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31
IX/1/2018
InterdIscIplInarIa archaeologIca
natural scIences In archaeology
homepage: http://www.iansa.eu
Archaeometric study of Roman tesserae from Salamanca (Spain).
Archaeology and geochemical analysis
Verónica Pérez de Dios
a
, Mª de los Reyes de Soto García
b*
,
Isabel de Soto García
c
, Rosario García Giménez
d
a
Department of Prehistory, Ancient History and Archaeology, Faculty of Geography and History, University of Salamanca, Calle de Serranos, 37008
Salamanca, Spain
b
Institute of Archaeology of Merida (IAM)-Spanish National Research Council- Junta de Extremadura- Plaza de España,15, 06800 Mérida (Badajoz), Spain
c
Department of Sciences, School of Agricultural Engineering, Public University of Navarre, Los Olivos Building, Campus Arrosadía, 31006 Pamplona, Spain
d
Departament of Geology and Geochemistry. Faculty of Science. Autonomous University of Madrid, Campus de Campoblanco C, Francisco Tomás y
Valiente 7, Módulo 06, 28049 Madrid, Spain
1. Introduction
The taste for decorative fooring is not a particular tradition
of the Roman Empire: in fact we need to go back in time
until Uruk’s period, at the Mesopotamian culture. The people
of Uruk were the frst culture to beautify walls and columns
with geometric mosaic-based terracotta cones of diferent
shades, whose decorative function evinced the extravagance
of the place and its owner (Rossi, 1971, pp. 6–34; Bertelli,
1988, pp. 9–44; Fiorentini, 2001, pp. 17–39; Palomar, 2011,
p. 54). The musivaria technique has not only become clear
in the archaeological remains but also in classical written
sources – Vitruvius and Pliny and the Edict of Diocletian
itself. Therefore, mosaics are one of the decorative elements
used by the Romans to decorate their buildings. It consists of
small pieces, called tesserae, whose combination following
a model or set pattern were used to decorate pavement. The
tesserae are usually cubic in shape and made of diferent
materials such as marble, ceramic, stone or glass. The
diferent types of colours and sizes allow them to create
patterns – from the most simple to more elaborate – all of it
can be observed for example in the large villas of the Roman
Empire, such as the Roman villa of Casale in Sicily (Italy) or
the Roman villa of Noheda in Cuenca (Spain).
Traditionally in Spain, mosaic pavements have been
studied from an iconographic point of view. However, there
are only a few examples of studies of the characterization of
tesserae mosaics (Palomar, 2011; Flores
et al.
, 2011; de Soto
et al.
, 2014). The last few years, the rise of multidisciplinary
analysis has greatly increased interest in the realization of
archaeometric studies, although these types of analysis were
not very abundant in Spanish science and not implemented
widely until the 1980s (Garcia, Olaetxea, 1992, p. 266). These
studies, apart from their purely descriptive interest, were
intended to identify the petrography, disclose manufacturing
techniques of the tesserae, analyse the sources of supply
of materials to thus make inferences about trade relations,
and alleviate the problems of conservation of pavements
Volume IX ● Issue 1/2018 ● Pages 31–42
*Corresponding author. E-mail: reyesdesoto@gmail.com
ARTICLE INFO
Article history
Received: 17
th
February 2017
Accepted: 4
th
June 2018
DOI: http://dx.doi.org/ 10.24916/iansa.2018.1.3
Keywords:
tesserae
Roman villa
Salamanca
geo-chemical analysis
ABSTRACT
In this study an analysis was made of 37 samples of tesserae of diferent shades and colours from
Roman archaeological sites in the province of Salamanca (Spain), where pavements of
opus
tessellatum
have been located. The tesserae were characterized by Mass Inductively Coupled Plasma
Mass Spectrometry (ICP-MS), X-Ray Difraction (XRD) and Spectrometry, in order to determine their
origin, composition and technique of manufacture. Results could reveal to us the local production of
certain tesserae and the existence of a commercial network through the
Via de la Plata
and its main
roads in the Iberian Peninsula.
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IANSA 2018 ● IX/1 ● 31–42
Verónica Pérez de Dios, M
a
de los Reyes de Soto García, Isabel de Soto García, Rosario García Giménez: Archaeometric Study of Roman Tesserae
from Salamanca (Spain). Archaeology and Geochemical Analysis
32
(Butzer, 1989, p. 153; James, 2006). In this sense, the main
objective of our work is to analyse the petrographic origin of
the tesserae from the diferent mosaics known so far in the
province of Salamanca, to infer some historical conclusions,
and corroborate the existence of certain trade routes linked
to mosaic workshops.
In this article we will study 37 tesserae samples of diferent
colours from seven Roman sites in Salamanca (Spain): Los
Villares -LV- (Fresno Alhándiga); Alquería de Azán -AZ-
and Aldearrica -AL- (Miranda de Azán); San Julián de
la Valmuza -SJ- (San Julián de la Valmuza); San Morales
-SM- (Aceña de la Fuente); La Vega -LVE- (Villoruela); and
Saelices -SA- (Saelices el Chico).
The aim of this work is to study the origin, composition
and the technique of manufacture of the tesserae in order to
determine local production or the existence of a commercial
route connecting mosaic workshops. The samples were
analyzed using geochemical techniques such as Inductively
Coupled Plasma Mass Spectrometry (ICP- MS) and X-Ray
Difraction (XRD). Finally, the results were studied using
statistical techniques.
2. Geographic context
The Salamanca region is situated in central-western Spain. It
is bordered to the north by Zamora and Valladolid, to the east
by Ávila, to the south by Caceres and to the west by Portugal
(Figure 1). The studied area is geologically situated within
the Central Iberian Zone. It has an average altitude around
800 m but there are large variations throughout the province,
upward to 2,428 m in the Sierra de Béjar and down to 116 m
in the valley of the Salto de Saucelle. From a geographical
point of view, the province of Salamanca is divided into
three diferent areas (Lucena-Conde, García, 1976):
The mountains: located in the south of the region. The
altitude ranges between 1,000–2,500 m. Granites, quartzites
and slates are present in this area. The soils are acid and
siliceous-humid soils are abundant in the mountain areas.
The high plateau: located in the centre of the region and in
the W and NW. Slates and granites are abundant in this area
and thus soils are siliceous.
The Duero Basin: materials are sedimentary rocks such as
conglomerates, sands, clays and limestone from the Tertiary
era. The most productive soils of the Salamanca region are
located in this area.
Alongside these geographical features, the abundance of
waterways and the likely environmental conditions became
the central plateau in a favourable area for the proliferation
of new settlements in Roman times.
3. Antecedents: Roman
villae
with mosaic pavements of
the province of Salamanca (Spain)
Although the urban development of the modern city of
Salamanca began in the second half of the frst century AD,
the evolution of the Roman rural settlement is unknown.
However, we have one important archaeological fact, the
creation of the Silver Way, a reference point next to the
famous
mansios
and other rural settlements of various types
with productive and residential functions (Roldán, 1971;
Martín, Benet, Macarro, 1991; Salinas, 1998; Ariño, Díaz,
1999, pp. 153–192; Menéndez, 2000–2001; Chavarria,
2006, pp. 18–25). A recent archaeological survey carried out
in the province has corroborated the existence of numerous
agricultural and livestock establishments of the second order,
and obviously larger settlements that may be classifed as
villae,
thanks to the ostentatious archaeological material
recovered
(Pérez, 2014; de Soto, 2015). Although in the
Provincial Archaeological Inventory, most deposits of Roman
chronology are identifed as
villae
, it is necessary to compare
information based on ancient archaeological surveys. In order
to determine which settlements could safely be ascribed to
this category, we have analysed the databases in which it
is recorded all the archaeological material from the Roman
sites of Salamanca province. The aim was to locate certain
ostentatious materials, such as mosaic pavements, in order to
corroborate the existence of
villae
. The search results allowed
us to catalogue seven sites with mosaics foors, six of them
located in the valley of the Tormes River, and the last one
placed on the riverside of the Agueda.
3.1 Mosaic pavements in the West: the village of Saelices
Located in the municipality of Saelices el Chico and invading
most of the current town, crops and column bases of Roman
chronology were discovered when the foundation of a
house was made in the 1980s. Subsequent archaeological
excavation of the site brought to light several housing
structures in which appeared a mosaic of about 13 m
2
with
a geometric decoration that highlights Solomon’s knots that
form three rows of square lattices of 48.5×48.5 cm in which
various geometric motives, vegetable and chequered patterns
are inserted. The pavement, very similar to the north gallery
of the peristyle of the Roman villa of La Olmeda, used to be
part of a corridor of the urban part of the villa (Chamoso,
Hernandez, 1997; Chamoso, 2007).
3.2 Roman villas with mosaic pavements in the valley of
the Tormes River
In the municipality of Miranda de Azan, two settlements
have been located with mosaic pavements: Alquería de
Azán, the most relevant of these, and Aldearrica. The site of
Alquería de Azán was frst recorded by Father César Morán,
who documented slate graves and archaeological remains
attributable to Roman times (Morán, 2000, p. 58). Later,
in the 1980s, Benet and Santonja documented white, black
and red tiles of a small size that corroborated the existence
of at least one mosaic foor (Benet, Santonja, 1990, p. 286).
Meanwhile, at the site of Aldearrica, cited by Morán and
Maluquer, a scattering of Roman materials, including various
tesserae, were found (Morán, 2000; Maluquer, 1956).
North of Miranda de Azán, in the municipality of Doñinos,
the frst fgurative mosaic of the province of Salamanca was
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Verónica Pérez de Dios, M
a
de los Reyes de Soto García, Isabel de Soto García, Rosario García Giménez: Archaeometric Study of Roman Tesserae
from Salamanca (Spain). Archaeology and Geochemical Analysis
33
discovered in 1801 at San Julián de la Valmuza Roman’s
villa. The mosaic foor was published in 1832 by Cean
Bermudez in “Summary of Roman Antiquities of Spain”
(Cean Bermudez, 1832). In 1984–1985, the
villa
was
partially excavated and a new pavement of
opus signinum
and
opus tesselatum
was found (Garcia, Serrano, 1996,
pp. 35–38; Regueras, Pérez, 1997, pp. 17–19).
On the other bank of the River Tormes, east of the city
of Salamanca, remains of mosaic pavement were found at
La Aceña de la Fuente, El Cenizal (San Morales) and La
Vega (Villoria and Villoruela). The frst settlement has two
distinct parts: a residential sector (La Aceña de la Fuente)
and a necropolis in the sector where ashen spots are visible
(El Cenizal) (Excar, 1991–1992). In the frst of these, Luis
Angoso located numerous fragments of Roman pottery,
metal elements, mosaic tiles and 16 fragments of slates with
textual inscriptions and numerals. In 1985, a few kilometres
from this site, between the municipalities of Villoria and
Villoruela, a Roman villa called La Vega was discovered.
In that year, García Figerola and Angoso conducted an
archaeological excavation of 12×12 m in which were
discovered three foors of
opus tesselatum
with a geometric
decoration made up of small tesserae of diferent colours
(white, black, red and yellow) and an important piece of a
mural painting (García Figuerola, Angoso, 1986; Regeras,
Pérez, 1997, pp. 51–60).
In 2015, the archaeological survey undertaken in the south
of the province by Pérez de Dios led to the discovery of a
new Roman settlement with a mosaic foor, located in the
present municipality of Fresno Alhándiga. The subsequent
archaeological excavation revealed the existence of a large
settlement next to the Silver Way, where was documented
part of a thermal resort with three diferent rooms. The
frst room contained a geometric mosaic foor with fsh and
dolphins, while the other two rooms of smaller size had
opus signinum
and one
hypocaustum
in perfect condition of
preservation (Pérez, de Soto, 2017).
4. Materials and methods
A selection of 37 tesselatum samples from seven
archaeological excavations in Salamanca (Spain) (Figure 1)
have been studied in order to diagnose their origin. Tesserae
have been sampled from diferent parts of the mosaics with
the purpose of creating a colour representative sample set.
However, in order to minimize the impact and considering
the archaeological importance of the fnd, sampling has been
limited to this number of samples.
Mosaic tesserae are small parallelepipeds with an irregular
structure that could be made of diferent kind of materials
such as stone, marble, glass or pottery. Samples of red, green,
white, cream and blue have been selected with a rectangular
prism shape and average dimensions of 1.0×1.0×1.5 cm. In
addition, blue and cream samples usually have conchoidal
fractures. A summary description of each sample is included
in Table 1. The colour of the samples was studied using the
Munsell Soil Color Chart (Munsell, 1975).
These samples were characterized according to their
mineralogy and chemistry. In all cases, a minimal part of
the sample was taken to minimize damage to archaeological
objects. The samples were labelled “LV”, “AZ”, “SJ”, “AL”,
“SM”, “LVE” and “SA”, to indicate the location of the
archaeological site, followed by an identifcation number.
Figure 1.
Map with a geographical position
of the analysed sites 1 – Los Villares
(Fresno Alhándiga); 2, 3 – Alquería de
Azán and Aldearrica (Miranda de Azán);
4 – San Julián de la Valmuza (San Julián
de la Valmuza); 5 – San Morales (Aceña
de la Fuente); 6 – La Vega (Villoruela) and
7 – Saelices (Saelices el Chico).
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IANSA 2018 ● IX/1 ● 31–42
Verónica Pérez de Dios, M
a
de los Reyes de Soto García, Isabel de Soto García, Rosario García Giménez: Archaeometric Study of Roman Tesserae
from Salamanca (Spain). Archaeology and Geochemical Analysis
34
Table 1.
Samples studied: physical characteristics.
SampleArchaeological siteMunsell colorDescription of the sample
LV2Los Villares
Light grey (Gley 1 7/N)
Stone
LV3
Los Villares
Light grey (Gley 1 7/N)
Stone
LV4
Los Villares
White (2.5Y 8/1)
Stone
LV5
Los Villares
Light red (10 R 6/6)
Tegulae
LV6
Los Villares
Light red (2.5YR 6/4)
Stone
AZ1
Alquería de Azán
Light bluish grey (Gley 2 7/ 10B)
Stone
AZ2
Alquería de Azán
Very pale brown (10 YR 8/2)
Stone
AZ3
Alquería de Azán
White (7.5 YR 8/1)
Stone
AZ4
Alquería de Azán
Reddish yellow (5 YR 6/6)
Tegulae
AZ5
Alquería de Azán
Pink (7.5 YR 7/3)
Stone
AZ6
Alquería de Azán
White (10YR 8/1)
Stone
SJ1
San Julián de la Valmuza
Light red (2.5YR 7/6)
Tegulae
SJ2
San Julián de la Valmuza
Very pale brown (10 YR 7/3)
Stone
SJ3
San Julián de la Valmuza
Light grey (5YR 7/1)
Stone
SJ4
San Julián de la Valmuza
Yellow (10 R 8/6)
Stone
AL1
Aldearrica
Greenish grey (Gley 2 6/10BG)
Stone
AL2
Aldearrica
White (10YR 8/1)
Stone
SM1
San Morales
White (2.5YR 8/1)
Stone
SM2
San Morales
Brownish yellow (10 YR 6/6)
Stone
SM3
San Morales
Very pale brown (10 YR 8/2)
Stone
SM4
San Morales
White (2.5Y 8/1)
Stone
SM5
San Morales
White (2.5Y 8/1)
Stone
SM6
San Morales
White (Gley 1 8/1)
Glass
LVE1
La Vega
Reddish yellow (5 YR 6/6)
Tegulae
LVE2
La Vega
Reddish yellow (5 YR 6/8)
Stone
LVE3
La Vega
Pink (5 YR 8/3)
Stone
LVE4
La Vega
Light bluish grey (Gley 2 7/ 10B)
Stone
LVE5
La VegaLight greenish grey (Gley 8 10Y)Stone
LVE6
La Vega
Very pale brown (10 YR 8/3)
Stone
LVE7
La Vega
White (2.5YR 8/1)
Stone
LVE8
La Vega
White (2.5Y 8/1)
Stone
LVE9
La Vega
Pink (7.5 YR 7/4)
Stone
SA1
Saelices
Light grey (5YR 8/1)
Stone
SA2
Saelices
Very pale brown (10 YR 8/2
Stone
SA3
Saelices
White (2.5YR 8/1)
Stone
SA4
Saelices
White (2.5Y 8/1)
Stone
SA5
Saelices
Pink (7.5 YR 7/4)
Stone
4.1 Mineralogical Analysis
XRD is a powerful tool in characterizing archaeological
materials (Eiland, Williams, 2001). For this reason, the
mineralogical compositions of the samples were determined
by X-Ray difraction (XDR) using a SIEMENS D-5000 with
a Cu anode, operating at 30 mA and 40 kV, using divergence
and reception slits of 2 mm and 0.6 mm, respectively.
Peaks were identifed following the criteria proposed
by Schultz (1964) and Brindley and Brown (1984). The
characterization of the bulk samples was performed with
the Rielveld method (Rietveld, 1969; De Ruan and Ward,
2002). This method allows us to quantify the mineralogical
and amorphous phases of samples that cannot be analysed
by other experimental techniques (Viani
et al.
, 1999); for
this reason, this method has been used in archaeology for
the characterization of ceramics (Comodi
et al
., 2004;
Compaña
et al.
, 2010).
The minerals identifed were phyllosilicates, quartz
feldspar-K, plagioclase, calcite, pyroxene, dolomite and
amorphous material.
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Verónica Pérez de Dios, M
a
de los Reyes de Soto García, Isabel de Soto García, Rosario García Giménez: Archaeometric Study of Roman Tesserae
from Salamanca (Spain). Archaeology and Geochemical Analysis
35
4.2 Chemical analysis
Dissolution of the samples was performed as follows (García
Giménez
et al
., 2005): a minimum amount of the sample was
treated with hydrofuoric acid in an open vessel, and heated
on a hot plate until dryness. This was followed by the addition
of aqua regia, and heated again until dryness. The residue
was dissolved with 1 ml of concentrated hydrochloric acid
and diluted with water to the mark in the Tefon volumetric
fasks. Care was taken to keep contamination to a minimum.
Ultrapure water was used throughout and all reagents
used were of analytical grade. Chemical analyses of both
major and minor elements were performed by inductively
coupled plasma-mass spectrometry (ICP-MS) in a SciexElan
6000 Perkin Elmer spectrometer equipped with an AS91
autosampler. Inductively-coupled plasma spectrometry
is one of the most important chemical techniques for the
characterization of solid materials in recent studies and
is becoming more popular in archaeological studies, as
it provides information on a huge number of elements
(William, 2005).
A total of 69 elements were determined: Al
2
O
3
, CaO, K
2
O,
Fe
2
O
3
, Na
2
O, MgO, MnO
2
, and TiO
2
as major elements; Li,
Be, B, P, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb,
Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, Cs, Ba,
La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf,
Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Th and U as minor
and trace elements. Moreover, SiO
2
content was estimated.
Blank samples, standard samples and duplicated samples
were simultaneously performed as quality control.
4.3 Statistical Analysis
Several analyses were done using the following programs:
SPSS 16 Programme and Origin 75E version. The frst
step was to study the major element concentrations of the
samples using box and whisker graphics in order to interpret
the distribution of data. Later, a linear discriminant analysis
was used for hard classifcation purposes, trying to establish
possible connections between groups of samples and
variables. This procedure is useful for classifying the dataset
into groups.
5. Results and discussion
Table 2 shows the mineralogical results from the samples
obtained by XDR. It can be observed that there is a huge
variation in the mineral composition of the samples, because
the samples are from diferent archaeological sites from the
province of Salamanca, and in this region it is possible to
fnd sedimentary, igneous and metamorphic rocks (Lucena-
Conde, García, 1976). However, the majority of the samples
are characterized by a high content of carbonates (calcite
and dolomite) and the mean value of these minerals in the
samples is 88%. This type of mineral can be found in the
Duero Basin. As mentioned previously, this soil type can be
found in the Salamanca region. Thus far, these samples must
relate to a local production. In addition, traces of strontianite
(SrCO
3
) were detected in two samples (LV2 and LV3). Only
eleven samples presented values of carbonate content below
Figure 2.
Tesselatum samples. Views
from binocular loupe at 32× magnifcation:
A) SM6; B) SJ1; C) LVE4; D) SA1.
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IANSA 2018 ● IX/1 ● 31–42
Verónica Pérez de Dios, M
a
de los Reyes de Soto García, Isabel de Soto García, Rosario García Giménez: Archaeometric Study of Roman Tesserae
from Salamanca (Spain). Archaeology and Geochemical Analysis
36
Table 2.
Mineralogical composition of the samples. P – Phyllosilicate; Qtz – Quartz, FK – Potassium feldspar; FCNa – Sodium-calcium feldspar; Cal –
calcite; Dol – Dolomite; Kaolinite; Opal; Stronzionite; Amorphous material; R, X
2
.
Mineral (%)PQtzFKFCNaCalDolKaoliniteOpalStronzianite
Amorphous
material
R
χ2
LV235400560000.001
526.814.7
LV30.0010.00100980000.001
220.99.7
LV40.0010001088000
219.46.8
LV5283200370000
316.76.7
LV607550120000
833.38.6
AZ115602669000
220.99.7
AZ20000961000
322.38.7
AZ30000941000
517.37.0
AZ452190.0018120000
1031.37.6
AZ5101005566000
426.04.5
AZ65450.0010.001440.0010006
16.46.5
SJ10000970000
321.15.4
SJ213602668000
522.69.4
SJ30.001830790.0010001018.9
6.9
SJ40000940000
619.612.3
AL1562200.00160000
733.36.0
AL20.001508820.001000
520.98.3
SM100.00100930.001000
722.38.8
SM256000005230
1217.38.6
SM30703880.001000
217.36.7
SM40000953000
215.25.9
SM50000970.001000
321.87.0
SM6vidrio50000000
9523.210.2
LVE112620.001160.0010000
1012.56.8
LVE21039245100000
1217.25.4
LVE300000.00198000
215.17.2
LVE40.001400.0018210000
412.64.3
LVE5839250.001250000
1311.78.3
LVE685000700190
1617.66.4
LVE70000980.00100
0
215.46.3
LVE80.0010.001009700003
16.36.9
LVE9303000300000
1017.78.9
SA10.001500930.001000
212.68.4
SA20.001540102200001411.9
6.8
SA30000950.001000
512.67.9
SA40000980.001000
215.36.5
SA50.0018400000100
614.76.3
40% (LV5, LV6, AZ4, SM2, LVE1, LVE2, LVE5, LVE6,
LVE9, SA2 and SA5) and carbonates were not detected in
three of the samples (SM2, LVE1 and SA5) where the quartz
content was over 70%. On the other hand, the majority of
the samples showed low concentrations of phyllosilicates,
quartz, plagioclase and K-feldspar.
In relation to the amorphous and glass fractions, the
samples were fairly homogeneous (values between 2–16%,
with a mean value of 5%), except for sample SM6 which was
made of glass (95% of amorphous materials, see Table 2).
The samples were fairly homogeneous in view of the
major element concentration, due to the fact that high
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Verónica Pérez de Dios, M
a
de los Reyes de Soto García, Isabel de Soto García, Rosario García Giménez: Archaeometric Study of Roman Tesserae
from Salamanca (Spain). Archaeology and Geochemical Analysis
37
Figure 3.
Box and whiskers plot of the
major elements.
Figure 4.
Box and whiskers plot of the major elements using a division according to the archaeological site.
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IANSA 2018 ● IX/1 ● 31–42
Verónica Pérez de Dios, M
a
de los Reyes de Soto García, Isabel de Soto García, Rosario García Giménez: Archaeometric Study of Roman Tesserae
from Salamanca (Spain). Archaeology and Geochemical Analysis
38
Table 3.
Minor and trace elements concentrations (expressed in ppm).
LiBeBPScVCrCoNiCuZnGaGeAsSeRbSrYZrNbMoRuRhPdAgCdInSnSbTe
AZ10104594100283620924037236144210000000000
AZ200155121001022510001213332200000010000
AZ300214130002027410002112132700000010000
AZ4000319000504230000159523600000000000
AZ5010313360243326635029253124720000000000
AZ613233638121019417104875150824253430000010000
SJ100133120003028210003213323400000000000
SJ2000336450222832523033237223910000000000
SJ30063260003032110002714333000000000000
SJ400252430003033100004215434100000000000
AL11220140052603629731214051300125620000000000
AL200133620002047210004913022700000000
000
SM101323420002026100004912122300000000000
SM2351427771516114214417457301129010907134130000100000
SM300214020001015200003715344200000000000
SM400153670002032400004213855300000000000
SM500184721001018310001614841500000010000
SM6000305058790053222003225772600000000000
LVE10307453101229881432003010515994650000000000
LVE20001484602537320000581347689000000000280
LVE3012871101115185203908126229130000000160
LVE4000186717021049198010441568117412000000000
LVE50005846192925310260425301700000000080
LVE6361336452317813495388432721360151078122180000300000
LVE700344022002032230002613272100000
030000
LVE81809961831113121301040415611310001000320
LVE94424693176735252833431413202929598552000000640
SA10099191750007248927200006361317100000100520
SA25312753735248149337803215116806684154320000200000
SA30027199320000027000003816741900000080000
SA40012210000014000002514723700000030000
SA53314563235142121136154325231600171216142320000400000
LV2563967306441294213608040291008154341001000530
LV32211908104322133144304010020170563111001100240
LV456015108523720316380521251016242411003000680
LV55239694195664493636016120028589105081002000740
LV60002206121134194191282201870224333123321000301170310
variability was only found for the concentrations of Ca
and Si. These results can be observed with the box and
whiskers graph of the major element concentrations using
all the samples together (Figure 2). This representation
helps interpret the distribution of data. In this plot, each box
encloses the middle 50%, the median being represented as a
horizontal line inside the box. Vertical lines extending from
each end of the box (“whiskers”) enclose the data within 1.5
interquartile ranges. Values falling beyond the whiskers, but
within three interquartile ranges, are plotted as individual
points (suspect outliers). Further outside points (outliers)
are distinguished.
A detailed study was made by representing the major element
concentrations using a division according to the archaeological
site (LV, AZ, SJ, AL, SM, LVE and SA) (Figure 4). It shows
small variability between samples with regard to their content
of Na, K, Ti and Mn (except for the content of Mn in samples
from LV). By contrast, there was high variability in the contents
of Si and Ca for all samples in all the archaeological sites as
was demonstrated in the Figure 2. Concentration of Mg showed
high variability in the samples from AZ and AL and small
variability in the rest of the samples. In the case of Al, samples
from AZ, AL, LVE and SA showed high Al content variability
and small variability for samples from SJ, SM and LV. Finally,
image/svg+xml
IANSA 2018 ● IX/1 ● 31–42
Verónica Pérez de Dios, M
a
de los Reyes de Soto García, Isabel de Soto García, Rosario García Giménez: Archaeometric Study of Roman Tesserae
from Salamanca (Spain). Archaeology and Geochemical Analysis
39
high and medium variability in the content of Fe was obtained
in the samples of SA, AZ, AL and LVE and small variability in
the rest of the samples (Figure 4).
In the Tables 3 and 4 the values of the concentrations of
the minor elements are represented. It is very signifcant that
there are values > 1000 ppm of Sr in four of the samples (LV2,
LV3, LVE4 and LVE2), this fact being related to the presence
of stronzianite which was detected by the XRD in LV2 and
LV3. Another important fact is that high values of W (> 100
ppm) were obtained in samples LV2, LV3, LV4, LV5, LVE8
and LVE9. This fact is related to a special geological feature
of Salamanca Province, where there are some granitic rocks
containing mineralizations of high concentrations of W and
this element was extracted from some mines in the region
(Antona
et al
., 1990; Burkhardt
et al
., 1987; Pellitero
et al
.,
1976; Llorens, 2011). Therefore, these samples were clearly
manufactured in the Salamanca region.
The supply of materials in the Duero basin contrasts with
the samples analysed in the city of Ávila or the Roman village
of La Olmeda (Palencia). In Ávila, red tesserae were found
to have come from the Almaden area (Soto
et al
., 2014), and
in the luxurious village of La Olmeda tesserae came from the
Cantabrian mountains, localized between Alar del Rey and
Aguilar de Campoo (Regueras 2013, p. 178).
Table 4.
Minor and trace elements concentrations (expressed in ppm).
CsBaLaCePrNdSmEuGdTbDyHoErTmYbLuHfTaWReOsIrPtAuHgTlPbBiThU
AZ123611833514313021201010100000007082
AZ212022301000000000000100000002010
AZ313092101000000000000100000001010
AZ432576000000000000000200000001010
AZ523472237612624032101010100000005032
AZ60317611251010100000103000000001441
SJ102573000000000000000100000001010
SJ262983642317417023101010100000004032
SJ302752000000000000000200000002010
SJ402573000000000000000100000001010
AL1225816304143130201010102000000011062
AL202534000000000000000100000002
010
SM101692000000000000000100000003050
SM212915915101010101032700000000110254
SM301331000000000000000100000002070
SM402372000000000000000400000003050
SM511332301000000000000100000002010
SM682166800000000000000000000032000
LVE1657025567245140201010113000000010082
LVE20257737930000032000301000000010000
LVE3046434120000000000101000000016010
LVE422241631122120220101020200000008051
LVE503752800000000000000000000009000
LVE651994122320204020203126000000070135
LVE7417361502000000000000400000
004040
LVE8072120100000000000014700000006011
LVE912881430313212020101021303000000048062
SA1033332120000000000105000000021008
SA20352221121000400010211500000003056
SA32146312010000000000009000000013020
SA411332701000000000000300000007010
SA532255174260303040103132000000040202
LV211561326310212021101011333000000026241
LV319361215101010100010278000100027121
LV40845101410101000000089000030007121
LV516031632415312001101011312000000037062
LV6012436131010100000100100001502667022
image/svg+xml
IANSA 2018 ● IX/1 ● 31–42
Verónica Pérez de Dios, M
a
de los Reyes de Soto García, Isabel de Soto García, Rosario García Giménez: Archaeometric Study of Roman Tesserae
from Salamanca (Spain). Archaeology and Geochemical Analysis
40
To study the minor concentration of the samples in detail,
a linear discriminant analysis using SPSS was used for trying
to establish possible connections between groups of samples
and variables. The samples were divided into seven groups
depending on their archaeological site: LV, AZ, SJ, AL,
SM, LVE and SA. This analysis was used for the difculty
of classifcation in trying to establish possible connections
between groups of samples and variables and possible
connections among the ceramic samples. This procedure is
useful for classifying the dataset into groups. It generates
a small number of functions of quantitative measurements,
which are linear combinations of the standardized pattern
variables with weight coefcients. The procedure assumes
that the variables are drawn from a population with
multivariate normal distributions and that variables have
equal variances. Figure 5 shows that there are four groups.
The frst, G1, included all the samples from AZ, SJ, AL
and SM archaeological sites that are characterized by their
content of As, Zn, Sc, V, Ni and Co. G2 is represented by the
samples from LVE and one sample from SA, characterized
by their content of As, Zn, Sc, Cr and U. G3 contained only
the samples of the SA archaeological site (concentrations
of Ni, Nb, Ce, Cr, As and U). Finally, G4 is the group of
samples from LV by their concentration of As, Zn, Sc, V, Ni
and Co.
This means that even though there were seven
archaeological sites with mosaics in Salamanca, there were
only four areas of production of these materials. In addition,
these results show that in the archaeological sites closest
to the city of Salamanca, i.e. AZ, SJ, AL and SM, the same
material was used for the production of the mosaics and
samples could be shared between these archaeological sites.
6. Conclusions
Archaeometric techniques allow the characterization of the
materials used in the manufacture of mosaics. The results of
this type of analysis let the commercial relations between
quarries to be established, it being possible to observe great
movements of materials that were coming from remote areas
(Álvarez
et al
., 1985; Arana, Ramallo, 1986).
In our analysis we have been able to detect great variation
in the materials used for the production of the locally found
tesserae (stones, ceramics or glass). These very diferent
materials must respond mainly to two demands: the colour
requirement and the availability of raw materials. However,
88% of the samples come from the Duero basin, so the use of
easily available materials is evident. The vast majority of the
stone samples are limestone, something that could be linked
to the ease of carving in lieu of the other stones of the area
that were more difcult to work with. In addition, another
advantage of the limestone is its wide chromatic variety. Only
one of the samples was of dark blue-black glass (SM6). This
sample presents a composition of calcium silicate, a typical
composition found in Roman glass (Palomar, 2011, p. 59).
However, the comparison of our results with other studies on
glass tiles shows that there is not a great divergence in their
Figure 5.
Graphical representation of the
samples as a function of two canonical
discriminant functions.1 – Alquería de Azán
(AZ); 2 – San Julián de la Valmuza (SJ); 3
– Aldearrica (AL); 4 – San Morales (SM);
5 – La Vega (LVE); 6 – Saelices (SA) and
7 – Los Villares (LV).
image/svg+xml
IANSA 2018 ● IX/1 ● 31–42
Verónica Pérez de Dios, M
a
de los Reyes de Soto García, Isabel de Soto García, Rosario García Giménez: Archaeometric Study of Roman Tesserae
from Salamanca (Spain). Archaeology and Geochemical Analysis
41
production, but rather a standardization, as it corresponds to
the vitreous production in the Roman age (Ricciardi
et al
.,
2009, p. 2558).
Through this study we have been able to determine
that the majority of the samples analysed come from the
province of Salamanca. Based on the results obtained we
have documented four diferent groups. Group 1, to which
belong the two archaeological sites of Miranda de Azán AZ
and AL), San Julián de la Valmuza (SJ) and Aceña de la
Fuente (part of SM) can most likely be explained by their
geographical proximity. The very same mosaic workshop
would work for these four archaeological sites, or at least
the workshop would get its raw materials from the same
place. Group 2 is more complicated to analyse since it has
a similar sample to those documented in Group 3, i.e. the
group furthest away from all the others. However, we are
talking about the same geographic context articulated by the
Silver Way, so it would not be so complicated for them to be
an exchange of building materials within sites in Group 2.
The remaining two groups (groups 3 and 4), despite being
located in areas that are relatively geographically close,
have used diferent materials. The existence of these well-
diferentiated groups has led us to conclude the existence of
at least four workshops in the province of Salamanca, since
in the case of tesserae tiles of the same colours, not all of
them relate to the same areas of provenance.
Acknowledgements
We would like to glad to Alberto Bescós, Director of the
Salamanca Museum, for his kindness and willingness
to provide us with samples for the study, as well as Julio
Sánchez-Tabernero and Paco Boyero, for allowing us to
work on their property. We would also like to thank to the
Junta de Castilla y León for the economic support for the
archaeological excavation in Los Villares (Salamanca).
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