Journal of Archaeological Science 40 (2013) 2686e2701
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Journal of Archaeological Science
journal homepage: http://www.elsevier.com/locate/jas
Archaeological and archaeomagnetic dating at a site from the ager Tarraconensis
(Tarragona, Spain): El Vila-sec Roman pottery
Marta Prevosti a, Lluís Casas b, *, Josep Francesc Roig Pérez c, Boutheina Fouzai d, Aureli Álvarez b,
Àfrica Pitarch e
a
Institut Català d’Arqueologia Clàssica, Plaça d’en Rovellat, s/n, 43003 Tarragona, Catalonia, Spain
Universitat Autònoma de Barcelona, Facultat de Ciències, Departament de Geologia, Campus de la UAB, 08193 Bellaterra, Catalonia, Spain
CODEX Arqueologia i Patrimoni, Pl. Sant Fructuós 1, 43002 Tarragona, Catalonia, Spain
d
Université de Tunis El Manar, Faculté des Sciences, Département de Géologie, Campus Universitaire, 2092 Manar II, Tunisia
e
IBeA Research Group, Department of Analytical Chemistry, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Barrio Sarriena s/n, 48940 Leioa, Bizkaia,
Spain
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 August 2012
Received in revised form
11 January 2013
Accepted 24 January 2013
A very accurate archaeological dating of a Roman site in NE Spain (El Vila-sec) was made based on the
typology of pottery artifacts. Three different phases were identified with activity ranging from the mid1st century BC to the early-3rd century AD. Analyses of bricks from kilns at El Vila-sec produced data on
their stored archaeomagnetic vector. These data were compared with the secular variation curve for the
Iberian Peninsula and the SCHA.DIF.3K regional archaeomagnetic model. Both, the reference curve and
the model, produced probability distributions for the final period of use for two kilns from the second
archaeological phase that were not used during the third phase. At a 95% confidence level, both time
distributions cover a wide chronological range including the presumed archaeological age. Both the
Iberian secular variation curve and the SCHA.DIF.3K regional model proved to be suitable models for
dating the site, although on their own they do not produce a single unambiguous solution. This
archaeomagnetic approach could also be applied to neighbouring archaeological sites that have an
imprecise archaeological age.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Archaeomagnetism
Ceramic
Typology
Dating
Geomagnetic field modelling
Archaeodirection
Archaeointensity
1. Introduction
Since the appearance of archaeomagnetism as a dating tool for
archaeological sites (Aitken, 1974), such dating has been based on
the use of secular variation curves (SVC) (e.g. Le Goff et al., 2002;
Gómez-Paccard et al., 2006; Zananiri et al., 2007). These reference
curves are arbitrarily centred on a given location and involve the
relocation of data from sites that are both well-dated (to built the
SVC) and poorly-dated (to attempt archaeomagnetic dating) sites.
The relocation of archaeomagnetic data to a central location is
a procedure that involves an inherent error (Casas and Incoronato,
2007). The current trend in archaeomagnetic dating tools is to
develop geomagnetic models of the secular variation that can be
used to obtain an ad hoc SVC for the specific site we are interested
in. Despite this, the reliability of these models depends largely on
* Corresponding author. Tel.: þ34 935868365; fax: þ34 935811263.
E-mail address: Lluis.Casas@uab.cat (Ll. Casas).
0305-4403/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jas.2013.01.027
the availability of experimental data close to the area and time
under study (Casas et al., 2008). Consequently, archaeomagnetic
dating of sites from areas with a limited record of well-dated sites
appears to be somewhat imprecise and, in particular with regard to
archaeomagnetic intensities, large discrepancies can be found between model predictions and actual data determinations (Fouzai
et al., 2012; Schnepp et al., 2009). In contrast, in countries with
an extensive archaeomagnetic record, the models tend to agree
with the measured data (e.g. Catanzariti et al., 2012). Dating is thus
possible and in some cases the obtained ages can constrain the
presumed ones derived from archaeological evidence (GómezPaccard and Beamud, 2008).
In this paper we present the results from excavations at El VilaSec pottery (NE Spain). The site comprises twelve kilns and the
finds made inside them allow a very precise identification of when
they were abandoned. Archaeomagnetic dating has been attempted for some of these kilns to verify the accuracy of archaeomagnetic dating tools in NE Spain and to provide new
archaeomagnetic data from well-dated archaeological features.
M. Prevosti et al. / Journal of Archaeological Science 40 (2013) 2686e2701
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2. El Vila-sec pottery
2.1. Archaeological framework
El Vila-sec is an extraordinary archaeological site in the Tarraco
ager, due to the number of excavated and datable kilns, as well as
the variety of pottery produced there, including Dressel (Dr.) 2e4
amphorae (Dressel, 1899a; Sciallano and Sibella, 1993; López
Mullor and Martín, 2008), Hispanic terra sigillata, thin-walled
ware and oil lamps. The coastal strip of the Hispania Tarraconensis
province was an area famous for its wine in antiquity (López Mullor
and Aquilué, 2008; Prevosti and Martín i Oliveras, 2009). Amphorae
for wine bottling were manufactured at several potteries which
also produced building materials, coarse ware, terra sigillata and
sometimes even other types of fine ware. It is vital to know how
these potteries evolved in order to trace the development of economic life in Roman times and particularly in the Camp de Tarragona area, where such workshops are frequent and of
a considerable size (Prevosti and Guitart i Duran, 2010, 2011).
A well-checked final dating for the wine amphorae from Tarragona is of great archaeological interest, as it is crucial to know the
chronological periods of such an important economic activity as the
trade in Tarragona wine in Roman times (Járrega and Otiña, 2008;
Berni Millet, 2010, 2011; Járrega and Prevosti, 2011), which is
mentioned in classical sources (Prevosti, 2009).
2.2. The archaeological site, excavations and archaeological dating
The widening of the C-14 highway between the towns of
Alcover and Reus in 2006 and 2007 allowed the existence of
a pottery (figlina) with a total surface area of around 2300 m2 to be
confirmed in the area of Vila-sec (near Alcover). Roig (2009, 2010)
documented twelve kilns and numbered them from 1 to 12 (Fig. 1),
a numbering we use in this article. El Vila-sec is one of the largest
Roman potteries discovered in the province of Hispania
Tarraconensis.
Roig’s (2007, 2008, 2009, 2010) archaelogical excavations helped to identify two different zones with pottery kilns and to distinguish three chronological phases. In addition to the twelve kilns,
other parts of the site were also excavated, including six settling
basins for the clay, a kneading trough, tegulae channels, a large
store and some areas used to process, shape and dry pottery items.
The production from this industrial workshop consisted of tableware, cooking ware, thin-walled ware, architectural pottery, doliae,
Hispanic terra sigillata, probably lamps and especially amphorae of
the Dr. 2e4 and late Dr. 2e4 types.
The first identified phase of activity at the pottery consists of
seven kilns (1e7 in Fig. 1). No pottery associated with the construction of these kilns was found as the construction trenches
were dug directly into the geological substratum. However, their
stratigraphic position dates them to prior to the second identified
phase (mid-1st century AD).
The pottery produced during this first phase consisted of
building materials (tegulae, imbrex and antefixes), pondera, coarse
ware (especially plates, casseroles, bowls, bottles, jars, mortars,
basins and lids), lamps, Hispanic terra sigillata and especially thinwalled ware. The Hispanic terra sigilata ceramics includes (Fig. 2)
imitations of South Gaulish terra sigillata goblets of the forms
Dragendorff (Drag.) 24/25b and Drag. 27b (Dragendorff, 1895;
Ritterling, 1913; Oswald and Pryce, 1920; Mezquíriz, 1961, 1985;
Passelac and Vernhet, 1993; Roca and Fernández García, 2005;
Genin, 2007). The thin-walled ware includes (Fig. 3) Mayet 18 (10
BC to 60e70 AD), Mayet 21 (period of Augustus), Mayet 29 (period
of Tiberius), Mayet 30 (period of Tiberius), Mayet 33 (period from
Augustus to Claudius), Mayet 35 (period from Tiberius to Nero),
Fig. 1. General plan of El Vila-sec pottery; the kilns are numbered as in Roig (2009).
Mayet 36 (period from Tiberius to Nero), Mayet 37 (period from
Tiberius to Nero), Mayet 38 (30e35 AD to 75e80 AD) and López 54
(20 BCe50 AD) (Mayet, 1975; López Mullor, 1989), as well as other
unclassified shapes that are identifiable as beakers or bowls. An
early small-scale production of Dr. 2e4 amphorae associated with
Kiln 7 (Fig. 1) was also reported. The first phase can therefore be
dated to between the Augustan era and the middle of the 1st
century AD.
The second phase is characterized by the abandonment of all the
preceding kilns and the construction of a second group of four kilns
built in a line (Kilns 9e12, Fig. 1). These were located at the southeastern end of the site and there was also a series of structures
related to the production and storage of finished pieces. The construction of the kilns can be dated to the middle of the 1st century
AD, based on strata from the destruction of the previous kilns and
the levelling of the ground to prepare it for the new building works.
It is worth to mentioning the materials that characterize these
strata: i) Italic terra sigillata Conspectus (Consp.) 18.2 (15 BCe30
AD), Consp. 23.1 (25e75 AD), Consp. 33.1 (1e50 AD) and Consp.
36.3.2 (10e30 AD) (Ettlinger et al., 1990) forms; ii) South Gaulish
terra sigillata Drag. 15a1 (1e60 AD), Drag. 15b1 (60e120 AD), Drag.
17b (25e60 AD), Drag. 18a (15e60 AD), Drag. 24/25b (40e70 AD),
Drag. 27b (40e80 AD), Drag. 33a1 (20e60 AD), Haltern 5 (10e50
AD) and Ritterling 12 (40e70 AD) (Dragendorff, 1895; Ritterling,
1913; Oswald and Pryce, 1920; Passelac and Vernhet, 1993; Genin,
2007) forms; iii) African coarse ware Ostia II, 306 (14e117 AD)
(Berti et al., 1970; Hayes, 1972) form; iv) thin-walled ware Mayet 21
(25 BCe25 AD), Mayet 30 (10e50 AD), Mayet 33 (10e30 AD), Mayet
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Fig. 2. Hispanic terra sigillata made at El Vila-sec during the first phase (1e3, Drag. 27; 4e7, Drag. 24/25; 8e11, moulds). The inventory numbers correspond to Roig (2010). Due to
the high degree of erosion of fragments 5 and 8e11, the negatives of decorative motifs have been highlighted to make them easier to observe.
35 (15e60 AD), Mayet 37 (25e60 AD) and López 54 (20 BCe50 AD)
(Mayet, 1975; López Mullor, 1989) forms; v) Amphorae, particularly
from southern Hispania, i.e. Dr. 7e11 (25 BCe100 AD), Dr. 20B (30e
50 AD) and Haltern 70 (50 BCe75 AD) forms, and among those
produced in Tarragona, Oberaden 74 (1e30 AD), Pascual 1 (50 BCe
50 AD) and the ever-present Dr. 2e4 (25 BCe300 AD) (Pascual,
1977; Sciallano and Sibella, 1993; Raynaud, 1993a; López Mullor
and Martín, 2008); vi) also of note is a volute lamp, type Dr. 9B
(Claudian era) (Dressel, 1899b; Pavolini, 1987; Morillo Cerdán,
1989); and lastly a dupondius from Tiberian period (21/22e37 AD).
The pottery manufactured during this second phase is clearly
made up of Dr. 2e4 amphorae and, to a lesser extent, building
material (tegulae and imbrex) and coarse ware (storage pots,
cooking pots, casseroles and small jars) (Fig. 4).
The third phase of activity at the pottery is indicated by a series of
small alterations and functional changes visible at different points
of the site. These especially concern the construction of Kiln 8
(Fig. 1) and alterations carried out on Kilns 11 and 12 (Fig. 1) from
the second phase. Dating of the third phase is based on a soil level,
that corresponds to the backfill level of Stratigraphic Unit 3092
(Roig, 2010), which is related to the construction of an area
delimited by rows of late Dr. 2e4 amphorae placed top downwards
(Fig. 5). These amphorae date from the 2nd century and the first
half of the 3rd century AD (Járrega and Otiña, 2008). Within this
level, a number of pottery items was recovered reinforcing the
dating of this phase to the 2nd century AD: Hispanic terra sigillata
type Drag. 15/17 (30e300 AD), Drag. 37a (70e300 AD) and Drag.
37b (70e100 AD) (Mezquíriz, 1961, 1985; Roca and Fernández
García, 2005).
This third phase is associated with the production of late or
evolved Dr. 2e4 amphorae, although some manufacture of building
materials and coarse ware cannot be excluded. In fact, the kilns
M. Prevosti et al. / Journal of Archaeological Science 40 (2013) 2686e2701
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Fig. 3. Thin-walled ware made at El Vila-sec during the first phase (1e4, Mayet 37 form; 5e8, Mayet 30 form; 9 and 10, Mayet 35 form; 11e12, Mayet 33 form; 13 and 14 Mayet 29
form; 15, original bowl from El Vila-sec; 16 original beaker from El Vila-sec; 17, Mayet 21 form: 18, López 54 form). The inventory numbers correspond to Roig (2010).
from this phase are either too small (Kiln 8) for firing amphorae or
they have evidence of direct firing (Kilns 11 and 12) and therefore
amphorae would have been produced in Kilns 9 and 10.
This hypothesis cannot be confirmed until the completion of the
pending excavations.
The pottery structures were in filled and abandoned in the late 2nd
or early 3rd centuries AD. The materials that allow the dating of the
uppermost strata are: i) African Red Slip (ARS) ware terra sigillata
chiara A: Lamboglia (Lamb.) 2a (100e175 AD) and Lamb. 4/36A
(90e175 AD) forms and ARS coarse ware; among the finds are
worthy of mention are: Lamb. 10A (100e450 AD), Lamb. 10B (69e
250 AD), Ostia III, 267A (100e475 AD) and Ostia III, 324 (100e
300 AD) pots and the lid of an Ostia III, 332 type pot (69e450
AD) (Lamboglia, 1958; Berti et al., 1970; Hayes, 1972; Raynaud,
1993b,c); ii) Hispanic terra sigillata, with of particular note the
Drag. 15/17 (30e300 AD), Drag. 18 (50e200 AD), Drag. 24/25 (30e
150 AD), Drag. 30 (50e100 AD), Drag. 35 (50e150 AD) and Drag. 36
(50e300 AD), Drag. 37a (70e300 AD) and Drag. 37b (70e100 AD)
(Mezquíriz, 1961, 1985; Roca and Fernández García, 2005) forms.
This collection of pottery dates the abandonment of the structures
to the end of the 2nd century AD, although the find of an extremely
eroded Antoninian coin dated to the 3rd century AD extends the
duration of the final fill of the pottery workshop remains into the
next century.
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Fig. 4. Pottery made at El Vila-sec during the second phase (1 and 2, amphorae Dr. 2e4 with the stamp ITA; 3 and 4, jars; 5 and 6, pots; 7e10, covers; 11: casserole). The inventory
numbers correspond to Roig (2010).
In summary, we can state that the first phase (including Kilns 1e
7) can be dated to the time of Augustus and that they were
rendered inoperative by the building of the second occupation
phase (which includes Kilns 9e12) from the mid-1st century AD.
It should be noted that excavation of Kilns 9 and 10 could not be
completed and their chronological evolution is therefore not
known. However, Kilns 11 and 12 were fully excavated and therefore it is known that they were altered in the mid-2nd century AD,
within what we call the third phase. It was also at this time that
Channel-Kiln 8 was built. All five kilns (second and third phases of
occupation) were definitively filled in during the late 2nd - early
3rd centuries AD.
2.3. Detailed description of the sampled kilns
Unfortunately, the remaining kiln structures from the first
building phase were quite damaged. The walls of their combustion
chambers were eroded and crumbling and thus not easily
M. Prevosti et al. / Journal of Archaeological Science 40 (2013) 2686e2701
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Fig. 5. Late Dr. 2e4 amphorae made at El Vila-sec during the third phase. The inventory numbers correspond to Roig (2010).
samplable for archaeomagnetic purposes. Only Kiln 7 (Fig. 6) was
successfully sampled.
The structure of Kiln 7 was partly destroyed by later building
phases, so it was not possible to delimit the praefurnium, although it
is recognizable that the access to this opening was via its northern
side. An axial wall that divides the chamber lengthwise into two
was preserved within the firing chamber. The remains of five arches
were also visible; they supported the clay pipe grid, which was not
preserved. This kiln can be included within Cuomo di Caprio’s Type
II/c (rectangular with a double passageway) (Cuomo di Caprio,
1971e1972, 1985, 2007). Three loose and quite crumbly quadrangular bricks from the base of the arches from Kiln 7 were
removed and drilled in the laboratory, and finally cut to produce 23
standard cylindrical (w2.5 cm diameter) specimens for archaeointensity determinations.
Three of the four kilns built in the second building phase were
sampled. These are Kilns 9 (Fig. 7), 11 (Fig. 8) and 12 (Fig. 9). Kiln 9 is
circular with a diameter of about 3 m. There is a perimeter stone
and mud wall and an inner latericium or brick wall of the firing
chamber. As the excavation could not be completed, an accurate
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Fig. 6. Kiln 7 with indication of the sampled bricks. This kiln produced Dressel 2e4 amphorae between the Augustan era and the middle of the 1st century AD. The rectangular
structure with a double passageway corresponds to N. Cuomo di Caprio’s (1971e1972, 1985, 2007) Type II/c.
classification cannot be made, although it may be Type I/b (circular
with a clay pipe grid supported by a radial wall) from Cuomo di
Caprio’s (1971e1972, 1985, 2007) classification. Kilns 11 and 12
are rectangular; the preserved parts are those excavated in the
substratum i.e. the firing chamber and the praefurnium. A perimeter
wall and an axial inner wall were reported for both. The perimeter
walls are made of mud bricks, bricks and fragments of tegulae
bound with mud, whereas the axial walls of the firing chamber are
made of bricks bound with mud. Similarly to Kiln 7, on the three
walls that delimit the firing chamber of these kilns there are the
Fig. 7. Kiln 9 with indication of the drilled cores. This kiln has a perimeter mud stone wall and an inner firing chamber wall made of latericium or brick material. It is probably an N.
Cuomo di Caprio (1971e1972, 1985, 2007) Type I/b.
M. Prevosti et al. / Journal of Archaeological Science 40 (2013) 2686e2701
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Fig. 8. Kiln 11 with indication of the sampled areas and late restructuring features. This kiln was built during the second phase and rebuilt in the middle of the 2nd century AD. The
original rectangular structure with a double passageway corresponds to N. Cuomo di Caprio’s (1971e1972, 1985, 2007) Type II/c.
remains of the arches that supported the clay pipe grid, which
again was not preserved. In the case of Kiln 12, a ventilation hole or
air flue was reported in the western corner. According to the Cuomo
di Caprio’s (1971e1972, 1985, 2007) classification both kilns also
correspond to Type II/c (rectangular with double passageway). Kils
11 and 12 were restructured during the third building phase. Kiln 11
was made smaller, using the firing chamber from the previous
phase. At the rear, the upper part of the axial wall was dismantled,
a new mud-brick wall on its north-western side was built and the
firing chamber floor level was raised (Fig. 8). The result is a smaller
kiln with two openings on all sides of the earlier axial wall. Kiln 12
underwent a similar reorganization (Fig. 9): the eastern corner of
Fig. 9. Kiln 12 with indication of the sampled areas, the ventilation hole and late restructuring features. Like Kiln 11, it was build during the second phase and rebuilt in the middle
of the 2nd century AD. The original rectangular structure with a double passageway corresponds N. Cuomo di Caprio’s (1971e1972, 1985, 2007) Type II/c.
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the perimeter wall of the previous firing chamber was dismantled,
a new transverse wall was built to move the firing chamber forward, lengthening the axial wall with an extension made of bricks
and mud bricks bound with mud, and the floor level of the chamber
was raised. The result was that the firing was concentrated between
the old axial wall and the earlier north-eastern wall whereas the
gap between the earlier axial wall and the earlier south-western
wall was abandoned and filled with earth. With the current available data, it is not possible to determine whether the restructured
kilns (11 and 12) were still used to fire pottery or another kind of
material.
Oriented samples were retrieved from the internal opus latericium walls of Kilns 9 (Fig. 7), 11 and 12 for archaeodirectional determinations. For Kilns 11 and 12, sampling was performed on
structures not fired during the third archaeological phase: the
abandoned firing chamber below the new floor level (Fig. 8) and
the abandoned south-western passageway (Fig. 9). A portable
electrical drill with a water-cooled diamond bit was used following
the standard palaeomagnetic sampling procedure; each core produced a single specimen. The in situ azimuth and dip of the cores
were measured using a compass coupled to a core-orienting fixture. Due to the crumbly consistency of the walls, drilling had to
proceed delicately to avoid breaking the cylinder before taking its
orientation. If broken, the cylinder was carefully placed back in its
hole to mark its orientation. From three rejected, misoriented cylinders from Kiln 12 (the first sampled), small cylinders (5 mm
diameter, 3 mm length) were sub-sampled in the laboratory for
microwave archaeointensity determinations. Samples from Kilns 11
and 12 had the most constrained archaeological age as they correspond to structures fired during the second archaeological phase
(mid 1st century AD-mid 2nd century AD), but not during the third
one. In contrast Kiln 9 could actually have continued to operate
during the third archaeological phase.
To sum up, 62 specimens were obtained from 4 kilns at the Vilasec Roman pottery with a very precise archaeological age. Table 1
summarizes the details of all the sampled structures, their presumed archaeological ages and the number of samples taken.
3. Experimental methods and data analyses
Rock magnetic properties were measured at the Geomagnetism Laboratory, University of Liverpool for core samples
from Kiln 7 and for several rejected misoriented cores from Kiln
12. Plate-like samples were cut from the cores and their change in
susceptibility was measured as they warmed from liquid nitrogen
to room temperature. Hysteresis properties, isothermal remanent
magnetization (IRM) acquisition, remanence coercivity and magnetization versus temperature curves were obtained by using
a Magnetic Measurements variable field translation balance
(VFTB).
Three types of palaeomagnetic procedures were applied: conventional archaeodirection and archaeointensity determinations
(at the Palaeomagnetic Laboratory of Barcelona, SCT UB-CSIC); and
Table 1
Sampled features in El Vila-sec (Latitude: 41.24 N, Longitude: 1.17 E) with indication
of their presumed ages according to archaeological evidence, the sample labels, the
number of collected cores (N) and the applied archaeomagnetic technique.
Kiln
Presumed age
Label
N
Technique
7
9
11
12
12
50
50
50
50
50
K7
K9
K11
K12
K12
23a
5
9
9
3
Conventional archaeointensity
Conventional archaeodirection
Conventional archaeodirection
Conventional archaeodirection
Microwave archaeointensity
a
BCe50 AD
ADe225 AD
ADe150 AD
ADe150 AD
ADe150 AD
From 3 quadrangular bricks.
microwave archaeointensity determinations (at the Geomagnetism
Laboratory, University of Liverpool). Archaeodirectional analyses
consisted of stepwise demagnetization of the natural remanent
magnetization (NRM) and measurement of the magnetization left
after each step. Thermal demagnetization was performed in
a Schoenstedt TSD-1 demagnetizer and magnetization measurements on a 2G Enterprises superconducting rock magnetometer.
Results were presented as Zijderveld diagrams (Zijderveld, 1967).
Characteristic remanent magnetization (ChRM) directions were
calculated by principal component analyses (Kirschvink, 1980).
Three anomalous directions were obtained (one per kiln), possibly
due to an incorrect replacement of broken cylinders during sampling. These outliers were removed from the calculation of mean
directions. Mean directions for each kiln and merged mean directions were computed following Fisher (1953) statistics; concentration parameter k and confidence factor a95 were also
computed. The mean direction was compared with i) the SVC for
the Iberian Peninsula (Gómez-Paccard et al., 2006) and ii) SCHA.DIF.3K model (Pavón-Carrasco et al., 2009) predictions using
a Matlab dating tool developed by Pavón-Carrasco et al. (2011). The
Iberian SVC is defined at a central location (Madrid) and it was
computed by the hierarchical Bayesian method using archaeomagnetic directions with dates ranging from 775 BC to 1959 AD.
The SCHA.DIF.3K model was obtained by least-sums of absolute
deviation inversion of palaeomagnetic data using spherical harmonics (SCHA) and provides full geomagnetic field vector values
over the European continent and neighbouring areas from 1000 BC
to 1900 AD.
Archaeointensity analyses were performed according to the Coe
variant of a Thellier-type experiment (Coe, 1967), the NRM was
measured and gradually removed and replaced by a new thermal
magnetization (TRM). This was achieved by heating the samples
alternatively in a zero (Z) and a 50 mT applied (A) field in a Magnetic
Measurements MMTD-80 thermal demagnetizer. Besides the conventional Z/A steps, pTRM and pTRM tail checks (Riisager and
Riisager, 2001) were performed to ensure the absence of alteration and multidomain behaviour within the magnetic remanence
carriers. The remanent magnetization measurements were also
performed on a 2G Enterprises superconducting rock magnetometer. Cooling rate tests were performed at 560 C on all samples
following the procedure described in Chauvin et al. (2000). The
correction was applied only for samples with stable acquisition
capacity (r2 < 5% where r2 ¼ (TRM1
TRM3)/TRM1). Microwave
archaeointensity analyses followed a similar procedure, although
using microwaves instead of heat to directly generate magnons
(Shaw et al., 1999) to erase the NRM or to generate a new microwave thermoremanent magnetization (TMRM). The microwave
system used operated in the 14 GHz frequency range; throughout
the experiments the frequency was finely tuned to the ferromagnetic resonant frequency of the sample and after irradiation
allowed direct measurement of the magnetization. Both conventional and microwave archaeointensity results were represented as
Arai plots where NRM lost is plotted against TRM (or TMRM) gained,
both normalized to the initial NRM along with the pTRM and tail
checks (Yu and Dunlop, 2003). Additionally Zijderveld diagrams
were also plotted using the steps performed in zero field to check
the directional uniformity of the NRM vector. An overall archaeointensity value was computed for each kiln by fitting a Gaussian
function to the sum of all individual results (Fouzai et al., 2012).
Samples with negative pTRM checks or f values lower than 0.5
(Biggin and Thomas, 2003) were not used to obtain the overall
intensity estimate. Positive pTRM checks were defined as those
with a difference between the original pTRM and the pTRM check
lower than 10 percent of the total TRM acquired (Chauvin et al.,
2000).
M. Prevosti et al. / Journal of Archaeological Science 40 (2013) 2686e2701
2695
Fig. 10. Representative Zijderveld plots depicting the orthogonal projection of the remanent magnetization vectors during progressive demagnetization for specimens from Kiln 9
(left), Kiln 11 (middle) and Kiln 12 (right). Open (solid) symbols represent projections on vertical (horizontal) planes. Lines indicate the ChRM directions.
The full archaeomagnetic vector (including both mean direction
and intensity) was also compared with the previously described
SCHA.DIF.3K model. The use of an archaeomagnetic field model
avoids the need for relocation of data to a central location,
a procedure that involves an inherent error (Casas and Incoronato,
2007). However, the use of archaeomagnetic field models involves
regularization and smoothing to interpolate field values, which can
also be a source of error.
Fig. 11. Stereographic projection of the archaeomagnetic directions calculated for each sample. In (a) mean directions and a95 error circles were calculated and plotted separately for
each kiln, in (b) the individual results from the Kilns 11 and 12 were merged to produce a single mean direction and a95 error. N indicates the total number of samples from the two
kilns; D and I stands for declination and inclination; a95 and k, 95% confidence cone of the mean direction and precision parameter from Fisher statistics.
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M. Prevosti et al. / Journal of Archaeological Science 40 (2013) 2686e2701
4. Results and discussion
4.1. The mean archaeomagnetic vector
Representative Zijderveld diagrams of samples from Kilns 9, 11
and 12 are shown in Fig. 10. Usually almost all the temperature
steps were taken into account to compute the ChRM directions.
MAD values were always lower than 5. The obtained archaeodirections were plotted separately for each kiln in stereographic
projections (Fig. 11a). The three kilns produce slightly different
mean directions (Table 2), possibly due to the low number of
samples per kiln (four from Kiln 9, eight each from Kilns 11 and 12).
Archaeological evidence indicates that the three kilns are practically contemporary. Taking into account the sampled points, the
results from Kilns 11 and 12 can be ascribed to a very constrained
age (the second archaeological phase), whereas the four results
from Kiln 9 could correspond either to the second or the third
archaeological phase. Adding this to the fact that Kiln 9 only produced four scattered directions, it is reasonable to combine the
archaeodirections from Kilns 11 and 12 to produce a single mean
direction (Fig. 11b and Table 2) expressed as a declination of 350.5
and an inclination of 56.7 (a95 ¼ 3.4 ) that according to the
archaeological evidence would correspond to a narrow time window (mid-1st century AD to mid-2nd century AD).
Archaeointensity determinations based on a constant proportionality between remanence magnetization and magnetic field is
known to be limited to stable non-interacting single domain (SD) or
pseudo-single domain (PSD) remanence carriers (Fabian, 2001).
Using the magnetization parameters and coercitive field (from the
hysteresis loops) and the remanence coercivity (from a backfield
experiment) the Day plots revealed that all the measured cores can
be attributed to the PSD regime. However, temperature-dependent
susceptibility measurements indicate that six cores do contain MD
particles: the appearance of a characteristic peak (Fig. 12a) around
130 K (the Verwey transition) indicates low-Ti titanomagnetite in
MD state (Moskowitz, 1980). In fact, the PSD field in a Day plot can
also be ascribed to a mixture of SD and multidomain (MD) states
(Dunlop, 2002). Magnetization versus temperature curves indicate
a distribution of blocking temperatures for all analyzed cores. This
could be due either to a broad distribution of magnetic particle
sizes or to a distribution of Ti contents within the titanomagnetites
of the analyzed core. The degree of thermal reversibility after
heating at 700 C is variable (Fig. 12b); the darker materials tend to
show a loss of magnetic signal after heating, whereas for the
brownish materials the trend is towards increased magnetic signal.
The decrease in signal can be related to oxidation reactions. The
increase in magnetic signal could relate to the growth of magnetic
grains through agglomeration.
Archaeointensity specimens obtained from the same cores that
exhibit hints of MD behaviour were not analyzed. Representative
Arai diagrams of specimens from Kilns 7 and 12 with the corresponding Zijderveld diagrams are shown in Fig. 13. A total of 31
Table 2
Archaeomagnetic directional results.
Kiln
n/N
D ( )
I ( )
k
a95 ( )
9
11
12
All
11 and 12
4/4
8/8
8/8
20/20
16/16
347.8
351.1
349.9
350.0
350.5
55.3
53.5
59.9
56.4
56.7
67.9
229.9
95.7
109.0
117.2
11.2
3.7
5.7
3.1
3.4
Columns from left to right: Kiln, number of the sampled kiln; n/N, number of
specimens analyzed (n)/independently oriented samples taken into account in the
calculation of the mean direction (N); k and a95, precision parameter and 95%
confidence limit of characteristic remanent magnetization, from Fisher statistics.
Fig. 12. (a) Low temperature susceptibility dependence for samples from Kilns 7 (top)
and 12 (bottom) exhibiting hints of the Verwey transition. (b) Thermomagnetic curves
for samples from Kiln 12 exhibiting an increase (top) or decrease (bottom) of the
magnetic signal after heating at 700 C.
specimens were analyzed (eighteen from Kiln 7 and thirteen from
Kiln 12), only three exhibit non linear Arai plots (one from Kiln 7
and two from Kiln 12). The rest of specimens are listed in Table 3
(Kiln 7) and Table 4 (Kiln 12) with the corresponding archaeointensities. Only one specimen from the Kiln 7 set was rejected
due to negative pTRM checks, whereas three were rejected for this
reason from the Kiln 12 set, and two other K12 specimens were
rejected due to low f fractions. Fig. 14 shows the overall archaeointensity results for each kiln (71.39 14.27 mT for Kiln 7 and
70.11 7.00 mT for Kiln 12). Thus, despite being measured with
different techniques and although the two kilns are not exactly
contemporary according to the archaeological evidence, both show
virtually identical archaeointensities. This agrees with the SCHA.DIF.3K predictions of a low rate of intensity variations for the first
five centuries AD. From the two archaeointensities, the value
obtained for Kiln 12 can be directly linked to the obtained mean
direction to date the second archaeological phase.
4.2. Archaeomagnetic dating tools
The archaeomagnetic results ascribed to the second
archaeological phase were used to check the proficiency of the
available archaeomagnetic dating tools. Probability density
functions of possible dates for declination and inclination were
obtained comparing the relocated data with the Iberian SVC.
Both functions were then combined to obtain the most probable
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M. Prevosti et al. / Journal of Archaeological Science 40 (2013) 2686e2701
Fig. 13. Representative Arai plots of normalized NRM remaining against TRM (or TMRM) gained for specimens from Kilns 7 and 12. The corresponding Zijderveld diagrams are
plotted in the upper right corner of each Arai plot.
solution at 95% confidence level. The procedure is illustrated in
Fig. 15. The combined (declination and inclination) probability
distribution is split in three intervals (BC 203e132, BC 23eAD
507 and AD 1836e1900). The presumed archaeological age (AD
Table 3
Conventional archaeomagnetic intensity results for Kiln 7 obtained from linear Arai
plots. All specimens except K7-c21 (negative pTRM checks) were used to compute
the mean intensity value.
Brick
Specimen
F (mT)
FCR (mT)
s (mT)
N
f
g
q
1
1
1
1
1
1
2
2
2
2
2
2
2
3
3
3
3
K7-c1
K7-c3
K7-c4
K7-c5
K7-c6
K7-c7
K7-c8
K7-c9
K7-c10
K7-c11
K7-c12
K7-c13
K7-c14
K7-c20
K7-c21
K7-c22
K7-c23
72.13
84.21
70.78
83.46
85.47
71.83
60.17
64.46
55.64
85.12
84.53
74.72
64.59
58.42
83.13
80.80
58.43
68.67
74.39
e
76.47
e
e
e
56.64
51.60
76.64
e
e
e
e
e
80.38
e
3.90
7.84
8.06
21.34
5.22
5.89
16.98
10.30
10.70
18.40
2.71
5.11
52.6
19.2
16.9
16.9
6.2
7
5
5
4
4
4
5
7
6
5
5
3
3
4
5
5
4
0.83
0.65
0.77
0.52
0.80
0.65
0.52
0.69
0.75
0.74
0.74
0.68
0.65
0.58
0.74
0.50
0.50
0.82
0.68
0.67
0.51
0.62
0.62
0.44
0.78
0.77
0.50
0.66
0.44
0.48
0.55
0.50
0.74
0.62
18.5
10.7
8.8
3.9
16.4
12.2
3.5
6.3
5.2
4.6
31.2
14.6
1.2
3.0
4.9
4.8
9.5
Mean value
71.39 14.27
Columns from left to right: Brick, label identifying the brick provenance; Specimen,
label of the specimen identifying the kiln (K7) and the core number; F, raw intensity;
FCR, cooling rate corrected intensity; s, standard deviation of the intensity estimate;
N, number of heating steps used for the intensity determination; f, fraction of NRM
used for intensity determination; g, gap factor and q, quality factor as defined by Coe
(1967).
50e150) lies within the wider interval and concentrates about
15% of the obtained probability distribution (Fig. 15). The third
interval would indicate a modern remagnetization of the studied
structures. The same described procedure was applied to the
non-relocated data using the SCHA.DIF.3K model. The corresponding probability distributions are shown in Fig. 16. The
combined probability distribution presents similar features when
compared to the distribution obtained using the SVC: it is split
Table 4
Microwave archaeomagnetic intensity results for Kiln 12 obtained from linear Arai
plots.
Specimen
F (mT)
s (mT)
N
f
g
q
pTRM
K12-c1-1
K12-c1-2
K12-c1-3
K12-c1-4
K12-c1-5
K12-c1-6
K12-c2-1
K12-c2-2
K12-c2-3
K12-c2e4
K12-c2-5
65.52
66.51
68.37
71.91
53.86
51.23
67.02
71.26
70.53
78.58
72.54
3.48
21.00
7.32
6.95
11.09
4.27
9.00
173.53
2.74
4.24
27.78
6
4
6
5
4
4
5
3
8
6
4
0.53
0.72
0.57
0.55
0.55
0.51
0.61
0.18
0.80
0.64
0.32
0.70
0.63
0.71
0.53
0.54
0.48
0.69
0.46
0.84
0.69
0.64
18.85
3.2
9.3
10.4
4.9
12.0
7.5
0.4
25.7
18.5
2.6
U
╳
U
U
╳
╳
U
e
U
U
e
Mean value
71.11 7.00
Columns from left to right: Specimen, label of the specimen identifying the kiln
(K12), the core number and the specimen; F, intensity; s, standard deviation of the
intensity estimate; N, number of heating steps used for the intensity determination;
f, fraction of NRM used for intensity determination; g, gap factor and q, quality factor
as defined by Coe (1967); pTRM, result of the pTRM check test: passed (U), failed
(U) or not performed (e).
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M. Prevosti et al. / Journal of Archaeological Science 40 (2013) 2686e2701
Fig. 14. Computation of mean archaeointensities for Kilns 7 (a) and 12 (b). The dashed line is the sum of all the accepted individual archaeointensity results for the kiln and the solid
line is a Gaussian fitting to those sums.
Fig. 15. Probability-of-age density functions obtained with the Matlab tool from Pavón-Carrasco et al. (2011) comparing the Iberian SVC with the merged archaeodirectional results
from Kilns 11 and 12 relocated to Madrid; relocated intensity results from Kiln 12 were compared with the Bayesian SVC for western Europe. At the bottom: location of the site,
experimental archaeomagnetic vector probability function without intensity data (middle) and including them (right).
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M. Prevosti et al. / Journal of Archaeological Science 40 (2013) 2686e2701
Fig. 16. Probability-of-age density functions obtained with the Matlab tool from Pavón-Carrasco et al. (2011) comparing the SCHA.DIF.3K model predictions with the archaeomagnetic results (the merged archaeodirectional results from Kilns 11 and 12, and the archaeointensity from Kiln 12). At the bottom: location of the site, experimental
archaeomagnetic vector and probability function without intensity data (middle) and including them (right).
into three time intervals, the first (BC 79eAD 145) includes the
presumed archaeological age, the second (AD 184e479) concentrates a significant amount of probability and the third points
to a modern remagnetization (AD 1886e1900). In this case, the
presumed archaeological time window represents 10% of the
obtained probability distribution. The main difference from the
SVC-obtained distribution is that the SCHA.DIF.3K-obtained distribution contains more ups and downs; this dissimilarity reflects
the difference between the wiggled SCHA.DIF.3K model curves
and the more smoothed Iberian SVC.
Table 5
Archaeomagnetic dating results.
Kiln
Input
Dating tool
Main solutions at 95% confidence level.
(in bold letters the solution that
matches with the PAA)
Presumed archaeological
age (PAA)
K7
Intensity
SCHA.DIF.3K
BC 1000eAD 1202
AD 1279e1350
AD 1580e1703
BC 50eAD 50
K11 þ K12
Direction (relocated)
Iberian SVC
AD 50e150
Direction þ intensity
(relocated)
Direction
Iberian SVC þ western
European intensity SVC
SCHA.DIF.3K
Direction þ intensity
SCHA.DIF.3K
BC 203e132
AD 23e507 AD
AD 1836e1900
AD 19e197
AD 218e483
BC 79e145 AD
AD 184e479
AD 1885e1900
BC 226e159
BC 93eAD 167
AD 182e464
% of probability
within the PAA
5
15
22
13
15
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M. Prevosti et al. / Journal of Archaeological Science 40 (2013) 2686e2701
The intensity data can be also added to the dating procedure, the
Iberian SVC only predicts the evolution of the archaeomagnetic
direction and therefore the Bayesian SVC for western Europe
(Gómez-Paccard et al., 2008) has been used for the relocated data.
In contrast, the SCHA.DIF.3K simulates the full archaeomagnetic
vector. Despite its large error, it is worth mentioning that the
contribution of the archaeointensity data helped to constraint the
possible ages. Both, the probability distributions obtained combining the direction data with either the Iberian SVC (adding the
western European intensity SVC) or the SCHA.DIF.3K include a time
interval pointing to a modern remagnetization. The geomagnetic
field strength is presently around 45 mT, far below the measured
archaeointensities for Kiln 12 and thus, adding the intensity to the
dating procedure, the modern magnetization hypothesis can be
excluded. The fully combined (declination, inclination and intensity) probability distribution results in an increased concentration of probability within the presumed archaeological time
window: using the Iberian SVC (adding the western European intensity SVC) the time window contains 22% of the probability
(Fig. 15); using the SCHA.DIF.3K model the time window concentrates 15% of the probability (Fig. 16). Table 5 summarizes the dating
results obtained.
The measured archaeointensity for Kiln 7 alone (without the
corresponding directional data) cannot provide information on its
archaeomagnetic age because the probability distribution covers
almost the full investigated time period (see Table 5).
The archaeointensities obtained for Kilns 7 and 12 are significantly higher than the model predictions. This result was also
found for Roman sites in Tunisia (Fouzai et al., 2012) and seems to
suggest that during this period the intensity is not well resolved by
the model due to a limited intensity database.
5. Conclusions
A highly precise dating for the activity at the Vila-sec Roman
pottery workshop was determined using the typology of its ceramic
materials. A chronology of three different phases was established. A
first phase with seven kilns that were abandoned in the middle of
the 1st century AD, a second with four kilns active from the middle
of the 1st century AD to the middle of the 2nd century AD and
a third with three kilns (two of them rebuilt from second-phase
kilns) active up until the late 2nd or early 3rd century AD.
Structures from the first archaeological phase could not be
precisely dated by archaeomagnetic methods because of the major
uncertainty associated with the mean archaeointensity and the lack
of archaeodirectional data for Kiln 7. The age of the second
archaeological phase was checked with three archaeomagnetic
dating tools. Both the Iberian SVC and the SCHA.DIF.3K models have
a similar level of accuracy. Nevertheless, the SCHA.DIF.3K model has
the advantage of not requiring data relocation and also predicts
archaeointensities, which in this case allowed a modern remagnetization to be ruled out. Combining the western European intensity SVC with the Iberian SVC also allowed, using relocated data,
a modern remagnetization to be ruled out. Furthermore this combination resulted in the highest concentration of probability within
the presumed archaeological age (22%). In any case, none of the
archaeomagnetic dating approaches produced a single unambiguous solution pointing to the presumed archaeological age. We have
to keep in mind that a given value of the archaeomagnetic vector
can occur at different time periods. Besides that, secular variation
curves and geomagnetic models are not foolproof and only a continuous effort to collect reliable archaeomagnetic data from welldated structures will increase their accuracy.
Archaeointensities of almost contemporary structures were
determined by two different techniques (microwave and the
conventional Thellier method) and yielded similar results. This
again suggests that TMRM and TRM are equivalent (Shaw et al.,
1999; Hill et al., 2002; Casas et al., 2005). The obtained archaeointensities seem to imply that during this period the intensity is
not well resolved by the SCHA.DIF.3K model, due to a limited intensity database. This reinforces the conclusion that the collection
of reliable archaeomagnetic data from well-dated structures is
required.
Only with high quality data, can the archaeomagnetic approaches undertaken be applied to dating other archaeological sites
from the same region, particularly when they lack any datable
artefacts.
Acknowledgements
We would like to thank Elisabeth Schnepp for her helpful and
insightful comments. This research was funded by the Spanish
Ministerio de Ciencia e Innovación (Project HAR2010-16953) and
the Agencia Española de Cooperación Internacional para el Desarrollo (Spain-Tunisia bilateral project A1/039844/11).
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