Litharge from El Centenillo and Fuente Espi: A Geochemical and Mineralogical Investigation of Spanish Silver Processing in the Sierra Morena

Abstract

Galena is treated as the most important silver ore in antiquity and especially in Roman mining history, but many other silver mineralisation and phases occur in the Earth's crust that also contain valuable amounts of silver for exploitation. This study addresses the silver-containing sulfosalts and how to decide between the alternative ores when only metallurgical remains are preserved and the mining context is not evident. Numerous samples of ore minerals, slags, lead metal and stones were collected by one of us (C. Domergue) over several years in the Spanish Sierra Morena, including two Roman foundry sites: Cerro del Plomo and Fuente Espi, both in the mining district of Linares-La Carolina. Cerro del Plomo is closely associated with lead-bearing ore veins near the foundry, while the mines that supplied Fuente Espi with lead ore have not yet been archaeologically explored. The metallurgical remains from the two foundries were analysed for their microstructure, mineralogy and phase composition using microscopy, electron microprobe analysis and X-ray diffraction. It was hoped that the litharge in particular would provide information about the ores used. Metal inclusions of copper and lead were identified, both still containing some silver. The cooling history and stratigraphy of the litharge cakes were developed and parallels drawn with earlier cupellation models. The litharge cakes from Cerro del Plomo and Fuente Espi are comparable in terms of microstructure and phase composition. Chemical and isotope analysis will follow and be the subject of a separate publication.

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Metallography, Microstructure, and Analysis (2023) 12:262–275
https://doi.org/10.1007/s13632-023-00940-8
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PEER-REVIEWED PAPER
Litharge fromEl Centenillo andFuente Espi: AGeochemical
andMineralogical Investigation ofSpanish Silver Processing
intheSierra Morena
P.Krause1,2· S.Klein2,3· C.Domergue4· Chr.Berthold5· N.Jöns1
Received: 1 October 2022 / Revised: 23 December 2022 / Accepted: 3 February 2023 / Published online: 28 April 2023
© The Author(s) 2023
Abstract
Galena is treated as the most important silver ore in antiquity and especially in Roman mining history, but many other silver
mineralisation and phases occur in the Earth's crust that also contain valuable amounts of silver for exploitation. This study
addresses the silver-containing sulfosalts and how to decide between the alternative ores when only metallurgical remains
are preserved and the mining context is not evident. Numerous samples of ore minerals, slags, lead metal and stones were
collected by one of us (C. Domergue) over several years in the Spanish Sierra Morena, including two Roman foundry sites:
Cerro del Plomo and Fuente Espi, both in the mining district of Linares-La Carolina. Cerro del Plomo is closely associated
with lead-bearing ore veins near the foundry, while the mines that supplied Fuente Espi with lead ore have not yet been
archaeologically explored. The metallurgical remains from the two foundries were analysed for their microstructure, min-
eralogy and phase composition using microscopy, electron microprobe analysis and X-ray diffraction. It was hoped that the
litharge in particular would provide information about the ores used. Metal inclusions of copper and lead were identified,
both still containing some silver. The cooling history and stratigraphy of the litharge cakes were developed and parallels
drawn with earlier cupellation models. The litharge cakes from Cerro del Plomo and Fuente Espi are comparable in terms of
microstructure and phase composition. Chemical and isotope analysis will follow and be the subject of a separate publication.
Keywords Archaeometallurgy· Lead ore· Galena· Foundry· Litharge· El Centenillo· Cerro del Plomo· Fuente Espi· La
Carolina· Sierra Morena· Spain· Roman
Introduction
From archaeological point of view, the silver-rich lead
ore, galena, is of outstanding importance as the natural
geo-resource for silver. This is almost independent of the
archaeological period or the region that is the focus of each
study. However, this view on silver is very one-dimensional
from a mineralogical point of view, since very large varia-
tion of silver ore exists in different geological formations
[1]. Paragenesis is known with several elements (Te, Sb, Au,
This invited article is part of a special topical issue of the journal
Metallography, Microstructure, and Analysis on Archaeometallurgy.
The issue was organized by Dr. Patricia Carrizo, National
Technological University – Mendoza Regional, and Dr. Omid
Oudbashi, Art University of Isfahan and The Metropolitan Museum
of Art, on behalf of the ASM International Archaeometallurgy
Committee.
* P. Krause
Paul.Krause@ruhr-uni-bochum.de
1 Institut für Geologie, Mineralogie und Geophysik,
Ruhr-Universität Bochum, Universitätsstrasse 150,
44801Bochum, Germany
2 Forschungsbereich Archäometallurgie, Deutsches
Bergbau-Museum Bochum, Am Bergbaumuseum 31,
44791Bochum, Germany
3 Institut für Archäologische Wissenschaften, Ruhr-Universität
Bochum, Am Bergbaumuseum 31, 44791Bochum, Germany
4 Laboratoire TRACES (UMR 5608 CNRS), Université
Toulouse-Jean Jaurès, 5 Allées Antonio Machado,
31058ToulouseCédex9, France
5 Competence Center Archaeometry – Baden-Wuerttemberg
(CCA-BW), Eberhard Karls-Universität Tübingen,
Wilhelmstraße 56, 72074Tübingen, Germany

263Metallography, Microstructure, and Analysis (2023) 12:262–275
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Cu, As, Hg, S, Cl) and minerals, especially the co-existence
with sphalerite (Zn), galena (Pb), lead sulfosalts, and silver
minerals. Primary silver mineralisation occurs worldwide
in complex hydrothermal veins as sulphide minerals, sul-
phosalts or Bi–Co–Ni–Ag (Schneeberg–Kongsberg type)
or Ag–Co–Ni–Bi–As(-U) (“five-element”) associations, as
galena-hosted Pb from massive sulphide deposits and sub-
volcanic settings (Au–Ag) [2]. Secondary silver mineralisa-
tion forms in oxidation zones, e.g. as jarosite, a group of
hydroxy-sulphate multi-element sandy soils [3].
It is therefore overdue to study the metallurgical remains
of lead silver smelting in more detail archaeometrically,
especially if the remains are decoupled from the supplying
mine, further to find clues as to whether galena or other sil-
ver minerals were used to produce silver metal. The bouquet
of silver minerals and parageneses may also have resulted
in different smelting techniques. Even co-smelting of mix-
tures of sulphide and oxide silver minerals may have been
practiced [4], as the mineralogy of silver shows high poten-
tial for this process. Co-smelting is much easier to perform
than the laborious roasting and sulphide smelting steps [4],
and it does not require pre-roasting of sulphide ore and is
a self-operating process depending on the thermodynamic
conditions (e.g. the free energy of the reaction at certain
temperature). Litharge cakes are hence the most promising
material for this objective, as several oxides (e.g. Cu, Zn,
Fe, Mn, Sn, Sb, As, also Ca, Ba, Mg) are absorbed together
with lead oxide by the collecting clay, which can then better
reflect the mineralogy of the ore than other metallurgical
by-products.
The term "litharge" is originally derived from the Greek
words lithos (stone) and argyros (silver). In the first cen-
tury AD, the term was used by Pedanios Dioskourides in his
De Materia Medica and by Pliny the Elder in his Naturalis
Historia [5]. Several German names are (were) used; pre-
sumably, the most common one is “Bleiglätte” [6]. Others
were more specifically in context to pigments and paint-
ings (“Königsgelb”, “Bleigelb”, or “Goldglätte”). Although
they are now classified as toxic, the red and yellow pigments
are still produced as pigments today [7]. Litharge was also
used during Medieval times in medicine [5]. Mineralogi-
cally, litharge is a modification of divalent lead monoxide
Pb(II)O. Lithargite or litharge, α-Pb(II)O, is intense red as
a powder, tetragonal and stable at room temperature. Mas-
sicotite or massicot, the orthorhombic β-Pb(II)O, is almost
luminescent sulphur- to orpiment-yellow as a powder and
forms from α-Pb(II)O as a metastable phase at transition
temperature of 488°C [8]. The process of transformation
from the high- to low-temperature modification of Pb(II)O
is reversible, but the transformation takes place fairly during
cooling. Rapid cooling therefore preserves the high-temper-
ature phase, whereas slowly cooling causes the two modifi-
cations to co-exist. In nature, litharge occurs as massive to
earthy oxidation product of lead sulphide minerals (galena)
in gossans or mine dumps, which consist mainly of litharge
and only to a small extent of massicot [9]. Metallurgically,
litharge forms as a waste product during the cupellation of
silver as solid lumps or brick-like [5], or accidentally in
metallurgical setting [10, 11]. It was also described more
generally as a product of galena roasting [12]. In fact,
litharge cakes are produced in the cupellation process by
overflowing of the liquid litharge, which is then collected in
a crucible or collecting pit [5, 13, 14]. To make the process
work, a porous, calcium-rich clay is needed, which is mixed
with plant ash, bone ash or a mixture of both to absorb the
litharge and impurities in the process [10]. It hence com-
prises a heterogeneous mixture of clay components with lead
oxide [15]–[20]. On top of the lead oxide-clay mass, “bub-
bles of carbonic acids” develop [19]. This way, separation
and purification of silver can be reached in a single step.
Synthetically, massicot can be produced by heating a pre-
cursor, e.g. cerussite, which decomposes between 130 and
470°C [22]. Parallel to nature and metallurgy, mixtures of
α- and β-Pb(II)O can be reproduced experimentally [23, 24].
Great care must be laid on the procedures in the laboratory,
because massicot was reported to convert back to litharge
by grinding the litharge cake [23]. Questions arise, (1) how
the PbO modifications interact with clay component, (2) in
which of the components of the litharge cakes lost silver is
trapped, and (3) whether the elemental composition of the
silver can be used as a fingerprint for the ore smelted.
Archaeological andGeographical Setting
The samples presented in this paper are from two sites in
the eastern Sierra Morena, in the province of Jaén (Spain).
These are two lead foundries from the Roman period, both
located in the vicinity of silver galena mines in the district
of Linares-La Carolina. The first one, Cerro del Plomo, is
located 20km north of the town of La Carolina, in the mid-
dle of the mountains, on the territory of the El Centenillo
mine, below the mining settlement of the contemporary
period (nineteenth and twentieth centuries). It covered the
top of a hill. In 1968 and 1969, one of us (C. Domergue)
carried out archaeological excavations there, which revealed
that this foundry had experienced two distinct periods of
activity, first in the second and first centuries BCE and then,
in the first and second centuries CE [25, 26]. Lead ores
(galena) were processed here, from which silver-rich lead
was extracted. This lead was subjected to cupellation, which
allowed the silver to be extracted. During this process, the
lead was transformed into oxide (litharge), which could be
melted down again to obtain de-silvered lead (or soft lead),
which was commonly used in Roman times, primarily for
making pipes for water distribution in cities. The location

264 Metallography, Microstructure, and Analysis (2023) 12:262–275
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of the Cerro del Plomo foundry, opposite the entrance to a
mining gallery (Socavón Don Francisco) of a rich vein in
El Centenillo, the Mirador vein, leads us to believe that this
metallurgical factory may have given priority to processing
the galena from this vein. However, as other finds on the
same site suggest, ore from other mines must also have been
processed here. During the excavations, numerous samples
of galena, lead and litharge were collected, sometimes from
well-dated occupation levels. Many of these samples were
put to immediate use and were the subject of quantitative
and semi-quantitative elemental analyses [25]. Those that
remained constitute the material studied in this article.
At about the same time that the archaeological excava-
tions of Cerro del Plomo were taking place, in the vicinity
of La Carolina, between the cemetery and the entrance to
the town, at the place called Fuente Espi, important earth-
works were being carried out in view of the construction
of a showroom for the Santana car company. This work
uncovered the remains of an important lead foundry from the
Roman period, which was active in the first century CE [26].
Nothing was preserved, and no archaeological excavations
could be carried out, but samples of galena, lead, litharge,
and slag were collected. These are among those presented
below. Unlike Cerro del Plomo, which was obviously linked
to the El Centenillo mine, the Fuente Espi foundry was not
located in the territory of a particular mine. Specifically, it
was located in the heart of a mining district, not far from
mines such as El Castillo, La Torrecilla, Los Guindos and
had to process the lead ore from the latter.
Objects found in these two foundries attract attention.
They are used as lead seals, which had been used to seal
bundles carrying material considered valuable enough to
justify such care [27]. They bear various inscriptions, in
particular the abbreviation S.C.: it was thought to be devel-
oped into Societas Castulonensis, Castulo from the name
of an important ancient city in the region, Castulo. As for
the nature of the materials thus delivered to the foundries,
it has been assumed, among other interpretations, that they
could be ore concentrates, transported from other mines to
be processed in one or other of these two foundries. If this
is the case, the samples (galena, lead, litharge, slag) should
indicate different origins.
The Material
The samples originate from the private collection from long
years of research of one of us (C. Domergue). They were
handed over years ago to TRACES (CNRS laboratory at
Toulouse University) and stored in a dedicated room belong-
ing to the metal team (responsible Ch. Rico and S. Baron).
The selection of samples has been made on site with special
focus on litharge (Table1 and Fig.1) and ore from the Sierra
Morena.
Elemental andPhase Analytical Methods
From the Cerro del Plomo site, samples NA5, 5, and 53 (two
thin sections: 53.1 and 53.2) were prepared as thin sections
and were micro-analysed using reflected light microscopy.
From Fuente Espi - La Carolina samples were equally pre-
pared from litharge cakes numbers 63b, 64 and 67.
Reflected light microscopy was performed on the sec-
tions using a Zeiss Axiophot microscope with an Axi-
oCam Camera. For electron probe microanalysis (EPMA),
a Cameca SX Five FE electron microprobe at Ruhr-Uni-
versität Bochum was available. The following crystals were
used: LTAP, for Na, Mg, Al, Si; LPET for K, P, S; LLIF for
Ca, Ag, Pb, Mn, Ni, Zn; and LIF for Fe and Cu. Following
standards were used as available in the microprobe labora-
tory: Jadeite, Diopside, Spessartine, Orthoclase, Fluorapa-
tite, FeS2, Mn, Ni, Cu, ZnS, Ag2Te, PbS. Measuring condi-
tions were 20keV acceleration voltage and 40 nA probe
current. To obtain high spatial resolution, a fully focused
beam was used. X-ray diffraction (XRD) was used for phase
differentiation between litharge and massicot in the sampled
litharge cakes. Different zones of the cake were carefully
separated with a chisel and ground to powder with an agate
mortar. XRD phase analysis was performed for samples
53 and 63b at the CCA-BW, Tübingen using a Bruker D8
advance powder diffractometer equipped with a Cu-sealed
tube (40kV/20mA), a Göbel mirror optics, a 0.2mm diver-
gence slit, a fixed knife edge to suppress air scatter and a
VǺNTEC 1-detector in scanning mode [28]. Measurement
Table 1 Inventory list of selected material from the Toulouse collection. [NA = information not available]
Region 1 Region 2 Site Mine Sample Description 1 Description 2 Description 3 Dating
Sierra Morena Jaen Cerro de Plomo El Centenillo NA 5 Litharge Sondage P1 layer II NA NA
Sierra Morena Jaen Cerro de Plomo El Centenillo 5 Litharge Fonderie site NA NA
Sierra Morena Jaen Cerro de Plomo El Centenillo 53 Litharge Fonderie site NA NA
Sierra Morena Jaen Fuente Espi La Carolina 63 Litharge NA NA 1 ct. CE
Sierra Morena Jaen Fuente Espi La Carolina 67 Litharge NA NA 1 ct. CE
Sierra Morena Jaen Fuente Espi La Carolina 64 Litharge NA NA 1 ct. CE

265Metallography, Microstructure, and Analysis (2023) 12:262–275
1 3
range was 2–70°2Theta, and step size was 0.008° and step
time 405s. For better crystallite statistics, the samples were
rotated during the measurement. The PDF-2 database from
ICDD (International Center of Diffraction Data) was used
for phase identification.
Results
Microscopy oftheLitharge Cake Samples
By cutting the litharge cakes from both sites in slabs, a
gradual change in colour from red to yellow material can
be observed. Under the stereomicroscope, several compo-
sitional zones can be further differentiated: A dark red (I)
and a red zone (II), a reddish zone (III) in transition between
red and yellow areas of the cross sections, and a yellow to
brownish–black zone (IV) in the yellow area of the cross
sections. The boundary between zone I and II is very sharp
and distinct, whereas between II, III and IV the transitions
are smooth. In Fig.2, the different zones are indicated by
yellow lines. All four zones were observed in samples 5
(Cerro del Plomo), 63b and 64 (Fuente Espi). In samples 53
(53.2; Cerro del Plomo) and 64 (Fuente Espi), the yellow-
ish hue of the reddish zone III is evident. In NA5 (Cerro del
Plomo), zone II could not be determined macroscopically,
but was later microstructurally defined.
The main objective of the microscopy was a detailed
mineralogical description of zones I to IV (Fig.3). The
dark red zone I was observed in all samples except sam-
ple 53 (53.2; Cerro del Plomo). This zone is characterized
by lath-like crystals up to > 1mm in length. The reflec-
tive colour is creamy white to light grey with strong red to
orange internal reflections. They form a kind of crystallite
framework, sometimes in preferred orientation. Especially
in sample 64 (Fuente Espi), the lath-like crystals are over-
printed by alteration. The interstices between the lath-like
crystals are filled by a phase with grey reflection colour.
High pore volume contributes to the structure. In samples 53
(53.1; Fuente Espi) and 67 (Cerro del Plomo), small metal
inclusions of pink reflective colour with a size < 50µm were
identified. Based on their microscopic characteristics, they
consist of metallic copper. The boundary between zones I
and II is sharp and distinct. The phase composition of red
zone II becomes generally more complex compared to zone
I. Zone II is again porous and is characterized by a crystal
framework, but here, the creamy white crystals with red to
orange internal reflection tend to be smaller in size, acicu-
lar in shape and mostly occur weathered (Fig.3d). They
decrease in volume portion (observed in samples 5 (Cerro
del Plomo), 53 (53.1 and 53.2, Cerro del Plomo) and 63b
(Fuente Espi)) in favour to a grey matrix, in which individual
phases are almost impossible to recognize. The transition
between zones II and III is smooth. In Zone III, the propor-
tion of the white acicular crystals decreases further in favour
of the grey matrix. The transition from zone III to zone IV is
again smooth. White acicular crystals are absent in zone IV,
but the white phase appears as xenomorphs. Various phases
with a grey reflection colour dominate zone IV. There is a
Fig. 1 Litharge samples from Cerro del Plomo (NA 5, 5, 53) and Fuente Espi (63, 64, 67) as selected for analysis

266 Metallography, Microstructure, and Analysis (2023) 12:262–275
1 3
Fig. 2 Litharge samples observed with the stereomicroscope. (a) Sample 5. (b) Sample NA5. (c) Sample 53.1. (d) Sample 53.2. (e) Sample 63b.
(f) Sample 67. (g) Sample 64
Fig. 3 (a) Zone I with lath-like white crystals having red to orange
internal reflections. (b) Zones I and II with sharp and distinct bound-
ary (black line). (c) Visible change of microstructure between zones
I and II in sample 53.1. (d) Zone II with white acicular crystals and
increasing grey phase. (e) Zones II and III with smooth transition.
(f) Zone IV with metal inclusions in a matrix of grey phases (sample
53.2). Acicular crystals are absent

267Metallography, Microstructure, and Analysis (2023) 12:262–275
1 3
tendency for the proportion of the white phase to continu-
ously decrease from zone I to zone IV.
Metal inclusions are observed in all samples. In samples
5, 53 (53.1 and 53.2; Cerro del Plomo), and 67 (Fuente
Espi), angular inclusions < 50µm in size and with a pink
reflective colour can be identified. They occur preferable in
zones II to IV. In samples NA5 (Cerro del Plomo), 53 (53.2)
and 63b (Fuente Espi), metal inclusions with a striking white
colour were also spotted (Fig.3 f). These are surrounded by
black to dark grey rims of a different phase.
Microscopically, no differentiation between the samples
from the two foundries Cerro del Plomo and Fuente Espi
can be made. Equal in Cerro del Plomo and Fuente Espi
litharge cakes are the four different zones that vary from
dark-red over red to reddish and finally, yellow-brownish
black. The dark red zone comprises lath-like greyish-white
crystals and another open-space-filling phase characterized
by a medium to light grey reflection colour. From the dark
red (I) to the red zone (II), a sharp boundary is observable.
Visibly smaller, acicular white crystals in a greyish-white
matrix characterize the red zone (II), and a gradually transi-
tion to the reddish zone (III) is characteristic. The reddish
zone (III) consists of a grey matrix with metal inclusions
(Cu, Pb). In the zones II and III, the needle-like crystals
occur rarely and mostly, they are characterized by altera-
tion (Fig.3b). In zone IV, the needle-like phase is absent
and the particles with a white reflection colour lost their
Table 2 Comparison of PbO, Al2O3, SiO2, and FeO in phases A-C
PbO Al2O3SiO2Feo
Phase A Phase B Phase C Phase C
Norm. wt.%
Minimum value 94.08 70.21 83.34 0.42 1.85 0.44
25%-Quantile 99.82 97.48 90.65 0.78 3.52 2.33
Median 99.90 99.06 91.15 1.02 3.95 3.21
Average 99.78 96.97 90.54 1.19 4.53 3.47
75%-Quantile 99.93 99.55 91.62 1.41 5.05 4.03
Maximum value 99.98 99.96 92.20 4.46 11.36 10.10
Fig. 4 Box-whisker diagram showing the normalized PbO wt.% in
phases A, B and C. The median Pbo content decreases from phase A
to C

268 Metallography, Microstructure, and Analysis (2023) 12:262–275
1 3
recognizable idiomorphic form. Various phases with grey
reflection colour predominate in zone IV.
Electron Microscopy
Phase identification with electron microprobe analysis
focussed on the phase composition of the lath-like to acicu-
lar phase, the various grey phases and the metal inclusions.
Lead‑Rich Phases
Three major compositions of PbO were identified (Table2),
in which the lead content decreases continuously (Fig.4):
Phase A is the purest PbO (99.90 wt.%). It represents the
white lath-like to acicular phase in zones I-II. In zones II
and III, phase A forms a complex BSE-white to grey matrix
associated with additional phases. In samples NA5, 53
(53.2) from Cerro del Plomo and 63b from Fuente Espi,
phase A also forms the rim around metallic inclusions. Phase
B comprises of 99.06 wt.% PbO with contribution of CaO
and SiO2. It fills the interstitial gaps between the laths in
zone I as observed in samples 5 and 53 (53.1) from Cerro del
Plomo, and 63b and 64 from Fuente Espi. Phase C has the
lowest PbO content of the three (91.15 wt.%) with increased
contents of Al2O3 (1.40 wt.%), SiO2 (3.95 wt.%) and FeO
Fig. 5 Phase diagram SiO2-CaO-PbO. The Ca–Si–Pb–O rich phases
D to G plot in the C2S stability field and along temperature isoch-
rones. Phase D matches the lowest temperature isochrone starting at
1100°C and neighbouring the ganomalite field. Increasing theoreti-
cal temperatures result for phases E–G (according to liquidus projec-
tion of the “PbO”-CaO-SiO2 system [24]; C2S = (Ca1-x-Pbx)2SiO4)),
although this cannot meet the practical temperatures achieved in the
furnace
Table 3 Normalized standard oxides of phases D-G (the Ca–Si-Pb–O phases) and H-M (the Si-Pb–O phases with variable K, Al, Mg, Ca) within the 25%–75% quantile [in weight percent-
age; < 0.xx = below detection limit; “Samples” = obser ved in]
PbO SiO2Al2O3CaO MgO K2ONa2O P2O5CuO FeO Samples
Norm. wt.% from 25%-quantile to 75%-quantile
Phase D 70.80–78.50 12.67–18.12 0.11 – 0.65 6.89–10.02 0.04 – 0.30 < 0,02 < 0,04–0,05 < 0,03–0,30 < 0,09–0,13 < 0,07–1,81 All samples
Phase E 59.91–62.28 18.41–19.00 0,04 -0,06 18.64–20.13 0,02 -0,04 0,04–0,27 < 0,04–0,04 0,03–0,53 < 0,09 < 0,07 NA5, 53, 63b, 64, 67
Phase F 40.39–47.24 21.36–22.77 0,04–0,1 27.02–32.20 0,03–0,06 0,09–0,24 0,04–0,10 0,10–0,23 < 0,09 < 0,07 63b, 64, 67
Phase G 18.95–24.21 28.50–29.75 0,02–0,03 46.13–48.88 0,03–0,04 0,12–0,31 0,13–0,18 0,51–0,85 < 0,09 < 0,07 67
Phase H 64.31–68.27 9.55–11.18 12.55–15,71 6.08–7.18 0,35 -0,70 0,07–1.07 0,13–0,28 < 0,03–0,06 < 0,09 0,09 -0,18 53.2, 63b, 67
Phase J 82.31–83.90 10.41–13.44 2.31–5.08 0.11–1.04 < 0,02–0,24 < 0,02–0,06 < 0,04–0,06 < 0,03–0,06 < 0,09–0,19 < 0,07–0,66 5, 53.2, 63b, 64
Phase K 24,49–35.32 19.15–23.94 13.17–17.51 23.115–28.51 3.09–5.13 0,02–0,13 0,19–0,36 < 0,03–0,03 < 0,09–1,35 0.87–2.44 63b, 64
Phase L 48.55–55.91 19.73–21.52 0,16–0,45 11.77–17.22 11.02–11.33 < 0,02 < 0,04–0,07 0,31–0,37 0,09–0,19 0,08–0,34 63b, 67
Phase M 7.44–21.80 31.28–36.97 24.55–30.48 < 0,02–0,55 0,08–0,22 18.66–23.83 0,66–0,75 < 0,03–0,03 < 0,09 0,15–0,58 53.2, 63b

269Metallography, Microstructure, and Analysis (2023) 12:262–275
1 3
(3.21 wt.%); CaO decreases from phase B to phase C (0.29
wt.% to 0,01 wt.%). Phase C occurs exclusively in zones III
and IV of samples 5, 53 (53.2) from Cerro del Plomo and
63b, 64 and 67 from Fuente Espi.
Ca–Si‑Pb–O Phases
Four Ca–Si-Pb–O phases were observed in zones II,
III and IV (Fig.5). Based on the EPMA analyses and
the derived statistics (25% quantile to 75% quantile)
(Table3), phase D, E, F, and G can be defined (Fig.6).
The most obvious is a decreasing PbO content from D
to G (Fig.5). Phase D is the most common of the four
phases. In zones II and III, D is associated with phase
A. Together they form the white–grey matrix. There is
a tendency for the proportion of phase D to increase
from zone II to zone III and zone IV, and zonation of the
crystals is observed. Phase D occurs in various forms,
either xenomorphic, as partially rounded particles, as
rod-shaped to rectangular or squat crystals, or as clus-
ters. Often, phase D fills the interstices of phase A and
occurs in intergrowths with phase E, which is the second
most abundant Ca–Si–Pb–O phase in samples NA5, 53
(53.1 and 53.2) from Cerro del Plomo and 63b, 64 and
67 from Fuente Espi. Phases F and G were observed less
frequently. Phase F was observed in samples 63b, 64, 67,
and 53.2 (Fig.6f), while phase G exclusively was oberved
in sample 67 from Fuente Espi.
Si–Pb–O Phases withVariable K, Al, Mg, Ca
Phases dominated by Si, Pb and O and with more variable
contents of K, Al, Mg and Ca can be identified in zones
III and IV (phases H-M, Table3 and Fig.6).
Ca–Pb–O Phase
In samples 53 (53.1, 53.2) from Cerro del Plomo and 63b
and 67 from Fuente Espi, a further phase consisting of Ca,
Pb, and O with a wide variation in lead content was identi-
fied. The phase is characterized by a cathodoluminescence
effect.
Metal Inclusions
Metal inclusions of copper and lead are co-existent. Metallic
copper occurs in zones II-IV and predominantly in phases A
or C. Copper inclusions are present as (hyp)idiomorphous
crystals with resorption lacunae, and their size usually does
not exceed 30µm (Figs.7 and 8). Pure lead was identified
as globular inclusions. They have diameters of ~ 25–200µm
and are surrounded by phase A (PbO). The Cu and Pb
Fig. 6 BSE images. (a) Lath-like white crystals of phase A in zone I
(sample NA5). Phase B fills the interstices. (b) Crystals of phase A in
zone I, phase B fills the open space of sample 67 and surrounds pores
(black). (c) Boundary between zone I and II of sample 63b. Phase D
is significantly dominant in zone II, phase A changes from lath-like to
acicular crystals. (d) Zone III of sample 63b with typical microstruc-
ture. (e) Sample 64. Acicular crystals of phase A in zone II. Phase D
forms clusters. (f) Sample 53.2. Phase C, E, F and H. Phase E coexist
with phase D. [Scale bars: (a) 200μm, (b) 500μm, (c) 1000μm, (d)
500μm, (e) 1000μm, (f) 50μm]

270 Metallography, Microstructure, and Analysis (2023) 12:262–275
1 3
inclusions (Table4 and Fig.9) in the majority of samples do
not contain detectable silver. Only Cu inclusions in sample
53.1 contain 580ppm on average (n = 19 single measure-
ments), and Pb inclusions in sample NA5 contain 2030ppm
Ag on average (n = 10 single measurements). Detectable Ni
up to 0.3 wt.% only occurs in the metallic copper inclusions.
The co-occurrence of copper and lead inclusions here is a
key to the temperature range under which they were formed.
The (hyp)idiomorphous copper particles with resorption
lacunae, allow the conclusion that they were exposed to
temperatures not much higher than the melting temperature
of copper at 1083°C. The fact that the lead inclusions are
globular, i.e. that they were exposed to temperatures above
the melting temperature of the lead, is not surprising.
X‑ray Diffraction oftheLitharge Cakes
XRD analysis was performed to determine probable dif-
ferences in mineralogical composition between the dif-
ferent zones and to distinguish between the different
Fig. 7 Metallic Cu-inclusions
in zone IV of sample 64. The
inclusions are encircled. (a)
Cu inclusions with a striking
reflection colour under reflected
light. (b) backscattered electron
image of the identical copper
inclusions. The metallic Cu
does occur within the phase C
Fig. 8 Backscattered electron images: (A) sample 53 (53.2). Idiomorphic Cu inclusion with resorption lagunae in phase A. (b) Metallic Pb (BSE
white) inclusions are surrounded by PbO (BSE grey; phase A)

271Metallography, Microstructure, and Analysis (2023) 12:262–275
1 3
modifications of Pb(II)O (Fig.10). X-ray diffractograms
of samples 53_1 and 63b_1 (zone IV; Fig.10 a) show
comparable phase composition. Ganomalite, a calcium-
rich Pb9Ca6[(Si2O7)3(SiO4)3], can be identified. In nature,
ganomalite is a rare calcium-lead silica mineral belonging to
the sorosilicate group. It can form dull white to grey masses
in association with phlogopite and calcite, also with mac-
edonite, celsian, idocrase and carbonate [29]. Ganomalite
has a wide stability range in the system CaO–SiO2–PbO
[30]. Massicot and litharge are only identifiable (when
truly present) by a slight asymmetry in the main intensity
of ganomalite at 29°2Theta, which overlaps with the main
intensity of massicot, and by an enhanced background signal
in the main intensity region of litharge around 28.6°2Theta.
Ca-rich ganomalite is also a component in Zone III (samples
53_2 and 63b_2), but this time accompanied alongside much
higher peak intensities of litharge, the tetragonal PbO modi-
fication. Massicot occurs subordinately with a weak peak
intensity at about 30° 2Theta. Zone II (sample 53.3) and
zone I (sample 53.4) are nearly coincident and differ only
in peak intensity. Here, the predominant phase is litharge.
Especially for the samples from zone I and II, but also for
the two samples from zone III, it is important to point out the
strongly increased 00l intensities (001, 002) of the litharge
compared to the theoretical intensities from the database.
This is due to a preferential orientation in the powder sam-
ples prepared for XRD analysis of the optically observed
lath-like to acicular crystals of zones I and II. Also, the sig-
nificant decrease in the half-width of the 00l intensities
compared to the reflections from the other lattice directions
could indicate an anisometric crystallite form. Massicot
is present only in minute amounts, and ganomalite is not
detectable in Zone I. Cerussite, a lead carbonate phase, and
calcite were identified in varying amounts in the samples
from zones II-IV. Potentially, calcite contains some Mg, as
indicated by a slight shift in the main intensity. Both miner-
als are presumably secondary products of alteration.
Neither the metal inclusions can be resolved by X-ray dif-
fraction, nor is it possible to identify the different Si–Pb–O
phases H-M. The Ca–Si–Pb–O phases (phases D-G) were
identified as ganomalite in the diffractograms. The chemical
differences of the phases distinguished under the microscope
may be too small to identify them as individual species in
the XRD.
Discussion
The pyrometallurgically formed litharge cakes of Cerro del
Plomo and Fuente Espi show a high-resolution stratigraphy,
which can be subdivided into four zones. Most important
is that the crystallization behaviour of the main phase of
PbO varies: In the dark red zone I, it forms a framework of
large lath-like crystals, which allows the crystallization of a
Ca–Si–Pb phase only in the interstices. Pores have formed,
presumably due to degassing of volatile components. The
adjacent red zone II, separated from I by a sharp bound-
ary, is also dominated by the main PbO phase, but here the
crystals are acicular and much smaller. This allows pre-
cipitation of larger amounts of Ca–Si–Pb–O and Si–Pb–O
phases. Zones III (reddish) and IV (yellow, brown-black) are
also characterized by these phases, and PbO is difficult to
Table 4 EPMA analyses of the metal inclusions in the litharge samples from Cerro del Plomo and Fuente Espi
Sample nb. spots ana-
lysed (n = x)
Inclusion type Cu Pb Ag Zn Ni Fe Mn
Cu Pb Cu-Pb Weight percentage
Cerro del Plomo
53–1 19 x 99.37 0.45 0.06 < 0.03 0.10 < 0.06 < 0.02
53.2 30 x 99.65 0.22 < 0.03 < 0.03 0.13 < 0.06 < 0.02
NA5 10 x < 0.08 99.75 0.20 < 0.03 < 0.02 < 0.06 < 0.02
53.2 11 x < 0.08 99.97 < 0.03 < 0.03 < 0.02 < 0.06 < 0.02
53.2 16 x 74.51 25.33 < 0.03 < 0.03 0.12 < 0.06 < 0.02
53.1 32 x 73.52 26.32 0.05 < 0.03 0.09 < 0.06 < 0.02
Fuente Espi
63b 7 x 99.59 0.19 < 0.03 < 0.03 0.20 < 0.06 < 0.02
64 14 x 99.08 0.50 0.03 < 0.03 0.32 0.07 < 0.02
67 32 x 99.51 0.18 < 0.03 < 0.03 0.30 < 0.06 < 0.02
63b 8 x < 0.08 99.96 < 0.03 < 0.03 < 0.02 < 0.06 < 0.02
63b 1 x 88.15 11.64 < 0.03 < 0.03 0.20 < 0.06 < 0.02
64 19 x 75.66 23.20 0.03 < 0.03 0.21 0.89 < 0.02
67 5 x 63.52 36.20 < 0.03 < 0.03 0.26 < 0.06 < 0.02

272 Metallography, Microstructure, and Analysis (2023) 12:262–275
1 3
differentiate. The yellow and reddish coloured areas of the
litharge cakes visible to the naked eye can be explained by
the XRD results. Litharge, the low-temperature modifica-
tion of PbO, shows red internal reflections and therefore,
appears red, while Massicot, the high-temperature phase,
appears yellow. Litharge is predominant in the deep red or
red areas, while massicot is predominant in the reddish to
yellow–brown areas. Zone I is virtually all litharge, while
massicot increases from Zone II to Zone IV. At the same
time, the calcium-rich ganomalite phase increases, also vis-
ible under the microscope as grey masses in zones III and
IV. The ganomalite signals in the XRD spectra may be repre-
sentative of all Ca–Si–Pb–O phases present, which gradually
differ from each other in chemical composition.
Two models for the formation of litharge during the
cupellation process are described in the literature, one is the
overflowing of the Pb-Ag melt with air, where the forming
PbO is tapped off, the other is by the use of "hollow sticks"
[5]. The litharge cakes of this study were clearly formed by
the process of overflowing. In the cupellation furnace, the
oxidized and liquid PbO, which floated on top of the molten
lead-silver, was poured into crucibles or into a collecting pit
outside the cupellation furnace. Pouring subjected the liquid
to relatively fast cooling, as evidenced by the small size of
the acicular PbO component in zones II–IV and the increas-
ing presence of massicot: The metastable high-temperature
phase is preserved through fast cooling. In contrast, the high
pore volume as well as the extensively grown crystals of
zone I, which has its sharp and distinct boundary with zones
Fig. 9 Bivariate diagrams. (a)
Cu versus Ag in the copper
inclusions. (b) Pb versus Ag
in the lead inclusions [axis
minimum limit Ag = D.L.,
logarithmic scaling, single point
measurements by EPMA.]

273Metallography, Microstructure, and Analysis (2023) 12:262–275
1 3
II-IV, indicate that this was previously a foam on top of the
litharge-clay mass: the PbO crystals had sufficient space
to grow and develop to large and lath-like crystals. This is
also consistent with the dominance of the low-temperature
modification litharge in zone I, which re-modifies from the
metastable high-temperature phase massicot.
The nature of the metal inclusions revealed that in
addition to lead also copper was present in the system.
Fig. 10 X-ray diffractograms of zones I, II-IV and identified phases.
(a) Phases identified in zone IV (samples 63b_1 and 53_1). (b)
Phases identified in zone III (samples 63b_2 and 53_2). (c) Phases
identified in zone I (sample 53_4) and II (sample 53_3). (d) Split of
sample 53 for XRD measurements. (e) Split of sample 63b for XRD
measurements

274 Metallography, Microstructure, and Analysis (2023) 12:262–275
1 3
If detectable, silver could be determined in both copper
and lead inclusions, whereas the copper also contains
nickel. The presence of the high calcium content in the
D-G phases indicates the use of a Ca-rich collection clay,
as has been called for in the literature. Whether bone
or plant ash was used needs further discussion. EPMA
revealed up to 1.1 wt.% phosphorous in one of the litharge
samples from Fuente Espi. But bulk geochemical analy-
ses will be subsequently performed to verify the values.
Envisaged is an extended chemical and isotopic charac-
terization of the material. In a further step, the ore finds
from the collection will also be comparatively analysed.
The (electron) microscopic analyses did not reveal any
differences between the Cerro del Plomo and the Fuente
Espi litharge cakes. However, it remains to be seen if the
geochemical analyses and the comparison with the ore
finds will allow a further differentiation.
Conclusion
Several litharge cake samples from two Roman foundry
sites in the Spanish Sierra Morena were analysed by ste-
reo and reflected light microscopy, XRD and EPMA to
obtain elemental and phase characterization of the litharge
and conclusions about the ore used. In addition, the study
serves to compare the smelting processes carried out
in the two foundries. The litharge samples are strongly
stratigraphed. Within a cake, four compositional zones can
be distinguished, with lead content being depleted from
one to the other, while at the same time the silica- and
calcium-rich phases, including Al, K, and Mg, increase.
The different zones are visible to the naked eye by a colour
change from dark red to brown–black, by phase analy-
sis characterized by different PbO ratios of tetragonal
litharge and orthorhombic massicot phase. Lead and cop-
per are present as inclusions almost throughout the litharge
cakes. Spherical lead testifies to the involvement of molten
lead in the process. Hypidiomorphic to angular copper
inclusions with resorption lacunae are relics of the only
partially melted ore charge, indicating that the lead ore
must have contained copper-rich accompanying phases
that were associated with nickel according to elemental
analysis. Whether crushed ore or concentrates were used
in the foundries cannot be determined by examination of
the litharge. In any case, the remaining gangue is removed
in previous smelting steps in which the sulfidic material
is converted to lead-silver. It is expected that the gangue
material was previously converted to a slag.
The high-resolution stratigraphy of the litharge samples
can be linked to the available theoretical models. The coin-
cidence of stratigraphy and crystal size of the lead-rich
phases is evident. An overflowing process was performed
in which air is blown over a lead-silver melt in a cupella-
tion furnace and lead oxide is tapped into a separate con-
tainer or mould. Due to high degassing, a foam forms on
the top of the solidifying litharge cake, resulting in large
lath-like lead-bearing crystals on the top of the cooling
mass. As the crystals progressively grow into the cake, the
crystal sizes continuously decrease and the crystal texture
changes in favour of a fine-grained structure. Elemental
and mineralogical interpretation shows that litharge cakes
are not a homogeneous mass and therefore need to be
examined only when complete. If only small pieces are
provided for archaeometric analysis, the overall picture
may be missed.
The two foundry sites, "Cerro del Plomo" and "Fuente
Espi", from which the litharge samples were taken, are
assigned differently to their potential ore deposits. While
"Cerro del Plomo" is located opposite "Socavón Don Fran-
cisco" and is closely related to the rich vein of “El Cen-
tenillo”, the ore deposit that fed the Fuente Espi process
has not yet been discovered. However, analyses of litharge
samples from the two sites show no significant differences
in microstructure, phase composition or stratigraphy.
The planned continuation of the study on the remaining
foundry material, including ore, slag, and lead metal finds,
will provide a more complete picture of the two sites.
Acknowledgements The topic is related to a work package within the
Excellent Science-ERC project "Silver Isotopes and the Rise of Money
by Francis Albarède, Lyon. The study is part of a dissertation by the
first author of this paper (P. Krause). This is paper No. 1. The PhD
position were financed by the Deutsches Bergbau-Museum Bochum.
We thank Sandrine Baron, Lyon for providing access to Claude Domer-
gue’s collection. The Deutsches Bergbau-Museum financially sup-
ported the analyses. The Ruhr-Universität Bochum provided access
to their electron microprobe facilities. Our thanks go to Sandra Kruse
called Lüttgen in the preparation laboratory of the German Mining
Museum Bochum for sample preparation and polished sections, and to
Hannah Zietsch, Archaeometallurgy Research Unit, for redrawing the
trivariate diagram. We appreciate the helpful comments of the anony-
mous reviewers and the help of the editor, based on which we were able
to improve the manuscript even further.
Funding Open Access funding enabled and organized by Projekt
DEAL.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
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otherwise in a credit line to the material. If material is not included in
the article's Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.

275Metallography, Microstructure, and Analysis (2023) 12:262–275
1 3
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