001; Cierny et al. 2004). Perini (1988, 1989) had alreadyobserved that the copious slag remains of ancient copper extractive activity in the Trentino regionduring the Copper Age (CA), the Early Bronze Age (EBA) and the Late Bronze Age (LBA)appear to be really distinct and to follow different patterns of exploitation (Angelini et al.2013). In the central-eastern part of Trentino (Val d’Adige and Alta Valsugana), the CopperAge metallurgical sites are systematically located on the valley floors (Perini 1988, 1989): someexamples are Romagnano Loc, Vela Valbusa, Riparo Gaban and Acquaviva di Besenello (forrecent studies and a summary of the previous work, see Artioli et al. 2009a; Angelini et al.2013). On the other hand, the LBA smelting sites in the same area are for the most part locatedat a considerable altitude, often more than 1000m, such as the sites in Val di Cembra, Val deiMocheni and Fiera di Primiero, and in the Lavarone area. The observed areal distribution patternof the smelting sites and its relationships with the exploited mines is still to be fully decoded.Interestingly, on the Lavarone plateau more than 100 smelting sites yielding tons of slags have beenidentified (Cierny 2008), although there are no copper mines on the plateau, which is dominated bybarren carbonatic rocks (Preuschen 1973; Perini 1988; Pearce and DeGuio 1999; Pearce 2007). Oneof the most interesting and systematically excavated LBA smelting sites on the plateau is the met-allurgical site of Luserna (Nicolis et al. 2007). The site is located in a small flat area, perched onthe slopes of the long ridge that dominates the underlying Astico river valley. Spectacular heapsof copper smelting slags occur in and around the site (Marzatico and Perini 1993; Nicolis et al.2007; Cierny 2008), although the archaeological excavation apparently recovered very few traces

*Received 5 August 2014; accepted 22 October 2014†Corresponding author: email [email protected]

Archaeometry 58, 1 (2016) 96–114 doi: 10.1111/arcm.12160

© 2015 University of Oxford

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of copper ores and rawmetal, and surprisingly scarce evidence of furnaces, tuyeres or crucibles. Thesmelting copper slags thus are the overwhelming evidence of the metallurgical process and themajor feature of the site, besides the topsoil stratigraphy, which shows abundant layers of coaland burnt soil.

The LBA slags from Trentino and Alto Adige have repeatedly been investigated in recent decades,because of their importance in prehistoric metallurgy and because at least one type of slag shows avery distinctive morphology (the so-called ‘flat slags’, or ‘Plattenschlacke’), which is hardly observedanywhere else (Cattoi et al. 1995; Anguilano et al. 2002; Herdits 2003; Metten 2003; Cierny et al.2004; Burger et al. 2007; Bourgarit et al. 2010) and the technical significance of which is uncertain.

Cattoi et al. (1995) analysed 11 slags from the Luserna site, three of which were macroscop-ically classified as flat slags, while the remaining eight were classified as coarse slags. Flat andcoarse slags (‘Schlackenkuchen’ and ‘Plattenschlacke’) are the two main types of slags system-atically present at all LBA smelting sites in the area, although to date the distinction is ratherqualitative and no measurable parameters have been proposed for a reproducible and systematicclassification. Based on a limited number of samples, the mineralogical and chemical investiga-tion of Cattoi et al. (1995) suggested that the relative amounts of fayalite (larger in the flat slags)and quartz (larger in the coarse slags) can effectively be used to discriminate the two groups ofslags. Furthermore, the authors proposed two possible smelting processes to explain the featuresof the slags: one implies that the different slags were produced in different metallurgical steps;the other implies that the slags were produced during a unique metallurgical step, but in differentpositions within the furnace. The subsequent investigations by Anguilano et al. (2002) andMetten (2003) essentially confirmed the mineralogical and chemical distinction of the slagsproposed earlier. At the smelting site of Acqua Fredda (near Trento, Italy), Metten (2003)analysed another group of slags called ‘slag sand’ or ‘Schlackensand’, which were allegedlyderived from the grinding of the coarse slags, for the purpose of beneficiation of thematte/copper content. Slag sands were not found at the smelting site of Luserna (Cierny 2008).

Despite the analytical efforts, the ambiguity in the reconstruction of the smelting processpersists to date, with authors favouring either the single-step (Metten 2003) or the multiple-stepprocess (Anguilano et al. 2002).

The present work aims to investigate a large number of slags (over 90 samples) from recentexcavations of the Luserna site, which were made available by the ‘Soprintendenza per i BeniLibrari, Archivistici e Archeologici of the Provincia Autonoma di Trento’. The aims of ourmulti-analytical work are: (i) to propose identifiable and reproducible parameters to classifythe slag samples; and (ii) to increase the knowledge on the physico-chemical parameters of theslags (viscosity, copper extraction efficiency, operating temperatures etc.) in order to provide asolid physical background for the interpretation of the extraction process.

THE SLAG SAMPLES

The slags studied in this work were recovered from the excavations started in 2005 and conductedby Dr Franco Nicolis of the ‘Soprintendenza per i Beni Librari, Archivistici e Archeologici dellaProvincia Autonoma di Trento’. The excavations were conducted by using several test pits(30×30cm2) systematically covering the site in order to investigate the entire smelting area. The fullcontents of each pit were recorded and systematic sampling of the slags was performed. Based onthis statistical survey, the archaeologists of the ‘Soprintendenza per i Beni Librari, Archivistici eArcheologici of the Provincia Autonoma di Trento’ (Franco Nicolis and Elena Silvestri) selected93 slags as representative of the slag assemblage. As a preliminary, on the basis of the macroscopic

LBA copper smelting slags from Luserna (Trentino, Italy) 97

© 2015 University of Oxford, Archaeometry 58, 1 (2016) 96–114

features of the slags (thickness, surface roughness and fluidal features, type of inclusions, internaltexture, presence of voids etc.), a redundant typological classification was developed, consistingof the following eight groups: coarse, flat, massive, flat–thick, flat–coarse, flat–thick–coarse,coarse–massive and fluid. Taking into account all of the discriminant parameters, the coarse-, flat-and massive-type groups of slags show the most significant differences (Fig. 1).

The coarse type (Fig. 1 (a)) is composed of slags with irregular shapes, high porosity and thediffuse presence of unreacted relicts of quartz and other minerals in the matrix. These slagsexhibit inhomogeneous texture and rough surfaces and they lack flow features. They correspondto the so-called ‘slag cakes’ (or ‘Schlackenkuchen’) described in the literature (Anguilano et al.2002; Herdits 2003; Metten 2003; Cierny et al. 2004).

The flat type (Fig. 1 (b)) has the typical features of the Plattenschlacke slags as described byHerdits (2003), Cierny et al. (2004) and Bourgarit et al. (2010). It is characterized by across-thickness limited to few millimetres (10mm at most), a lack of visible porosity, a homoge-neous texture and very smooth surfaces, sometimes showing features of rapid cooling on one side(cooling ripples). A specific feature of the flat slags is that the unbroken edges are always curvedand thicker than the internal part, suggesting that slag solidification occurred on a liquid materialwith low wettability in contact with the reactor material.

The massive type (Fig. 1 (c)) has not been interpreted as a distinct group in previous studies of theLBA Alpine slags, and has been variously included in either the coarse type (Anguilano et al. 2002)or the flat type (Metten 2003), mostly depending on the slag thickness. The macroscopic features ofthe massive slags are a combination of those of the other two groups: they have a smooth surface, asin the flat slags, and an irregular surface, as in the coarse type. They also display a thick core with ahomogeneous texture, like the flat slags, and a surface porosity, like the coarse slags.

Figure 1 Types of slags: (a) coarse, (b) flat and (c) massive.

98 A. Addis et al.


Each of the other five macroscopic slag typologies shows a subset of one or more characteristicstypical of the main three groups. The flat–thick slags are similar to the flat slags, but thicker. Theflat–coarse type displays the same macroscopic features as the flat slags, but with one irregular andunsmooth side. The flat–thick–coarse type has a thickness similar to the flat–thick type and anirregular surface like the flat–coarse slags. The coarse–massive type shows a compact core likethe massive slags and an irregular surface similar to the coarse type. The fluid type displays flowtextures. These preliminary typological groups did not withstand subsequent discrimination basedon measurable physico-chemical data and they were finally included in the three main groups.

EXPERIMENTAL METHODS

The slag samples were first examined for macroscopic features (colour, texture, shape, thickness,weight, presence/absence of visible quartz, charcoal, and voids) and subsequently divided intoseveral morphological types.

The apparent densities of the samples were measured with a hydrostatic weighting apparatususing the following expression (Wenk and Bulakh 2004):

ρ ¼ Wair

Wair �Wwaterð Þ�ρwater T°Cð Þ; (1)

where Wair is the weight of the slag in air, Wwater is the weight of the slag in water and ρwater (T°C)is the density of the water at the operating temperature. The measured densities (Table 1) werecompared with those calculated from the bulk chemical compositions through commonlyadopted models (Lange and Carmichael 1987; Bottinga and Weill 1972) by using thePetrolog-3 software developed by Danyushevsky and Plechov (2011).

Based on the measured densities, three groups of slags were distinguished, and the mostrepresentative samples of each density group were selected and prepared in thin section for stan-dard minero-petrographic analysis through reflected- and transmitted-light optical microscopy(RL-OM and TL-OM).

The same selected slag samples were also analysed by X-ray powder diffraction (XRPD),using a PANalytical X’Pert PRO diffractometer. Phase identification and semi-quantitative phaseanalysis (Table 2) were performed by means of the X’Pert HighScore Plus version 3.0a softwarepackage, using the reference intensity ratio method (RIR; Chung 1974).

Based on the measured densities and mineralogical analyses, a reclassification of the slagtypology was performed in order to correlate the macroscopic observations with the miner-alogical and physical features. The most representative samples of each classified groupwere then selected for X-ray fluorescence (XRF) bulk chemical analysis. The XRF analyseswere carried out on 3 g of pressed powder by using a WDS sequential Philips PW2400spectrometer. The quantitative analyses (Table 3) were performed using the SuperQ softwarepackage.

The samples were also prepared for electron microscopy as thin polished sections, coatedwith a thin layer of carbon as the conductive material. The analyses were carried out using aCamScan MX 2500 scanning electron microscope (SEM), working in high-vacuum mode,employing a LaB6 filament source and equipped with an energy-dispersive spectrometer(EDS) for chemical analysis. Backscattered electron images and micro-chemical analyseswere acquired using the CamScan Helios 5.2.22 software and the SEM Quant PhiZAF soft-ware, respectively.



EXPERIMENTAL RESULTS

Density

Initially, the density measurements were carried out on all eight preliminarily defined slag typol-ogies, although it was soon apparent that some of them (flat–coarse, flat–thick–coarse, fluid andcoarse–massive) were composed of too few samples and could not be considered as statisticallyindependent groups. The densities of the massive, the coarse and the flat–thick slag types(composed of more than 10 samples each) do not show a normal distribution. With more than50 samples, the flat slag type is the largest group and its data can be adequately modelled by anormal distribution. During selection of the slag samples, care was taken to ensure that thenumber of investigated slags in each group was representative of the actual occurrence of the slagtypes in the archaeological record.

The following observations can be made on the basis of the density measurements (Table 1). (a)The coarse type shows the lowest density range and a mean value around 2.62g cm�3, which isclose to the theoretical density of quartz (ρQz=2.6g cm�3). (b) The massive and flat types showdensity values around 3.6–3.7g cm�3, although they show slightly different intervals and meanvalues (smallest for the flat slags). (c) Two far outliers were identified: one flat slag (ρ=4.17g cm�3)and one massive slag (ρ=2.54g cm�3); the latter turned out to be a fragment of burned rock that hadbeen wrongly classified as a slag. (d) The densities of the samples that had preliminarily beenclassified in the groups with a small number of samples (i.e., flat–thick, flat–coarse and flat–thick–coarse) are well within the range of densities of the flat or massive types.

On the basis of the optical microscopy observations and the XRPD analyses of the mostrepresentative samples of the slag types, it is apparent that the mineralogical composition isroughly consistent with the observed density values, taking porosity into account.

Table 1 Summary statistics for the density values measured on the Luserna slag samples. Archaeological typology:typology macroscopically identified by the archaeologists. Type: Count, total number of slags for each group; SD,

standard deviation; Min., minimum; Max., maximum; LQ, lower quartile; UQ, upper quartile; IQR, range interquartile;ρmod, modelled density values for the melt (see text for details)

(a) Macroscopic typologyName Count Mean Median SD Min. Max. Range LQ UQ IQR

Coarse 9 2.59 2.63 0.17 2.34 2.79 0.44 2.44 2.70 0.26Coarse–massive 1 2.89Flat 57 3.57 3.56 0.11 3.37 4.15 0.79 3.51 3.60 0.10Flat–coarse 1 3.67Flat–thick 13 3.60 3.66 0.19 3.32 3.98 0.65 3.45 3.73 0.28Flat–thick–coarse 1 3.76Fluid 1 3.12Massive 10 3.61 3.73 0.40 2.54 3.89 1.34 3.67 3.77 0.09Total 93

(b) Slag typeName Count Mean Median SD Min. Max. Range LQ UQ IQR ρmod

Coarse 10 2.62 2.64 0.19 2.34 2.89 0.55 2.44 2.74 0.30 2.81Massive 19 3.68 3.73 0.17 3.32 3.98 0.66 3.60 3.77 0.17 3.32Flat 61 3.56 3.56 0.09 3.37 3.78 0.41 3.51 3.61 0.10 3.25Total 90

100 A. Addis et al.


The coarse slags are mainly composed of quartz, although there is a significant amount offayalitic olivine. The presence of fayalite (ρFa = 4.6 g cm�3) should indeed increase the meandensity values of the samples (a slag composed of 75wt% quartz and 25wt% fayalite shouldbe around ρFa25%–Qz75%=3.2 g cm�3), although it is effectively counterbalanced by the presenceof substantial porosity within the sample.

The same can be assumed for the massive and flat slags. The observed values could be ex-pected for slags essentially composed of 50wt% fayalite and 50wt% quartz (ρFa50%–Qz50%=3.6g cm�3), whereas they are mainly composed of fayalite and magnetite (ρMa=5.2g cm

�3) withnegligible quartz, pointing to theoretical density values around ρ=4.7gcm�3. The observed valuesfor the archaeological slags (ρ=3.5–3.6g cm�3) again indicate substantial porosity with the samples.

In the attempt to quantify the measured density values of the slags and to assess their values in theliquid state, the densities of the melts were calculated using the petrological model developed byLange and Carmichael (1987) and Bottinga and Weill (1972), based on the bulk chemical compo-sition of the slags (Table 3). The calculated values (ρmod) are reported in Table 1. The density valuesmodelled on the chemical composition are all much closer to the mean experimental values thanthose that are grossly assessed from the mineralogical phase composition and discussed above.The mineral-based values are all overestimated by about 20–25% with respect to the actualvalues, whereas the chemical composition-based values are within about 10% of themeasured mean values, and they fit even better considering that the silicate melts are tobe considered 10–20% less dense than the compositionally equivalent crystalline solids(Best 2003). Accordingly, the modelled density values for the highly porous coarse slagsare slightly in excess with respect to the measured slag density, whereas the reverse is truefor the rather compact massive and flat slag types.

Mineral composition, texture and cooling rates

Quartz, olivine and magnetite are the most abundant minerals detected in the slags of Luserna. Inthe coarse slags, quartz is the main component (75wt%), followed by olivine (14wt%) and lowamounts of pyroxenes, cristobalite and magnetite (Table 2). Quartz occurs as millimetre-sizedgrains. Such large amounts of quartz would be anomalous in modern copper slags, in whichthe quartz content usually ranges from 30 to 35wt% (Živković et al. 2009; Fernández-Calianiet al. 2012). In principle, the abundant quartz in the coarse slags may have derived from aquartz-dominated gangue closely mixed with the copper ores. However, the closest deposits thatwere exploited for copper, such as the Calceranica and Vetriolo mines (Addis et al. 2012; Addis2013), typically have a fine-grained silicatic gangue (Frizzo 2004), which is inconsistent with thelarge grain size of the quartz observed in the coarse slags. Moreover, the fairly constant quantityof quartz in the coarse slags (75±6wt%) detected in Luserna and in the coarse slags of two otherrecently excavated LBA sites in Trentino (Transacqua and Segonzano) points to a carefully con-trolled addition, possibly gauged to the quality of the ore charge. Thus it is concluded that mostof the quartz was intentionally added during the slagging process (see ‘Interpretation of the metal-lurgical process’). The small but ubiquitous amounts of cristobalite indicate that high temperatures(possibly in excess of 1200 °C) were reached in the slagging process.

The massive and the flat slags are mainly composed of olivine (massive≈ 72wt%;flat≈ 77wt%), with substantial amounts of magnetite (massive≈ 18wt%; flat≈ 12wt%) and py-roxene (massive≈11wt%; flat≈10wt%). Quartz is generally absent. In the massive slags,magnetiteoccurs as large lumps and aggregates, and is often associated with small copper and matte droplets. Inthe flat slags, by contrast, the magnetite is mostly finely dispersed within the olivine matrix.



The olivines frequently show growth zoning. This non-equilibrium feature is typical of solidsolution crystals growing under changing temperature conditions. The micro-chemical analyses(EDS) showed that the crystal cores are enriched in the high-temperature forsteritic componentwith respect to the rims. A rough estimate of the crystallization temperature of the magnesium-rich cores can be obtained by comparison with the forsterite–fayalite (Fo–Fa) binary diagram(Bowen and Schairer 1935). In the coarse and the massive slags, the maximum estimated temper-ature of the Fo-rich core is about 1330 °C (Fo34–Fa66; samples LU-G74 and LU-M5). In the flatslags, the maximum estimated temperature is about 1350 °C (Fo37–Fa63; samples LU-P61 andLU-P34). These temperature estimates refer to a purely binary olivine system. The presence ofother chemical components (see Table 3) may shift the crystallization temperature signifi-cantly. For example, the presence of Ca, Al and Si may decrease the crystallization temper-ature of a fayalite-rich olivine by up to≈ 100 °C, whereas the presence of trivalent iron,

Table 2 The mineralogical phases of the flat slags expressed in weight percentages (wt%) andcalculated by the XRPD–RIR method: SD, standard deviation; �, below the detection limit

Sample Quartz Olivine Cristobalite Magnetite Pyroxene

CoarseLU-G56 71 14 3 3 9LU-G57 66 17 3 5 9LU-G58 81 15 2 2 –LU-G59 80 18 2 – –LU-G69 76 15 3 1 5LU-G70 70 11 4 – 15LU-G72 82 10 2 1 5LU-G74 72 14 3 – 11

Mean 74.8 14.3 2.8 1.5 6.8SD 5.9 2.7 0.7 1.8 5.3

MassiveLU-M4 – 71 – 17 12LU-M5 – 54 – 32 14LU-M47 – 68 – 17 15LU-M60 – 68 – 25 7LU-PSR38 – 64 – 18 18LU-PS45 – 84 – 12 4LU-PS65 – 86 – 14 –LU-PS64 – 79 – 7 14

Mean – 71.8 – 17.7 10.5SD 10.8 7.7 6.2

FlatLU-P68 7 74 – 19 –LU-P6a – 81 – 12 7LU-P6b – 82 – 13 5LU-P9 – 72 – 6 22LU-PR34 – 75 – 18 7LU-P61 – 79 – 5 16

Mean 1.2 77.2 – 12.2 9.5SD 2.9 4.1 5.8 8.0

LU-F52 70 16 3 5 6

102 A. Addis et al.


Table

3XRFbulk

chem

ical

compositio

nsof

thecoarse

slagsof

Luserna,expressedin

wt%

:SD

,standard

deviation;

K,meanvalues

oftheviscosity

indices(SD≤0.2)

Sample

SiO2

TiO

2Al 2O3

Fe 2O3

MnO

MgO

CaO

Na 2O

K2O

P2O5

Pb

Cu

SCu/S

Coarse(K

=0.57)

LU-G

7447.15

0.18

5.46

38.34

0.09

1.09

3.19

0.17

1.28

0.18

0.22

1.02

1.08

0.94

LU-G

7254.54

0.30

8.03

27.39

0.20

2.95

2.34

0.11

0.97

0.21

0.09

1.25

1.18

1.06

LU-G

7056.85

0.16

4.75

27.57

0.12

1.66

3.01

0.11

0.95

0.12

0.06

1.55

2.79

0.56

LU-G

5654.89

0.22

6.38

29.07

0.19

2.19

3.92

0.19

1.45

0.16

0.03

0.42

0.50

0.84

LU-G

5860.26

0.15

3.91

26.96

0.12

1.72

1.84

0.05

0.67

0.10

0.49

0.52

1.24

0.42

LU-G

M57

47.95

0.18

5.25

36.62

0.14

2.14

2.98

0.10

1.02

0.15

0.09

1.43

1.44

0.99

Mean

53.61

0.20

5.63

30.99

0.14

1.96

2.88

0.11

1.06

0.15

0.16

1.03

1.37

0.75

SD

5.12

0.06

1.43

5.10

0.04

0.63

0.72

0.08

0.27

0.04

0.17

0.47

0.76

0.62

Massive

(K=1.71)

LU-PS64

34.52

0.24

6.55

44.34

0.24

3.24

6.77

0.16

1.36

0.17

0.06

0.77

0.89

0.87

LU-PS65

28.41

0.19

5.41

55.55

0.20

2.76

3.23

0.07

0.95

0.14

0.09

1.16

1.04

1.12

LU-M

4725.45

0.18

4.96

57.54

0.19

2.52

3.56

0.04

0.76

0.16

0.08

2.36

1.47

1.61

LU-PSR38

26.04

0.19

5.03

56.97

0.23

3.18

3.73

0.05

0.70

0.13

0.11

1.76

1.11

1.59

LU-M

525.77

0.18

4.64

56.43

0.26

2.33

3.73

0.10

0.87

0.14

0.55

1.35

1.16

1.16

Mean

28.04

0.20

5.32

54.17

0.22

2.81

4.20

0.08

0.93

0.15

0.18

1.48

1.13

1.31

SD

3.81

0.03

0.74

5.54

0.03

0.40

1.45

0.05

0.26

0.02

0.21

0.61

0.22

2.77

Flat(K

=1.39)

LU-P68

35.09

0.25

7.33

45.22

0.23

3.86

4.05

0.10

1.13

0.15

0.06

0.64

1.20

0.53

LU-P9

35.61

0.17

4.83

47.48

0.24

2.52

6.14

0.12

1.38

0.16

0.03

0.43

0.44

0.98

LU-P61

33.87

0.21

5.86

47.31

0.22

2.92

5.39

0.12

1.29

0.16

0.30

0.35

0.96

0.36

LU-P6

31.23

0.22

5.34

52.45

0.24

2.85

4.22

0.08

1.07

0.14

0.08

0.36

0.90

0.40

LU-PR34

28.55

0.23

5.61

53.47

0.26

2.59

4.45

0.04

1.35

0.26

0.36

0.54

0.89

0.61

Mean

32.87

0.22

5.79

49.19

0.24

2.95

4.85

0.09

1.24

0.17

0.17

0.46

0.88

0.52

SD

2.95

0.03

0.94

3.58

0.01

0.54

0.89

0.03

0.14

0.05

0.15

0.13

0.28

0.46



which is indicated by the occurrence of magnetite in the slags, does not significantly affectthe calculated temperatures (Morton and Wingrove 1969, 1972). We should thus considerthe above temperatures as an upper limit for the crystallization of olivine in the slags, whereas a min-imum temperature of ~1200–1250°C can be inferred for the melting conditions. There is no evidencefor significantly different temperature conditions during the production of the different types of slag.

Another distinctive feature of the olivines is the variety of their textures (Fig. 2) that are commonlyinterpreted to reflect different cooling conditions (Faure et al. 2003, 2007). The olivine observed in theporous, quartz-dominated coarse slags is mostly polyhedral (euhedral to subhedral forms, no internalstructure and a solid core). Based on the experimental studies of Donaldson (1976), these olivine mor-phologies suggest low cooling rates (<0.5 °Ch�1). The massive slags are mostly characterized bypolyhedral and hopper olivines (i.e., skeletal crystals with hollow cores and/or embayed edges), whichindicate cooling rates from 0.5 to 40°Ch�1. The olivines in the flat slags are mainly chain-like andrandomly distributed in the matrix, suggesting high cooling rates (>300°Ch�1).

The time taken for solidification of the different types of slag can be estimated from Newton’sLaw of Cooling, taking the velocity involved in the formation of the olivine habit into consider-ation. Indeed, the cooling rate (x) is proportional to the difference between the temperature of theobject (T) and the surrounding temperature (Ta):

dTdt

¼ �k T � Tað Þ: (2)

Solving the differential equation (2), the functionmodelling the cooling time of the slags is as follows:

T tð Þ ¼ T � Tað Þe�kt þ Ta: (3)

In Figure 2, the cooling rates for the different olivine morphologies are plotted as a function of thetime needed to reach 100 °C (the approximate temperature of the slags near to the solid state) startingfrom 1300°C (the approximate temperature of the liquid slags). If we assume that the slags weretapped—that is, air quenched (the upper parts of the slags were brought in rapid contact with theair)—the hopper and prismatic olivines may take about 90 and 180h, respectively, for theirformation. Even considering a thick slag, for which the cooling could have taken a long time fromcore to surface, these times are not consistent with a tapping process: at the surface of the slag, thecooling times are considerably shorter than the ones calculated. Thus it is inferred that the massiveand the coarse slags were cooled slowly within the furnace and/or under a substantial layer ofmaterial, possibly coal or ashes, that hindered the transfer of heat. By contrast, the brief time needed

Figure 2 Olivine formation velocities and the time needed in order to reach the solid state of the slags.

104 A. Addis et al.


for the formation of the chain olivine (≈1h) indicates that the flat slags cooled rapidly, either aftertapping outside the furnace or at the surface of a reactor in contact with a molten fluid.

Cu–Fe sulphides

Observations of the sulphides and copper droplets under RL-OM were performed on 30 represen-tative slag samples. For each type of slag, the most typical sulphides were studied with ascanning electron microscope, and more than 150 chemical point analyses were collected usingan energy-dispersive X-ray spectrometer (see supplementary Table S1 available online).

In order to simplify, the mean chemical compositions of the most representative sulphides ofeach type of slag are plotted in the ternary Cu–Fe–S diagram (Fig. 3).

The Cu–Fe sulphides have different compositions in the three slag types. In the coarse slags, thesulphides mostly consist of large spheroidal segregations with internal symplectitic intergrowths ofchalcopyrite and bornite (defined here as Matte A) in pyrrhotite (maximum diameter 2mm; Fig. 3(b)). In addition, small grains of chalcopyrite were observed (Fig. 3 (a)), which represent relictsof the ore that was used as the charge. This observation confirms the suggestion of Cattoi et al.(1995) that copper was extracted from chalcopyrite-rich ores.

Figure 3 The Cu–Fe–S system at 700 °C (modified after Yund and Kullerud 1966). Bright-field reflected light images:(a) chalcopyrite (Cp); (b) symplectitic intergrowths of chalcopyrite (Cp) and bornite (Bn) in pyrrhotite (Po); (c) covellite(Cv); (d) symplectitic intergrowths of chalcocite (Ch) and bornite; (e) backscattered electron images of copper droplets(in red) observed in the flat slags. Su, Cu–Fe sulphides; Ol, olivine; Ma, magnetite.



The sulphides observed in the massive slags mostly consist of small symplectitic intergrowthsof chalcocite and bornite (Fig. 3 (d)) and crystals of covellite, defined here as Matte B (Fig. 3 (c)).In addition, a few droplets of Matte A and a number of small copper droplets with a mean diameterof 100μm are also observed. The copper droplets are systematically associated with magnetite.

The total number and density of Cu sulphides and copper prills observed in the flat slags are sub-stantially lower than those observed in the coarse and themassive slags. This is in agreement with thebulk contents of Cu and S measured by XRF (see Table 3). In the flat slags, the sulphides are finelydispersed within the olivine matrix (maximum diameter of matte particles 50μm). Their compositionmostly ranges from chalcopyrite to bornite (Matte A), with many particles showing an intermediate,non-equilibrium composition. A few Fe-rich sulphides were observed, which were invariably asso-ciated with the Cu–Fe sulphides. No covellite, digenite or chalcocite was identified. The flat slagsanalysed also show the ubiquitous presence of very small (2–10μm) copper prills (Fig. 3 (e)).

The systematic presence in the slags of sulphides having variable Cu/S ratios (in the range 1:1 to 2:1)indicates the use of a prevailing Cu-rich sulphidic ore in the charge (Tylecote 1987). A systematic in-crease in the Cu/S and Cu/Fe ratios of the residual sulphides is observed along the sequence coarseslag→massive slag→flat slag, which indicates a possible de-sulphurization and matting route of thesmelting process. It is especially important to note that the massive slags show a large number of prillsof bornitic composition (Addis 2013), which are almost absent in the flat slags. Apparently, themain aimduring the production of the massive slags was the efficient breakdown of chalcopyrite into highlyenriched bornite matte, not copper. On the other hand, the scarcity of bornite droplets in the flat slagssuggests that themain aim of the stepwas the decomposition of bornite and the final reduction of copper.

Chemical composition and viscosity

The bulk chemical compositions of the slags are interpreted using the CaAl2Si2O8–SiO2–FeO dia-gram as a base (Schairer 1942; see Fig. 4). The compositions of the six, quartz-dominated coarse slags

Figure 4 Bulk chemical composition of the slags plotted into the ternary CaAl2Si2O8–FeO–SiO2 diagram, modified af-ter Schairer (1942).

106 A. Addis et al.


cluster towards the SiO2 apex. The presence of abundant relict quartz in the samples, however, rulesout an equilibrium crystallization, which would imply temperatures in excess of 1400 °C. In fact, ex-cluding the modal quartz component, all the coarse slag compositions would plot within the fayalitefield, as already noted by Metten (2003). The bulk compositions of the massive and the flat slags, de-void of restitic quartz, are also in the fayalite field (Fig. 4). Based on the CaAl2Si2O8–SiO2–FeOphase diagram, apparent crystallization temperatures in the range 1100–1150 °C can thus beestimated for all the slag types (Fig. 4). Since this simplified diagram does not consider the MgOcomponent, these temperatures should be considered as minimum crystallization temperatures.

The viscosity of the molten slags can be assessed from the measured chemical compositions,as proposed by Bachmann (1982). The viscosity of the liquid slag is expressed as an index (K),calculated as follows:

K ¼ CaO þ MgO þ FeO þ MnO þ K2O þ Na2OSiO2 þ Al2O3

: (4)

The K-index ratio contains the viscosity-reducing oxides in the numerator and the viscosity-increasing oxides in the denominator, so that the higher the index, the more fluid the slag.

The K-index values calculated for the Luserna slags vary from 0.6 to 1.7, consistent with therange of values reported by Bachmann (1980) for other prehistoric copper slags.

The actual viscosity (η) can then be estimated using the following equation:

log η poiseð Þ ¼ 26:46� 23:67 T °Cð Þ�0:001� �� 3:12K þ 3:84 T °Cð Þ�0:001

� �3 þ 0:59K2; (5)

which is based on experimental measurements on modern copper slags (Bachmann 1980;Lutz et al. 1988).

The viscosity values for the three slag types were calculated for different temperatures in therange 1200–1400 °C based on the K indices calculated from the normalized mean chemicalcompositions reported in Table 3. The coarse slags, which have the lowest viscosity index, arethe most viscous slags (η=1244 poise calculated at 1200 °C and η=183 poise at 1400 °C).The calculated viscosities of flat slags are 31 poise at 1200 °C and 5 poise at 1400 °C. The mas-sive slags, which have the highest viscosity indices, are the most fluid slags (η=12 P at 1200 °C;η=2 P at 1400 °C). Considering that the viscosity of a slag must not exceed 5 poise in order to betapped (Rehder 2000), the massive slags are virtually the only slag type that could be easilytapped. Conversely, the high amounts of SiO2 and Al2O3 in the coarse slags make fluid tappingoperations impossible.

Oxygen fugacity

The oxygen partial pressures that are required to oxidize an element or to reduce its oxide atdifferent temperatures can be calculated from Ellingham-type charts (Ellingham 1944; Artioli2010). At 1200 °C, the oxygen partial pressure needed to form copper metal and magnetite,which occur in both the massive and the flat slags, is in the range 10�5–10�9 atm. At 1400 °C,the range is 10–3.8–10–6.5 atm. These values are consistent with those reported by Metten(2003) for the slags of Acqua Fredda. Based on the structure of the smelting furnaces excavatedat Acqua Fredda (Cierny et al. 2004) and the composition of the slags found inside them, Metten(2003) identified two furnace zones characterized by different oxidizing atmospheres at 1200 °C: (1)an upper furnace zone (pO2= 10

–4.5 atm), characterized by the phase association of delafossite(CuFeO2), magnetite, cuprite (Cu2O) and copper; and (2) a lower furnace zone (pO2= 10

–4.5–



10-9.5 atm), characterized by the presence of magnetite and copper metal. The absence ofdelafossite and cuprite in all of the analysed slags from Luserna suggests that the slaggingprocesses were mostly carried out under reducing conditions.

THE INTERPRETATION OF THE METALLURGICAL PROCESS

The early descriptions of a multi-stage smelting process appeared in the extractive metallurgytexts of the sixteenth century. Bachmann (1982) provided a full description of the multi-stepoperations used to treat the primary sulphidic ore: (1) beneficiation of the Cu–Fe sulphide ore;(2) partial roasting (T≈ 800 °C) in order to remove the sulphur of the sulphidic ore as sulphurdioxide; (3) slagging, namely the first smelting operation (T> 1200 °C), in which the roastedore and the quartz were heated in order to remove the iron; (4) matting as the second smeltingoperation, in which the product of the slagging stage is further converted into sulphides thatare more enriched in Cu; and (5) refining of the produced matte to obtain raw copper. Theroasting and slagging operations could be repeated between successive steps to improve theefficiency of the process. The large amount of experimental evidence derived from the analysesof the Luserna slags is consistent with a multi-stage process similar to that described above.Some of the results are especially compelling and supportive of this hypothesis:

• the mineralogy and composition of the three slag groups are consistent with materials producedin separate chemico-physical steps;

• the olivine morphology and texture indicate different cooling rates from a high temperature forthe different slag types;

• it is not possible to derive one slag type (namely the flat slags) from the same batch of themassive slags by simple tapping in the fluid form; and

• the size, relative amount and composition of the residual sulphides in the three slag typesstrongly indicate that the production of each slag group was aimed at specific achievementsalong the chain.

On the basis of these observations, three different operating steps are proposed for the copperextraction process at Luserna, each one constrained by the specific mineralogy and chemistry ofthe three types of slag: coarse, massive and flat.

The first step corresponds to the slagging process, during which chalcopyrite ores (or theirroasted equivalent) are heated above 1200 °C with excess quartz in order to produce a matte(Matte A) that is only enriched in Cu at a low level, and a quartz-dominated highly viscous coarseslag. The coarse slags must have been left to cool down in the furnace, because of their highviscosity.

Concerning the deliberate addition of quartz during the slagging step, several points must bediscussed. In contrast to the model proposed by Tylecote (1987), in which quartz was added tocool down the liquid slags in order to facilitate the separation of the slags from the moltenmatte through some kind of tapping, we suppose that quartz was added in order to obtain aquartz-oversaturated system. Such an excess of quartz helped to promote the separation of ironfrom the sulphidic ore, producing a fayalitic slag without any tapping process (as reportedabove, the tapping condition was not satisfied during this step). As Piel et al. (1992) andMetten (2003) have observed, quartz-rich slags were found at numerous LBA Alpine smeltingsites. Excluding any intentional addition, these authors asserted that the quartz came from thehost rocks of the ores (Hauptmann 2007). Regarding this point, during the last decade we con-ducted a substantial programme of sampling and analysis of the Alpine copper mines within

108 A. Addis et al.


the Alpine Archaeocopper Project (AAcP, http://geo.geoscienze.unipd.it/aacp/welcome.html).The aims of the AAcP are to define the mineralogical, petrological and geochemical aspectsof the ancient copper Alpine mines, and to develop a database of elemental and isotopic tracersfor the provenancing of ancient copper metals and smelting slags (Artioli et al. 2009b, 2010;Nimis et al. 2012). The extensive analyses of several Alpine copper districts allowed us toidentify the Calceranica and/or Vetriolo mines in Valsugana as the possible sources that sup-plied the Luserna site (Addis et al. 2012; see also isotopic data in a forthcoming publication).The fine-grained silicate gangue of these mines (Frizzo 2004) is inconsistent with the coarseslags of Luserna, in which the quartz added is mainly characterized by large grains. This iseven more so if the charge underwent a beneficiation process. Furthermore, the consistentamount of quartz measured in the coarse slags of other LBA Trentino sites (Transacqua andSegonzano, which are located in different Trentino districts and were supplied by other mines)strongly suggests a standardized addition of excess quartz, which implies a standardized LBAAlpine slagging process.

During the second step, the Matte A produced in the first step was used as a charge to produce amatte (Matte B) that is enriched in Cu at a high level, which is abundantly entrapped within the mas-sive slags. This step is the main matting stage. Due to their high fluidity, the massive slags were suit-able for being tapped, although the macroscopic slag features (i.e., the absence of flow features) andevidence of slow-cooling rates (i.e., polyhedral olivine morphology) suggest that tapping was notperformed. Good fluidity, however, certainly ensured efficient slag–matte separation.

The fayalitic flat slags, matte and raw copper are the products of the third step, which started bysmelting the highly Cu-enriched Matte B. Our interpretation is that the sulphides observed in theflat slags are not the main product of the process, but that they represent sulphides (Matte A) with alow level of Cu-enrichment, at the surface of the stratified liquid, which precipitates copper at thebottom. In essence, this step decomposes the starting enriched Matte B into copper +MatteA+ a thin layer of flat slag for Fe trapping. The macroscopic and microscopic features of theflat slags (i.e., extremely low thickness, fluidal borders, a corrugated upper surface in con-tact with air, a very smooth lower surface in contact with liquid and the rapid coolingfeatures of the olivine) support their interpretation as a thin veneer of fayalitic slag formingat the surface of the molten matte, with copper separating by gravity at the bottom.

It is not possible to assess whether the different steps were repeated in multiple cycles, andwhether roasting steps were added between them to aid de-sulphurization and increase thereactivity of the matte.

The velocity and duration of the smelting steps

The separation of the components in the liquid state can be modelled by the simplified relativemovement of droplets within a viscous matrix. The sinking velocity (or buoyancy) is controlledby the temperature, viscosity and density differences between the materials forming the drop andthe matrix. The buoyant force for separation—that is, the sinking velocity of a sphere movingthrough a viscous fluid—is described by Stoke’s Law:

V ¼ gd2Δρ18η

; (6)

where drops having diameter d and density ρdrop move through a fluid with density ρfluid andviscosity η at the specific acceleration of gravity (g). Δρ (= ρdrop – ρfluid) is the density contrast be-tween the particle and the surrounding medium, assuming equilibrium and constant temperature.



http://geo.geoscienze.unipd.it/aacp/welcome.html

Table4

The

separatio

nrates,andthetim

eneeded

forseparatio

n,of

aparticleof

matteandmetallic

copper

with

adiam

eter

d(μm):C,coarse;

M,massive;F,flat;Matte

A,

matte

with

acompositio

nCp 5

0–B

n 50;Matte

B,matte

with

acompositio

nBn 3

3.3–C

v 33.3–C

h 33.3

(a)Separatio

nrates(cmh�

1)

d(μm)

C—Matte

AM—Matte

BM—Cu

F—Cu

F—Matte

A

1200

°C1400

°C1200

°C1400

°C1200

°C1400

°C1200

°C1400

°C1200

°C1400

°C

1000

320

319

1994

746

4909

278

1917

86586

500

15

80499

187

1227

69479

22146

250

01

20125

47307

17120

537

500

01

52

121

50

110

00

00

00

00

00

(b)Tim

eneeded

forseparatio

n(h)

C—Matte

AM—Matte

BM—Cu

F—Matte

AF—Cu

d(μm)

1200

°C1400

°C1200

°C1200

°C1200

°C1400

°C1200

°C1400

°C1200

°C1400

°C

1000

≈5

≤1≤1

≤1≤1

≤1≤1

≤1≤1

≤1500

≈20

≈3

≤1≤1

≤1≤1

≤1≤1

≤1≤1

250

≥48

≈12

≤1≤1

≤1≤1

≤1≤1

≤1≤1

50≥4

8≥4

8≈9

≈5

≈5

≤1≈5

≤1≈1.5

≤110

≥48

≥48

≥48

≥48

≥48

≈14

≥48

≈18

≈36

≈5

110 A. Addis et al.


Considering an operating temperature range of 1200–1400 °C, the separation velocity ofcopper and matte particles with either Matte A or Matte B composition sinking through the slagswas calculated using the physical parameters (density and viscosity) resulting from the presentinvestigation (Table 4 (a)). The calculations were performed for a series of particle sizes (diam-eter in the range 10–1000μm) and selecting the appropriate particle and slag composition in eachstep; that is, Matte A separating from coarse slag, Matte B separating from massive slag, andcopper separating from flat slag. Because of the much higher viscosity of the coarse slags, theslowest of these processes turned out to be the separation of Matte A from the coarse slag(Table 4).

On the basis of the calculated velocities and the observed thickness of the different types ofslags, it is possible to tentatively estimate the duration of the smelting operations (Table 4 (b)).In the coarse slags (maximum thickness≈ 15 cm), the mean diameter of the matte particles isabout 1mm, which indicates that matte prills with a diameter> 1mm could effectively separatefrom the slags. Therefore, the duration of the process was at least≈ 5 h at 1200 °C, decreasing toless than 1 h at 1400 °C. In the massive slags, the matte particles have diameters in the range50–250μm. Accordingly, the duration of the high-temperature processes was 1–9 h at 1200 °Cand 1–5 h at 1400 °C.

The copper prills observed in the flat slags have a maximum diameter of 10μm. Copper prillswith diameter<10μm require more than 30 h at 1200 °C and 5h at 1400 °C. A copper/flat slagdirect separation is thus implausible. It is much more sensible to assume a matte matrix, whereslag droplets float to the upper crust and copper droplets sink to the bottom.

Even if the separation velocities were obtained using several parameters affected by approxi-mations (the viscosity K index, the melting behaviour of the different components etc.), thecalculated values are useful limiting values, which can be combined with the estimated durationsto constrain the process.

CONCLUSIONS

The large body of archaeometric evidence collected on the copper smelting slags from the LBALuserna site form the basis for the interpretation of the copper extraction process. The interpre-tation is essentially based on the properties of the examined slag materials, since little evidenceis reported from the smelting sites of the area, with the remarkable exception of the Acqua Freddasite, where a series of furnaces has been excavated.

It is proposed that the three different slag types found at the Luserna site (coarse, mas-sive and flat) can be distinguished on the basis of chemical, physical and mineralogicalparameters, and that each slag group is the product of a distinct metallurgical step. Ac-cording to the literature schemes, the three steps may be defined as slagging, mattingand smelting. Each of them involved specific operating conditions and yielded differentproduct materials.

The matting and the copper production operations carried out at Luserna seem to haveachieved a rather efficient level of copper extraction, as suggested by the remarkable fluidityof the slags. Physical modelling of the operations based on the calculated viscosities anddensities of the materials indicates that each of the separate steps recognized in the processlasted for a minimum of several hours at the maximum operating temperature (above 1200 °C), so that the complete smelting process implied extensive manpower, massive fuel con-sumption and a carefully controlled sequence of operations.



ACKNOWLEDGEMENTS

The ‘Soprintendenza per i Beni Librari Archivistici e Archeologici della Provincia Autonoma diTrento’, especially in the persons of Franco Nicolis and Paolo Bellintani, is thanked for makingthe slags from the Luserna site available for investigation. We extend our thanks to ElenaSilvestri for her collaboration during the selection of the samples and the early measurementsof the density. Laura Chiarantini provided slag samples for calibration of the XRF spectrometer.Alan Williams helped on an early draft of the paper. David Bourgarit and Andreas Hauptmanncontributed to endless discussions on Alpine prehistoric metallurgy.The work contains part of the result of the Ph.D. dissertation of A. Addis at the University ofPadova, financially supported by the Cassa di Risparmio di Padova e Rovigo (CARIPARO).

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SUPPORTING INFORMATION

Additional supporting information may be found in the online version of this article at thepublisher’s website.

Table S1 The chemical composition of the sulphides observed in the slags of the Luserna siteobtained by means of EDS point analyses and expressed in wt%.

114 A. Addis et al.


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