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Some information on the firing temperatures, one of the most intriguing aspects in the investigations on ancient pottery, has been inferred by the XRD, ATR‐FTIR and TG/DTA data on shards excavated in archaeological salt‐making installations (Molino‐Sanchón and Santioste) located in Villafáfila wetland (Zamora, Spain). From such data it can be concluded that oven and bowl plinths pastees were fired at temperatures above those used in the bowls for brine evaporation. The temperature at which these pottery pastes were fired varies between 600 and 800 ºC.
Citation preview
1
Investigations on the Prehistoric salt exploitation in Villafáfila
(Zamora, Spain) salinas: evidences on the low fired earthenware
pottery used
P. Martín‐Ramos, J. Martín‐Gil, F.J. Martín‐Gil
CyL Heritage Conservation Laboratory (LICOPCYL), Higher Technical School for Agrarian Engineering,
Avenida de Madrid, 57, Palencia‐34004, Spain
G. Delibes‐de‐Castro
Department of Archaeology, University of Valladolid
Key words: salt exploitation; earthenware; firing temperatures; Villafáfila archaeologica
Abstract
Some information on the firing temperatures, one of the most intriguing aspects in the
investigations on ancient pottery, has been inferred by the XRD, ATR‐FTIR and TG/DTA data on
shards excavated in archaeological salt‐making installations (Molino‐Sanchón and Santioste) located
in Villafáfila wetland (Zamora, Spain). From such data it can be concluded that oven and bowl
plinths pastees were fired at temperatures above those used in the bowls for brine evaporation. The
temperature at which these pottery pastes were fired varies between 600 and 800 ºC.
Introduction
Among the mineral substances extracted since the ancient times, salt was one of the most precious.
It was used for preservation of meat and fish, as well as for tanning, cuppelation, religious and
magical rituals, parturition, and funerary activity.
The general term salina describes the site or the installation where salt‐making takes place through
brine evaporation as a result of a natural or artificial processing (Petanidou, 1997). In salinas, brine
evaporation has been generally assisted by solar irradiance, although there is archaeological
evidence that ebullition has been also in use since Prehistoric Times until Medieval Times
(Escacena‐Carrasco and Rodríguez de Zuloaga, 1994; Rodríguez‐Rodríguez, 2000; Valiente‐Cánovas
et al., 2002, WARP Conference 2005; Coles, 1984).
Villafáfila (41º49’N 005º37’W), in Castilla‐León, Spain, is an inland wetland of 2,854 ha, included in
the Ramsar sites with international importance, where salt‐making installations in Molino Sanchón
and Santioste sites were active since Copper Age and Bronze Age (Delibes de Castro et al., 2009).
Pottery artifacts found in these installations (figure 1) are similar those reported in Mesopotamia,
Bosnia, Romania, Poland and Turkey (figures 2‐5).
Our approach is to analize the clayey materials that resulted of the archaeological research in order
to see how the processes of salt extraction were carried out.
2
Methods
X‐ray diffraction data (XRD), infrared analysis (ATR‐FTIR spectra) and thermal analysis (TG and
DTA curves) were used in a complementary way in the determination of the chemical and
mineralogical composition of clayey samples from Molino‐Sanchón and Santioste sites and to gain
some information on the firing temperatures, one of the most intriguing aspects in the investigation
on ancient pottery.
Diffraction patterns. The XRD patterns (figure 6) were recorded on a Philips PW1710 BASED
diffractometer with Cu anode equipped with PC‐APD software for data collection and calculation of
average crystallite size. Generator conditions were 40 kV in voltage and 30 mA in current. Metal
oxide composition is reported in table 1.
Infrared analysis. Each simple was prepared for infrared analysis using the potassium bromide
pellet method: half a gram of ceramic powder was scraped off each shard and crushed. The fine
divided ceramic (0,2‐0,5 mg) was mixed with 300 mg reagent grade KBr. The sample was dispersed
throughout the KBr by grinding manually in an agate mortar for about 10 min. The mixture of
sample and KBr was pressed in an steel die at a pressure of about 10 tons to form a transparent
pellet, having a 0.8 cm diameter and a thickness of about 1 mm. The pellet was transferred to the
ATR‐FTIR spectrometer for analysis. ATR‐FTIR spectra were performed using a spectrometer
BRUKER IFS 66, equipped with a DLaTGS detector. The spectra were obtained covering the 4000‐
400 cm‐1 range with a spectral resolution of 1 cm‐1 (figure 7). The spectra are reported in figure 6 and
their characteristic frequencies are reported in table 2.
Thermal analysis. The thermogravimetric analysis (TG) and differential thermal analysis (DTA) of
the pottery materials under study were carried out using a Mettler Toledo TGA/SDTA 851e/ SF/1100
apparatus, covering the 25 – 1000 ºC range. TG curves are shown in figure 8. DTA curves including
heating ‐cooling‐heating cycles are shown in figure 8.
Results and discussion
Mineralogical composition of the shards according XRD and chemical analysis. Both, pottery
shards from Molino‐Sanchón and furnace walls fragments from Santioste are fired carbonate‐
bearing clays of Vindoboniense origin. The clayey materials are composed of quart, feldspar,
plagioclases, illite and kaolinite with sparse pedogenic carbonates and organic material ash (figure
6). Carbonates have been characterised as calcite (CaCO3), dolomite (MgCO3) and natron. Natron is
a sodium carbonate (Na2CO3) which is formed when saline continental water rich in bicarbonate
anion is evaporated. In both sediments and waters from Villafáfila lagoons, natron is a secondary
product since most of the sodium present is as halite (NaCl). Our analysis on the Barrillos Lagoon
water (one of the places where the mud could be drawn) have given 2300 mg/L in Cl‐, 1500 mg/L in
Na+, 400 mg/L in CO3H, 100 mg/L in Ca2+ and 50 mg in Mg2+.
From the table 1, concerning to metal oxide composition of pottery fragments by XRD, different
types of pottery fragments can be recognized in basis the Ca content: those Ca‐rich, as the bowl
samples; those Ca‐poor, as the bowl plinth shards; and those Ca‐intermediate, as the oven fragments;
By analyzing Si, Al and Fe contents, can be observed that SiO2+Al2O3+Fe2O3 values vary from 88
%wt for plinth shards to 35% for bowl samples, being intermediate for oven fragments. Thus,
through their metal oxide composition, three types of pottery fragments can be distinguished
without difficulty.
3
The bowl samples are thick pastes that show irregular or reducing firing with inorganic and organic
degreasers, sometimes including carbonized organic components. Their surfaces are grey, black or
dark brown. The main constituents of both the bowl and their residue are quartz and calcite, as
reported. The absence of any firing mineral in these pastes is a clear indication of a low firing
temperature (beyond 600 ºC). In other hand, their high content of P2O5 (1.74 % in bowl and 2.23 % in
bowl residue) is an uncommon fact in primitive ceramics but has been found in some Roman and
Egyptian archaeological sites. Phosphorus could came from either an (Al,Fe)‐phosphate
concentrated in the organic humus of soils (Rodica‐Mariana et al., 2009) or from Ca‐phosphate
(apatite or bone powder) incorporated by the potter as food cooking residue with strengthening
purposes.
Concerning to the bowl plinths or holders, their inner side shows a red colour due to their
composition as a ferruginous clay (4.3 % in Fe2O3). The grey colour of the outer skin of the bowl
plinth can be explained by the high content of organic matter and their yellowish tonality by
deferrugination. The outer side of plinth has illite, kaolinite and a relatively high content in natron
(2.1% Na2O).
In connection with the samples taken of the oven, we found compositions very close to those of the
bowl plinths: quartz, illite and some amount of carbonates and kaolinite for the inner side and
quartz, kaolinite and illite for the outer side.
The presence of peaks at 2θ = 40.5º and 50.5º in the XRD spectra of all the samples suggest the
presence of KCl. This species can be formed in detectable amounts from straw combustion above
400 ºC (Olander and Steenari, 1995).
ATR‐FTIR spectra. It is not difficult to draw among the constituents of the bowl from Molino
Sanchón site the presence of calcite, indicated by the peak at 1420 cm‐1 (figure 7). Concerning the
bowl residue, their spectrum shows together with the presence of carbonate (711 cm‐1) the presence
of orthoclase (1010 cm‐1) and straw hemicelluloses (982 cm‐1) as minor constituents. The band at 1396
cm‐1 could be assigned to goethite (whose presence suggest reducing conditions).
The spectra of bowl plinth samples show the presence of quartz (775 cm‐1), aluminosilicates (972
cm‐1) and calcium oxide (1461 cm‐1) but does not contain firing minerals.
The two samples excavated from the Santioste oven contain carbonate (1425 cm‐1), illite (1640 cm‐1)
and diopside (871 cm‐1). Diopside is a pyroxene formed as a result of thermally induced reactions,
i.e., those are formed during firing at temperatures between 600 °C and 780 °C.
Thermal analysis. Since 1993 (Misiego‐Tejeda et al.) is well known that successive heating‐cooling
cycles on a ceramic material lead to a thermal hysteresis that results in loss of all thermal effects
previous to the heating temperature reached. In the DTA curves from Molino Sanchón samples we
observe absence of thermal effects before 700 °C (except for the endothermic at 92 °C corresponding
to desorption of water) and the persistence, after a cycle heating‐cooling‐heating, of those who are
above that temperature. This finding is suggestive that the materials studied can be defined more as
earthenware or low temperature fired clays bodies as ceramics themselves.
The highest percentages of weight loss during the first heating have been exhibited by the bowl
residue and then by the bowl itself (figure 8). The bowl residue in TG showed maximum weight
loss between 700 °C and 813 °C which was sensitized by an endotherm in DTA at 800 °C. The bowl
experienced its maximum weight loss between 600 °C and 730 °C and was sensitized, in DTA, by an
endotherm at 720 °C (figure 9).
4
The clay bodies of the bowl plinth, both inside and offside, have proved to be very thermally stable.
In the course of heating to 1000 °C did not lose significant weight and TG records have proved
virtually flat. Their DTA curves showed diffuse thermal effects prior at 600 °C (attributed to
decomposition of the most common clay minerals) and well defined thermal effects at 875 °C and
about 990 °C (figure 9).
The sample for the inside of the oven suffered, in TG, significant weight loss between 686 °C and
745 °C, which was accompanied in DTA by an endotherm at 723 °C. The sample of the outdoor of
the oven gives, in the course of the heating DTA curve, a fine endothermic peak at 600 °C and by
continuing the heating, another at 723 °C in correspondence with the weight loss recorded between
673 °C and 732 °C. The thermal effect at 600 °C should be attributed to the softening or pre‐melting
of kaolinite (figure 9).
Conclusions
The studied shards of this report are pottery and that pottery is in the form of low fired
earthenware. The true pottery is made by forming clay into a desired shape, allowing it to dry and
heating it in a very hot oven, called a kiln, at a sufficient temperature, and for a sufficient period of
time until the clay particles fuse together. If insufficient firing is gained the clay body is only a
fragile pile of microscopic rocks held together (a “greenware”) and little feldspathic glass is
produced (as supported by XRD and FTIR in our fragments). The thermal history of the shards of
this study, evidenced through DTA curves, showed that they never were fired above 800 °C. Thus,
although the availability of the alkaline metal ions (fluxes) from the feldspar present in the studied
bodies encourages bonding of the outer layers of the refractory quartz particles to the surrounding
feldspathic glass matrix, a coherent skeletal internal structure lacks. This low feldespathic glass
content explains the poor properties observed in the clay bodies: mainly, low density, low strength
and friability. On the possibility that sintering has taken place, we think that is possible because the
kaolinite in a clay body that would ordinary melt at 1200 °C sinters at temperatures as low as 600
°C. Sintering appears to happen not so much because of melting, but because of diffusion of the
rapidly moving atoms between the neighbouring refractory particles. Villafáfila potters could made
use of this characteristic in a low temperature firing which is now called a “biscuit bake”.
5
References
Petanidou, T. (1997) Salt – Salt in European History and Civilization, Hellenic Saltworks S.A.,
Athens.
Escacena‐Carrasco, J.L. and Rodriguez de Zuloaga, M. (1994) La Marismilla – Una salina neolítica
en el Bajo Guadalquivir, Revista de arqueología, 9(89), 14‐24.
Rodríguez‐Rodríguez, E. (2000) Historia de las explotaciones salinas en las Lagunas de Villafáfila.
Cuadernos de Investigación Florián de Ocampo, Diputación de Zamora, Zamora.
Valiente‐Cánovas, S., Ayarzagüena‐Sanz, M., Moncó‐García, C. and Carvajal D. (2002) Excavación
arqueológica en las Salinas de Espartinas (Ciempozuelos) y prospecciones en su entorno, Archaia, 2,
33–45.
VV. AA. Archaeology from the Wetlands: Recent Perspectives: Proceedings of the 11th WARP
Conference, Edinburgh 2005. Society of Antiquaries of Scotland (6 May 2007). ISBN‐10: 0903903407.
Coles, J. (1984) The archaeology of wetlands. Edinburgh : Edinburgh University Press, 1984, 111 p.
ISBN 0852244894.
Delibes de Castro, G. (2009). La explotación de la sal en la edad del Bronce: el testimonio de las
salinas de Villafáfila (Zamora). Conferencia en el MARQ (9th Dec 2009). Alicante
Moga, I. (2009). Salt Extraction and Imagery in the Ancient Near East. Journal for Interdisciplinary
Research on Religion and Science, No. 4, January 2009
Erdoğu, B., Özbaşaran, M., Erdoğu, R. and Chapman, J. (2003). Prehistoric Salt Exploitation in Tuz
Gölü, Central Anatolia: Preliminary Investigations, Anatolia Antiqua.
Buccellati, G. (1991), Salt at the Dawn of History: The Case of the Bevelledrim Bowls), in E. van Donzel et
al. (eds.), Resurrecting the Past. A Joint Tribute to Adnan Bounni, Nederlan
Historisch‐Archaeologisch Instituut te Istanbul, İstanbul, 1991.
Olander, B. and Steenari, B.‐M. (1995). Characterization of ashes from Wood and Straw. Biomass
and Bioenergy,8(2), 105‐115
Misiego‐Tejeda, J.C, Marcos‐Contreras, G.J., Sarabia‐Herrero, F.J., Martín‐Gil, J. and Martín‐Gil,
F.J. (1993). Un horno doméstico de la Primera Edad del Hierro de “El Soto de Medinilla” y su
análisis por ATD. Bol Semin Estud Arte Arqueol (BSAA), 59, 89‐111. ISSN: 02109573
Ion, R.M., Ion M.L., Fierascu R.C., Servan, S., Dumitriu, I., Radovici, C,, Bauman, I., Cosulet, S.
and Niculescu. V.I.R. (2009). Thermal analysis of Romanian ancient ceramics. J Therm Anal Calorim.
DOI 10.1007/s10973‐009‐0226‐x
6
Figure 1. Pottery shards involved in the salt exploitation in Santioste (Zamora, Spain)
Figure 2. Bevelled‐rim bowls from Qraya, Mesopotamia (apud Buccellati, 1991)
7
Figure 3. Pottery ladles and reconstruction of a hypothetical salt‐making installation
(apud Buccellati,1991).
8
Figure 4. Salt‐pots from Bosnia, Romania, Poland and Turkey (apud Erdoğu, 2003).
9
Figure 5. Plinths (or holders) of clay that were used to hold ceramic vessels in ovens (apud Valiente‐Canovas, 2002)
10
Figure 6. XRD patterns of pottery shards and oven wall fragments from sites of Prehistoric salt exploitation
in Villafáfila, Zamora, Spain
Molino Sanchón ‐ Bowl residue
Molino Sanchón ‐ Bowl
Bowl plinth (inside) Bowl plinth (outside)
Santioste – Oven (inside) Santioste ‐ Oven (outside)
11
Figure 7. ATR‐FTIR spectra of pottery shards and oven wall fragments from sites of Prehistoric salt
exploitation in Villafáfila, Zamora, Spain
3368
.77
1396
.44
1008
.58
870.
62
711.
46
100015002000250030003500
W avenumber cm-1
0.0
0.1
0.2
0.3
0.4
AT
R U
nits
Molino Sanchón ‐ Bowl residue
3380
.19
1419
.71
981.
66
100015002000250030003500
Wavenumber cm-1
0.0
0.1
0.2
0.3
0.4
AT
R U
nits
Molino Sanchón ‐ Bowl
3385
.27
977.
23
775.
35
100015002000250030003500
W avenumber cm-1
0.00
0.05
0.10
0.15
0.20
0.25
AT
R U
nits
Bowl plinth (inside) 33
89.5
8
1461
.41
967.
80
775.
20
100015002000250030003500
Wavenumber cm-1
0.00
0.05
0.10
0.15
0.20
0.25
0.30
AT
R U
nits
Bowl plinth (outside)
3386
.44
1639
.84
1420
.51
977.
30
870.
83
100015002000250030003500
W avenumber cm-1
0.0
0.2
0.4
0.6
0.8
1.0
AT
R U
nits
Santioste – Oven (inside)
3380
.06
1637
.94
1428
.93
976.
64
870.
86
100015002000250030003500
W avenumber cm-1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
AT
R U
nits
Santioste ‐ Oven (outside)
12
Figure 8. TG curves of pottery shards and oven wall fragments from sites of Prehistoric salt exploitation in
Villafáfila, Zamora, Spain.
13
Molino Sanchón - Bowl residue
Molino Sanchón - Bowl
Bowl plinth (inside)
Bowl plinth (outside)
Santioste oven (inside)
Santioste oven (outside)
Figure 9. DTA curves of pottery shards and oven wall fragments from sites of Prehistoric salt exploitation
in Villafáfila, Zamora, Spain
14
Table 1. Chemical composition (% in weight) for pottery shards from Molino Sanchón and for an ancient
fournace in Santioste.
Fe2O3 MnO TiO2 CaO K2O SiO2 Al2O3 P2O5 MgO Na2O
Bowl residue 1.6 0.14 0.24 33.9 1.4 28.4 5.8 2.23 2.3 0.2
Bowl 1.8 0.12 0.24 29.8 1.5 28.2 6.4 1.74 2.0 0.1
Bowl plinth (inside) 4.2 0.06 0.78 2.8 3.4 69.4 14.6 0.08 1.5 0.9
Bowl plinth (outside) 3.3 0.04 0.70 3.3 2.6 73.1 12.1 0.07 1.3 2.1
Oven (inside) 3.5 0.06 0.60 8.5 2.9 58.5 12.1 0.29 2.0 0.9
Oven (outside) 3.4 0.05 0.65 4.0 2.7 70.3 12.1 0.10 1.6 1.5
Peak assignments:
711 cm‐1 Characteristic for calcite, dolomite and natron
775 cm‐1 Characteristic for quartz and orthoclase
871 cm‐1 Characteristic for diopside
977 cm‐1 Assigned to Si‐O‐Al mixtes vibration modes from aluminosilicate moities
982 cm‐1 Characteristic for gehlenite. Characteristic of arabinosyl units from straw
1009 cm‐1 Si‐O‐Si vibration. Characteristic for orthoclase (alkaline feldspar)
1396 cm‐1 Characteristic for natrón and goethite (b‐FeOOH)
1420 cm‐1 Characteristic for CO3 vibrations from calcite/dolomite
1461 cm‐1 Characteristic for CaO in the structure
1640 cm‐1 Characteristic for illite
3369 cm‐1 Characteristic for gibsite (hidrargilite), Al(OH)3
3380 cm‐1 Molecular water and Al‐OH stretching vibration from Al2O3.xH2O amorphous forms
Table 2. Predominant ATR‐FTIR peaks (in cm‐1) for pottery shards from Molino Sanchón and
for an ancient fournace in Santioste.
Bowl residue 711 871 1009 1396 3369
Bowl 982 1420 3380
Plinth, inside (red) 775 977 3385
Plinth, outside (grey) 775 968 1461 3389
Oven, inside 871 977 1420 1640 3386
Oven, outside 871 977 1429 1638 3380