Autochthonous or Allochthonous, the Prehistoric Pottery of Cueva de Los Postes

Abstract

Cueva de Los Postes is located in the southern part of the Spanish Extremadura region, in the Fuentes de León municipality. This study analyzed pottery found during archaeological excavations in Cueva de Los Postes. The aim was to determine whether the raw materials had common or separate origins and determine whether or not the origin of the pottery raw material was regional. In this regard, several pottery fragments were found in different phases (Neolithic and Copper Age) of the Holocene occupation of Cueva de Los Postes. These were analyzed using ATR-FTIR (Fourier Transform Infrared spectroscopy). The analyzed samples demonstrated no strong differences in their composition. In order to identify a possible origin for the kaolinite of the analyzed pottery, six samples of clay sediments were taken from inside and around the cave and subjected to ATR-FTIR. This was done to compare their compositions and check for a possible correlation with the pottery. The analyzed pottery, from different stratigraphic units, shows homogeneity in raw material. This finding, and the analyses made of the regional sediments, confirm that the raw material was regional and that there was a continuous use of it.

1. Introduction

The composition of pottery and the investigation of the origins of its raw material is the basis of this research. For this research, pottery from the Cueva de Los Postes was analyzed, as were clay sediments taken from in and around the cave. The cave is located in the Tentudìa region in the autonomous community of Extremadura within the province of Badajoz, in the town of Fuentes de León [1,2,3] (Figure 1).

This study, using ATR-FTIR (Fourier Transform Infrared spectroscopy), aimed to identify any homogeneity or heterogeneity in the compositions and possible surface treatments of pottery fragments that came from the excavation carried out in the Cueva de Los Postes. To identify a possible choice of sourcing location based on the raw material used, six sediment samples were taken for FTIR analysis and subsequently compared with the analyses performed on the pottery fragments. These samples were taken from the ground inside and around the cave.

1.1. Geomorphological and Archaeological Context

The site of Cueva de Los Postes sits on a hill above the confluence of the Montemayor and Santa Cruz rivers, which then joins the Guadalquivir River and flows into the Atlantic Ocean. The Cueva de Los Postes covers an area of approximately 180 m². Its irregular structure is divided into two main halls with a general E-W orientation [3,4,5,6]. This cave is one of the nine caves located in this natural park of Fuentes de León, a karstic complex. The complex is a Cantabrian limestone formation that has undergone many metamorphic and aquatic alterations over time. The erosion of this limestone matrix has made possible the formation of the red clayey soil found in and around the caves in this area [7,8].

The excavation began in 2004 [9,10,11]. In Cueva de Los Postes, a stratigraphic sequence ranging from the Pleistocene period up until the medieval age has been highlighted [1,2,3,7,8,12].

The Cueva de Los Postes in Fuentes de León stands as a pivotal site for unraveling the recent prehistory of the southwest Iberian Peninsula. Its function was exclusively funerary, enabling the documentation of collective burial rituals practiced by Neolithic societies in this region, which persisted largely unchanged through to the Copper Age. This period saw the emergence of new materials, such as metals and new typologies of pottery, alongside shifts in the size of lithic industries evident in burial offerings.

This research has been carried out in different fields of investigation and has shown that humans frequented the site during the Mesolithic, Neolithic, and Copper Age periods and that it was exclusively linked to funerary rituals. The presence of pottery in the layers of the excavation relating to the last two periods (Neolithic and Copper Age) should therefore be interpreted as votive offerings or part of funerary equipment [8,12,13,14,15,16,17,18].

Regrettably, our understanding of the residential contexts of the human groups interred in the Cueva de Los Postes remains limited. Despite extensive surface prospecting efforts, no evidence has emerged in the vicinity of the cave to delineate the types of settlements utilized by these groups. Nevertheless, in the Extremadura region, linked to the middle basin of the Guadiana River, archaeologists have excavated several Copper Age settlements, offering valuable insights into the settlement patterns of Chalcolithic societies in the southwest Iberian Peninsula [19,20,21,22].

1.2. Los Postes Sectors

The site of Cueva de Los Postes was divided, during excavation, into different sectors. The principal sectors of the cave are Area 1, the Hueco Eulogio sector, and the external sector.

1.2.1. Area 1

The Area 1 sector is located in the first chamber of the cave. Within it, there are different sectors or zones, all presenting the same stratigraphic sequence. The different clusters of Holocene occupation of the cave were identified thanks to radiocarbon dating carried out largely in Area 1 of the cave [7,9]. In the Epipaleolithic to Mesolithic stratigraphic units (20-10 SU), occasional and collective burial depositions of nomadic hunter-gatherer groups are evident.

Stratigraphic unit 9, in which pottery fragments were found, has been dated to the end of the 6th and the beginning of the 5th millennium BC. This layer is the first in the region to contain remains of Neolithic groups. There are contemporary occurrences in different areas of the Iberian Peninsula, but not in the Tentudìa region [23,24,25,26,27,28,29,30,31]. Stratigraphic unit 9 is a natural formation and presents brief and infrequent human activity connected to funerary deposits and related purposes. This element would lead to or support a theory of abandonment of the cave for almost a millennium between the last hunter-gatherers and the first farmers. This scenario has already been highlighted in other sites in Extremadura [30].

Stratigraphic units 8 and 7 of Area 1 date back to the middle and final Neolithic (4th millennium BC). Stratigraphic units 6 and 5 are, within reasonable doubt, attributed to the end of the Neolithic period. The doubt behind the chronological attribution is due to disturbances related to illegal excavations carried out before the official ones as well as the previous use of the cave for pastoral purposes, all of which have strongly compromised the more recent stratigraphic units [9]. For stratigraphic units 8 and 7, different dates have been proposed. The proposed dates highlight different moments from the end of the 5th millennium and include the entire 4th millennium BC. In this part of the stratigraphy, the chronological sequence does not correspond to the stratigraphic sequence. This leads to the assumption that the sedimentary deposit is related to different burial events and not to chronological events, which is why absolute dates are considered fundamental in this context.

1.2.2. Hueco Eulogio Sector

The Hueco Eulogio sector is in the rear chamber of the Cueva de Los Postes. The study of the materials, the dating (C14 AMS and U/Th), the paleoenvironmental analyses [12], and the sedimentological characterization allowed us to define four major stages which characterize the anthropic occupation of the cavity. Stage 1 covers the first four stratigraphic units, which have a sedimentary thickness of about 40 cm which is heavily affected by both anthropogenic and animal (burrows) activity. This has led to a combination of different contexts and consequently of archaeological material in which the presence of bisque and glazed pottery of modern chronology have been identified. In Stage 1, there is a notable presence of material from Roman chronologies alongside objects of indigenous tradition, which are directly associated with the artifactual assemblages documented in the excavations of the nearby Celtic fort of Capote in Higuera la Real (Badajoz) [32,33,34,35]. Stage 2 groups together the UE (stratigraphic units) 5, 6, 7, 8, 9, and 10 with an approximate sedimentary strength of 1.20 m. This is the most characteristic occupation of the Cueva de Los Postes, defined chronologically by the dating (Poz-44045) 4140+/−35 BP/Cal. 2875BC–2620BC, which was obtained on a human tooth from the UE 3/subn. 5 of the test pit of the Sala del Fondo. This is archaeologically equivalent to UE 5 of the Sala de Entrada and (Poz-13703) 5455 ± 40 BP/Cal. 4370BC–4230BC. These data come from a charcoal sample from the interior of a circular stone structure of uncertain functionality that appeared in UE 8 of the test pit of the Sala de Entrada. In this second major period of occupation, the Cueva de Los Postes basically acted as a collective funerary deposit. The human remains (more than 50 individuals) are distributed in the deepest part of the cavity, creating a formless accumulation with no apparent order, the result of the successive addition of human remains to the burial deposit. There is a variety of remains associated with these burials, several grave goods made of pottery vessels, all of them highly fragmented and exclusively handmade. The small and medium-sized vessels with oval or hemispherical shapes and straight or slightly rippled rims stand out in terms of percentage. Decorations are very scarce and when they do appear, they are in the form of series and bands of incised lines and to a lesser extent, impressions and red ochre coatings on the exterior of the vessels. In addition to pottery, lithic tools such as geometric, knives, blades, laminites, concave-based arrowheads, halberds, axes and polished adzes, bone tools (trowels, awls, combs, smoothers), and decorative tools (necklace beads made of various materials and perforated seashells) were discovered.

Stage 3 integrates stratigraphic units 11, 12, 13, and 14 with a sedimentary depth of 0.9 m. Like the previous stage, this is a funerary-type occupation chronologically defined by the following dating (all obtained from charcoal samples): UE 12: (Poz-14616): 7360+/−50 BP/Cal. 6370 BC–6080 BC; EU 13: (Poz-18774): 7440+/−50 BP/Cal. 6420 BC–6220 BC; EU 14: (Poz-33226): 7780 ± 60 BP/Cal. 6770BC–6460B. This would be the first occupation of the Cueva de Los Postes as a funerary deposit. In this sense, the ritual is similar to the type still in use a millennium later during phase 2, except the grave goods for these burials are different. In this stage, the ceramic vessels have completely disappeared, and the ones present are pieces of lithic industry which include geometric flint elements (very few in number) and a series of macrolithic pieces made from pebbles collected from the banks of the nearby Rivera de Santa Cruz. In addition to these pieces, there are also three small limestone slabs decorated with symbolic and figurative motifs [9].

Stage 4 is a very different occupation to the previous stages from a functional and chronological point of view. Dating was only possible in a test pit in the Sala del Fondo known as “Hueco Eulogio”. This is an occupation sealed by a thick limestone crust, which has yielded a U/Th dating of 192,986 +/−15,451–13,837. This unit, known as Pleistocene 1, approximately 0.3 m thick, has so far yielded a notable set of faunal remains with the presence of bovids, ursids, and chelonians, as well as a pair of lithic objects on black flint and a human phalanx of what is currently the first and only hominid remains in Extremadura, indicating human occupation of the cavity during the Middle Paleolithic. This find shed some light on the presence of the first Neanderthals in the southwest of the Iberian Peninsula. Little can be learned so far about their physical and cultural characteristics, as the studies are still in their preliminary stages and the sample size is so far very small.

1.2.3. External Sector

The external sector is a surveyed trench located outside the cave 80 m SE of the cave entrance. In this survey, several stratigraphic units were identified, which yielded several ceramic fragments, lithic tools and a human burial that has been dated to 4205 +/−30 BP (Poznan 140758).

2. Materials and Methods

2.1. Sampling

This research involved invasive methodology, removal of a sample not exceeding a few milligrams in weight, from the inner and outer surfaces of pottery sherds taken from Cueva de Los Postes. Twelve pottery fragments were analyzed. These were taken from different parts of the cave and from varying stratigraphic units dating to different phases of the Holocene occupation. Six sediment samples were taken in and around Cueva de Los Postes to compare the composition of the raw materials (clay) and final products (pottery). The pottery analyzed do not show significant differences when observed macroscopically (using color, texture, and porosity, among other characteristics). The microscopic observation of the composition of the pottery was instead investigated using FTIR-ATR instrumentation (Sciencetech Inc., London, ON, Canada). From these samples, a total of 24 compositional analyses of the pottery and 6 compositional analyses of the sediments were carried out using ATR-FTIR spectroscopy analysis.

2.1.1. Pottery

The samples of pottery analyzed (Figure 2) were marked with the POST formula (indication of the site investigated), consecutive numbering, and letters E and I indicating, respectively, the external or internal portion of the pottery form to which the fragment belonged. The list of the samples selected for this work and their location in the cave is as follows:

Samples POST-1E and POST-1I come from the pottery fragment with the inventory number 219 from the excavation campaign of 2008. This was found in the Hueco Eulogio sector, stratigraphic unit 1. It is a non-decorated handmade fragment belonging to a keeled shaped pottery. Samples POST-2E and POST-2I come from the pottery fragment with the inventory number 273 from the excavation campaign of the year 2008. This was found in the sector “Sumidero”, i.e., drain. It is a non-decorated handmade fragment of pottery with a rim. Samples POST-3E and POST-3I come from the pottery fragment with the inventory number 2517 from the excavation campaign of the year 2004. This was found in the western sector, in the excavation grid “Zanja inicial” or starting ditch. It is a handmade fragment belonging to a pottery body without decoration. The samples POST-4E and POST-4I come from the pottery fragment with inventory number 2339 from the excavation campaign of the year 2014. This was found in the outer sector in stratigraphic unit 1. It is a handmade fragment belonging to a pottery body and does not have decoration. The samples POST-5E and POST-5I come from the pottery fragment with inventory number 2248 from the excavation campaign of the year 2014. This was found in the eastern sector, in the excavation grid ‘J3’, stratigraphic unit 1. It is a non-decorated handmade pottery sherd with a rim. The samples POST-6E and POST-6I come from the pottery fragment with the inventory number 2332 from the excavation campaign of the year 2015. This was found in the Hueco Eulogio sector, in the excavation grid ‘West extension’, stratigraphic unit 3-SUBN 4. This is a handmade fragment belonging to a pottery body featuring a surface treatment. Samples POST-7E and POST-7I come from the pottery fragment with inventory number 1880 from the excavation campaign of the year 2007. It was found in the western sector, in the excavation grid ‘Detrás columna’, i.e., behind the column, stratigraphic unit 9. It is a non-decorated handmade fragment belonging to a pottery body. Samples POST-8E and POST-8I are from the pottery fragment with the inventory number 1879 from the excavation campaign of 2007, in the western sector, in the excavation grid ‘Detrás columna’, stratigraphic unit 8. It is a non-decorated handmade fragment belonging to a pottery body. Samples POST-9E and POST-9I are from the pottery fragment with the inventory number 1878 from the excavation campaign of 2007. It was found in the western sector, in the excavation grid ‘Detrás columna’, stratigraphic unit 6. It is a handmade fragment belonging to a pottery body and features an external ashlar projection which acts as a handle or grip to carry the vessel. Samples POST-10E and POST-10I come from the pottery fragment with inventory number 2907 from the excavation campaign of the year 2009. It was found in the eastern sector, in the excavation grid ‘K1’, stratigraphic unit 2. It is a non-decorated handmade fragment belonging to a pottery body. Samples POST-11E and POST-11I come from the pottery fragment with inventory number 2240 from the excavation campaign of the year 2014. It was found in the eastern sector, in the excavation grid ‘J3’, stratigraphic unit 3. It is a non-decorated handmade pottery sherd with a rim. Samples POST-12E and POST-12I come from the pottery fragment with the inventory number 1862 from the excavation campaign of the year 2012. It was found in the sector Heuco Eulogio, in the excavation grid ‘West extension’, stratigraphic unit 2. It is a handmade fragment belonging to a pottery body.

2.1.2. Sediments

The 6 sediment samples were taken inside the Cueva de Los Postes, from the ground above the cave, from one of the nearby caves (which has particularly clayey soil), and from an area known until the last century as a source of raw clay material (Figure 3). The sampled sediments were named with the first three letters of the site of sampling, a two-number code to identify the year of sampling, and a numerical code to indicate the succession of sampling within the site. The codes are therefore:

POS22/01 A sediment sample taken from the clay soil around Cueva de Los Postes. POS22/02 A sediment sample taken from the clayey soil in a stratigraphic unit dated to the Holocene, inside the Cueva de Los Postes. CAB22/01 A soil sampled from inside Cueva de Los Caballos, located inside the Fuentes de León Natural Park and presenting particularly clayey soil. Sampling was carried out in square C2 according to the grid system used during the excavation, with deposition dated to the Holocene. CAB22/02 A soil sample taken from inside Cueva de Los Caballos, located inside the Fuente de León Natural Park and presenting particularly clayey soil. Sampling was carried out in the square B4 according to the grid system used during the excavation, with deposition dated to the Holocene. JAR22/01 A sediment sample taken at the quarry of Vale de la Jara (8.5 Km far from Los Postes), known and used by the inhabitants of Fuentes de León until the last century as a source of raw clay material. This first sample was taken from soil that presented itself, at macroscopic observation, as having a more yellowish coloration. JAR222/02 Another sediment sample taken from the quarry of Vale de la Jara. This sample was taken from soil that was, upon macroscopic observation, of more reddish coloration than sample JAR22/01.

The sampling points of these six clay sediment samples were chosen by proximity to the site of Los Postes.

2.2. Analytical Method

The samples were analyzed using ATR-FTIR—Attenuated Total Reflection-Fourier Transform Infrared spectroscopy. ATR-FTIR spectroscopy returned spectra that were collected using a Bruker Alpha FT-IR (Bruker, Billerica, MA, USA) and Opus 7.5 software. The spectrometer employed an Attenuated Total Reflection (ATR) sampling device. The ATR-FTIR spectrometer was equipped with a global source—a KBr beam splitter (Spectral Systems, LLC., Hopewell Junction, NY, USA) and a lanthanum deuterated α alanine doped TriGlycine sulfate detector (Laser Components USA, Inc., Bedford, NH, USA) (at room temperature). The ATR sampling device worked with an internal diamond reflection element (IRE) in a single reflection configuration. Spectra were recorded in the spectral range of 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹, 24 scans. The returned spectra must be processed and interpreted using database information to allow for reading and subsequent discussion.

Fourier Transform Infrared spectroscopy is an analysis used to identify materials through the study of the vibrations of chemical bonds. This analysis is based on the absorption of infrared radiation by the materials. The infrared radiation is focused through a spectrometer onto the sample to determine and measure both the wavelengths absorbed by the material and the intensity of the absorption. A mathematical operation called the Fourier transform converts these data into a spectrum. Both qualitative and quantitative information can be provided by the study of these spectra. The wavelengths absorbed by the sample are characteristic of the chemical groups present in the sample. This is a molecular analysis; the absorption intensity at a defined wavelength indicates the concentration of the chemical group responsible for the absorption or the molecule present in the sample. Each peak represents a molecule. The peaks related to molecules of water can be found between 4000 and 3000 cm⁻¹. Organic elements produce peaks between 3000 and 2000 cm⁻¹. In the part of the spectrum that returns lower values (approximately 900–400 cm⁻¹) we find oxides, silicates, aluminum, phosphates etc. [36,37].

3. Results

Spectra obtained from the ATR-FTIR analysis of the Los Postes pottery, as seen in Figure 4 and Figure 5, and sediments, as seen in Figure 6, are presented below.

3.1. Pottery

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POST-1E and POST-1I

FTIR spectroscopic analysis allows us to identify the raw material of the sample as being from the kaolinite-serpentine group. This conclusion is based on the observation of the spectra, which in the case of the outer portion POST-1E show a typical peak of aluminum silicates discriminating for the kaolinite clay type at 989 cm⁻¹. The water peak typical of serpentine appears at 3370 cm⁻¹. In the POST-1I inner surface, the same ceramic type is identified with a typical peak at 982 cm⁻¹ and a water peak at 3360 cm⁻¹. The inner surface also reveals the presence of organic substances (peaks at 2924 and 2853 cm⁻¹) that could be due to the use of the pottery vessels or to post depositional actions due to the environment of the Cueva de Los Postes.

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POST-2E and POST-2I

The composition of POST-2E fits into the kaolinite-serpentine group. It shows the typical peak of the kaolinite clays type mineralogical group at 978 cm⁻¹ and the water peak at 3400 cm⁻¹. In contrast, the inner surface POST-2I is identified as meta-kaolinite because it has the typical kaolinite ceramic type peak at 988 cm⁻¹ and two of the three peaks of water (at 3530 and 3396 cm⁻¹) that identify kaolinite, leading to the sample’s identification as meta-kaolinite.

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POST-3E and POST-3I

This sample presents itself as kaolinite on both the outer and inner surfaces. In fact, on both surfaces it presents the three peaks typical of kaolinite waters, together with the peak typical of the aluminum silicates of kaolinite type clays. Specifically, POST-3E presents the peaks of the aluminum silicates at 999 and 916 cm⁻¹ and those of the kaolinite waters at 3694, 3618, and 3384 cm⁻¹. POST-3I, on the other hand, has the typical kaolinite aluminum silicates peak at 994 cm⁻¹ and the typical kaolinite group water peaks at 3695, 3619, and 3400 cm⁻¹.

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POST-4E and POST-4I

The analysis of this sample leads to the identification of kaolinite for the outer surface and serpentine for the inner surface. The outer surface, POST-4E, has typical aluminum silicates peaks at 999 and 914 cm⁻¹ and water peaks at 3694, 3619, and 3364 cm⁻¹. This surface also has organic residues identified by the presence of a peak at 2929 cm⁻¹. POST-4I, on the other hand, is serpentine because it has the aluminum silicates peak typical of the kaolinite type clay group at 984 cm⁻¹ and the serpentine water peak at 3368 cm⁻¹.

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POST-5E and POST-5I

The observation of the FTIR spectra of the POST5 sample highlights the meta-kaolinite composition of the external part and kaolinite-serpentine group for the interior. In this respect, the typical peaks of kaolinite aluminum silicates at 988 cm⁻¹ and of two of the three typical kaolinite waters at 3600 and 3386 cm⁻¹ (which lead to it being identified as meta-kaolinite and not kaolinite) are evident on the outer surface of POST-5E. POST-5I, on the other hand, has a typical peak for aluminum silicates at 987 cm⁻¹ and a typical peak for serpentine clay waters at 3399 cm⁻¹.

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POST-6E and POST-6I

This pottery fragment is composed of meta-kaolinite on both surfaces. The aluminum silicates component peaks are at 984 cm⁻¹ in POST-6E and 983 cm⁻¹ in POST-6I, while the typical water peaks are 3616 and 3385 cm⁻¹ on the outer surface and 3616 and 3384 cm⁻¹ on the inner surface. In this sample, the outer surface shows the presence of organic substances identified in peaks 2925 and 2851 cm⁻¹. This sample shows, at a macroscopic observation, a layer that seems to be made of Almagra spread on both surfaces (inner and outer) (Figure 2) [38].

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POST-7E and POST-7I

This pottery fragment is meta-kaolinite on both surfaces. The outer surface, POST-7E, has typical peaks of aluminum silicates at 984 to 908 cm⁻¹ and kaolinite type clay mineralogy group waters at 3694 and 3618 cm⁻¹. The inner surface, POST-7I, has peaks typical of kaolinite type clays at 998 and 911 cm⁻¹ and its waters at 3694 and 3619 cm⁻¹.

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POST-8E and POST-8I

The pottery fragment analyzed is meta-kaolinite on both its outer and inner surfaces. The typical peaks of the aluminum silicates of the kaolinite group are found in POST-8E at 984 cm⁻¹ and in POST-8I at 983 cm⁻¹, and those of the waters for POST-8E at 3620 and 3386 cm⁻¹ while in POST-8I they are found at 3613 and 3389 cm⁻¹.

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POST-9E and POST-9I

This sample is identified as serpentine for the outer surface and meta-kaolinite for the inner surface. POST-9E has typical kaolinite aluminum silicates peaks at 986 cm⁻¹ and typical serpentine water at 3384 cm⁻¹. POST9-I has typical aluminum silicates peaks at 995 cm⁻¹ and two of the kaolinite water peaks at 3616 and 3386 cm⁻¹. This surface also reveals the presence of organic substances at peaks 2923 and 2851 cm⁻¹.

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POST-10E and POST-10I

Both surfaces are shown as serpentine. POST-10E has the typical peaks of the aluminum silicates component at 981 cm⁻¹ and that of serpentine water at 3406 cm⁻¹. This surface also reveals the presence of organic compounds at peak 2978 cm-1. POST-10I, on the other hand, shows the typical peaks of the aluminum silicate at 963 and 916 cm⁻¹ and that of the typical water found in serpentine clay at 3395 cm⁻¹.

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POST-11E and POST-11I

This sample is composed of serpentine clay for the outer surface and meta-kaolinite for the inner surface. This is evident from the typical peaks of the aluminum silicates and the water peaks. In POST-11E, the peak for aluminum silicates is 990 cm⁻¹ and the water peak for serpentine clay is 3383 cm⁻¹. POST-11I, on the other hand, has the aluminum silicates peak at 990 cm⁻¹ and two of the three peaks typical of kaolinite group waters at 3607 and 3376 cm⁻¹.

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POST-12E and POST-12I

This pottery fragment shows the same composition, meta-kaolinite, in both examined surfaces. The typical aluminum silicates peaks of the kaolinite mineral group are found in POST-12E at 990 cm⁻¹ and in POST-12I at 991 cm⁻¹, the relative peaks are at 3613 and 3382 cm⁻¹ for POST-12E and 3600 and 3387 cm⁻¹ for POST-12I.

3.2. Sediments

The sediment samples taken in the soil above Cueva de Los Postes (POS22/01) and within it (POS22/02) return very different spectra. Sample POS22/01 shows typical and discriminating peaks of the different mineralogical groups of the clays at 996 and 913 cm⁻¹ for the aluminum silicates component and at 3617 and 3371 cm⁻¹ for the associated waters. The sample taken inside the POS22/02 cave yields different typical peaks, the aluminum silicates peaks here being at 1007 and 916 cm⁻¹ and the water peaks at 3692, 3618, and 3367 cm⁻¹.

The sediment samples taken at the Cueva de Los Caballos, CAB22/01 and CAB22/02, present the discriminating peaks for clay with values greater than 1000 cm⁻¹. In the case of CAB22/01 this peak is coupled with a peak at 915 cm⁻¹. Furthermore, the waters in CAB22/01 present two peaks at 3694 and 3617 cm⁻¹ and in CAB22/02 at 3689, 3615, and 3292 cm⁻¹.

The samples JAR22/01 taken from the Vale de la Jara clay quarry show typical peaks of the aluminum silicates of the kaolinite group clays at 996 and 912 cm⁻¹ and of their waters at 3617 and 3362 cm⁻¹. The second sediment sample taken from the quarry is different than the first one. In JAR22/02 the typical and discriminating peaks of the aluminum silicates of the clays and waters are found at 1030, 1002, and 910 cm⁻¹ and 3694 and 3622 cm⁻¹ respectively.

4. Discussion

The analysis by ATR-FTIR spectroscopy on the pottery led to the conclusion that they do not show strong differences in the peaks typical of aluminum silicates, i.e., those in the range between 1010 cm⁻¹ and 900 cm⁻¹ (around 1010–1000+o-4 cm⁻¹ are silicates, around 988–999+o-4 cm⁻¹ are aluminum silicates and around 910–916+o-4 cm⁻¹ are aluminum). The large differences among the analyzed pottery are evident in the peaks typical of water. The number of water peaks, which vary in the analyzed samples, indicates the use of raw material of different clays for the production of the analyzed pottery, even though the analyzed clays are found to belong to the same mineralogical group. The clays used to make the analyzed pottery are from the kaolinite-serpentine group and are kaolinite, meta-kaolinite, and serpentine. The identification of these three types of clays was possible through the identification of peaks typical in the ceramic spectrum. In the case of kaolinite, the presence of four peaks that characterize this clay has been identified, as have the aluminum silicates peak typical of this clay type (around 988–999+o-4 cm⁻¹ T(Al,SI)-O T=Al ou Si(Kao) which are aluminum silicates) and the typical three water peaks (around 3696–3698+o-4 cm⁻¹, 3617–3621+o-4 cm⁻¹, and 3358–3377+o-4 cm⁻¹). Meta-kaolinite is identified as kaolinite missing one of the water peaks that is necessary to consider a clay a kaolinite. In contrast, serpentine is identified by the water peak typical of this clay type (3360–4006+o-4 cm⁻¹). This raw material clay is found in calcium-poor soils.

The pottery surfaces composed of kaolinite include POST-3E, POST-3I, and POST-4E.

The pottery surfaces composed of meta-kaolinite include POST2-I, POST-5E, POST-6E, POST-6I, POST-7E, POST-7I, POST-8E, POST-8I, POST-9I, POST-11I, POST-12E, and POST-12I.

The samples belonging to the serpentine clays group are POST-1E, POST-1I, POST-2E, POST-2I, POST-4I, POST-5I, POST-9E, POST-10E, POST-10I, and POST-11E.

In order to highlight the differences that emerged between the inner and outer surfaces in some samples, the spectra were compared with each other as in Figure 4 and Figure 5 and

Table 1. Table in which only the relative peaks of water and the main aluminum silicates peaks found in the clays are shown. Peaks of different intensities are identified in the spectra, reported in the Table as very strong (vs), strong (s), medium (m), weak (w), very weak (vw), and shoulder (sh). (Edited by Sara Garcês and Virginia Lattao).

	Aluminum Silicates					Organic Substances		OH Kaolinite-Serpentinite Group
	Al-Oh		T(Al,Si)-O T=Al o SI(Kao)	T(Al,SI)-O T=Al ou Si	Si-O Stretching
POST-1E			989 vs					3370 sh
POST-1I			982 vs			2853 w	2924 w	3360 sh
POST-2E			978 vs					3400 sh
POST-2I			988 vs					3396 w	3530 sh
POST-3E	916 m		999 vs					3384 vw	3618 w	3694 sh
POST-3I			994 vs					3400 sh	3619 vw	3695 vw
POST-4E	914 m		999 vs				2929 vw	3364 sh	3619 w	3694 vw
POST-4I			984 vs					3368 sh
POST-5E			988 vs					3386 sh	3600 w
POST-5I			987 vs					3399 sh
POST-6E			984 vs			2851 vw	2925 vw	3385 sh	3616 w
POST-6I			983 vs					3384 sh	3616 w
POST-7E	908 s		984 vs						3618 w	3694 vw
POST-7I	911 m		998 vs						3619 w	3694 vw
POST-8E			984 vs					3386 sh	3620 vw
POST-8I			983 vs					3389 sh	3613 w
POST-9E			986 vs					3384 sh
POST-9I			995 vs			2851 vw	2923 vw	3386 sh	3616 w
POST-10E			981 vs				2978 vw	3406 sh
POST-10I	916 vs	963 vs						3395 sh
POST-11E			990 vs					3383 sh
POST-11I			990 vs					3376 sh	3607 w
POST-12E			990 vs					3382 sh	3613 w
POST-12I			991 vs					3387 sh	3600 w
POS22/01	913 w		996 vs					3371 sh	3617 w
POS22/02	916 m			1007 vs		2118 w		3367 sh	3618 w	3692 vw
CAB22/01	915 m			1005 vs			2521 vw		3617 w	3694 vw
CAB22/02				1005 vs				3292 sh	3615 w	3689 vw
JAR22/01	912 m		996 vs					3362 sh	3617 w
JAR22/02	910 m			1002 vs	1030 vs				3622 w	3694 w

. This difference between inner and outer surfaces is evident in the presence of different amounts of water and in the use of different clays between the inner and outer surfaces of the pottery body. The differences are evident in samples POST-2, POST-4, POST-5, POST-9, and POST-11.

In the case of samples POST-2, POST-9, and POST-11, the raw material serpentine was utilized for the external surface. An additional peak of water evident in the internal surface identifies the raw material as meta-kaolinite. The higher concentration of water in the inner surface could be related to the firing process.

In the case of sample POST-4, the outer surface is identified as kaolinite and the inner surface as serpentine. In contrast, we identified the outer surface of sample POST-5 as meta-kaolinite and the inner surface as serpentine. These results could be connected to a probable surface treatment related to the pottery’s use and/or to the firing process following this treatment. There are different examples of surface treatments which are sometimes used in the preparation of a specific type of pottery. As an example, the potter may first create the pottery vessel and fire it, then add another layer to the surface and fire it again [39,40]. Macroscopic observation does not show a surface treatment. The only sample in which it is possible to identify this element in both surfaces is POST-6, and only one of the samples analyzed, POST-9, has a decorative grip element to its design (Figure 2).

The comparison between the different spectra in Figure 4 and Figure 5 allows us to observe that the composition of the analyzed pottery is substantially homogeneous, with differences in the presence of water, a factor that could be connected to the preparation and firing of the pottery body.

Another element that can be seen from the spectra reading is that, in some samples, there is a concentration of organic substances. This concentration is only found in a few samples and is not only found on the inner surfaces but also on the outer surfaces of the pottery fragments. This factor is considered to be due to post depositional actions inside the cave. The presence of organic residues found in the ATR-FTIR spectra is believed to be the result of the use of this pottery and its past origins or alternatively the result of post depositional action and soil contamination (which is considered to be the more plausible explanation in this context). This explanation is supported by the low concentration of organic substances found on the internal surfaces (samples POST-1 peak 2924 and 2853 cm⁻¹ and POST-9 peak 2923 and 2852 cm⁻¹) and external surfaces (samples POST-4 peak 2929 cm⁻¹, POST-6 peak 2925 and 2851 cm⁻¹ and POST-10 peak 2978 cm⁻¹). That the organic deposits are not concentrated on only one side of the two surfaces and are present only on some samples supports the hypothesis that the presence of organic deposits was not intentional but the result of post depositional action.

In order to compare the clay that comprises the pottery and to understand if the supply of raw material was found near the cave or in the area adjacent to it, six sediment samples were taken. Two of these samples were taken from an old quarry located about 8 km from the cave. The quarry was used until the end of the last century as a supply of clay. Additional samples were taken in the cave, outside the cave, and in one of the other caves in the Fuentes de León park, the Cueva de Los Caballos.

From observing the FTIR spectra of the six sediment samples (Figure 6), it is clear that they are not composed of the same material. Samples taken at the clay quarry of Vale dela Jara (JAR22/01 and JAR22/02) are not considered likely to be the same clay used for the creation of the analyzed pottery, but they are a part of the same mineralogical group, which is the kaolinite-serpentine group. The sample POS22/02, which was taken inside the Cueva de Los Postes, is quite different. It turns out to be a part of the geokaolinite-serpentine group, but it does not correlate with the composition of the analyzed pottery, something that can also be argued for the sediment sample JAR22/02. The sediment samples that are similar to the composition of the analyzed pottery are POS22/01, taken outside the cave, and JAR22/01. To make the comparison between the pottery and the sediments, the peaks that are typical of water found in the different clays (around values of 3000–4000+o-4 cm⁻¹), and those typical of the different clay groups (between values of 900 and 1000+o-4 cm⁻¹) were used to make this identification. All the pottery we analyzed show the typical peak of aluminum silicates between 980 and 990 cm⁻¹ with a range of +o-4 cm⁻¹ (Table 1).

In Table 1, only the relative peaks of water and the main aluminum silicates peaks found in the clays are shown. From Table 1, it can be seen that the pottery samples which peak around 910–916 cm⁻¹ do not peak around 1000 cm⁻¹ and are therefore not comparable with the POST22/02, JAR22/02, CAB22/01, and CAB22/02 sediment samples. However, a particular comparison is evident between sediment samples JAR22/01, taken at the quarry, sample POST22/01, taken outside of Cueva de Los Postes, and some of the pottery sherds (POST-3E, POST-4E, POST-7E, and POST-7I). These samples have peaks related to the aluminum silicates component around 985–999+o-4 cm⁻¹ and one around 910–916+o-4 cm⁻¹. An exception is made for sample POST-10I, which shows a pair of typical aluminum silicates peaks at 963 and 916 cm⁻¹ that were not found in the other analyzed samples.

The two samples taken at the Cueva de Los Caballos, due to the particularly clayey composition of the cave, are both considered to be non-comparable with the analyzed pottery. Sample POS22/02, i.e., the one taken within the Cueva de Los Postes, is also not considered comparable to the analyzed pottery due to its high percentage of limestone (i.e., peaks at 1419, 872, and 712 cm⁻¹).

Therefore, only the samples taken from the exterior of the cave (POS22/01) and (more likely according to the interpretation of the spectrum) sample JAR22/01 can be considered comparable to the analyzed pottery. This, however, only shows up as a factor that can be highlighted for pottery that has a peak around 990 cm⁻¹ and a peak around 911 cm⁻¹. The analyzed pottery shows peaks typical of the component of aluminum silicates around lower values, such as 990 cm⁻¹, and it never shows values above 1000 cm⁻¹. On the other hand, JAR22/02 is excluded because it presents a peak typical of aluminum silicates around 1002 cm⁻¹ and another around 1030 cm⁻¹ and as such is considered too different from the analyzed pottery. There is therefore compatibility between sediment sample JAR22/01 and the pottery identified as meta-kaolinite POST-7, but not for the other pottery identified as meta-kaolinite such as samples POST-2-I, POST-5E, POST-6E, POST-6I, POST-8E POST-8I, POST-11I, POST-12E, and POST-12I.

For the other pottery, no matches were found in the sediment samples analyzed for this purpose. However, it can be argued that the clay used in the composition of the pottery came from clays found in soils in the vicinity of the cave. The kaolinite-serpentine group of clay appears to be one of the main groups present in the region of the Fuentes de León Natural Park [41].

From the map of clay distribution in the Extremadura region, one can see how different types of clays are reported, as seen in Figure 7. According to García Ramos et al. [42], in the region, clays are derived from the weathering of Precambrian slate formations known as the “Serie Negra” (Black Series). Their mineralogical composition is predominantly montmorillonitic-kaolinitic, with low carbonate and organic matter content. Clay originates from Cambrian and Precambrian materials formed through the alteration of slate formations. Montmorillonite and kaolinite are the dominant clay minerals [43].

The results obtained do not allow us to conclude with certainty the true location of the supply of these clays for the creation of the analyzed pottery. Different varieties of clay were used to create the analyzed pottery. However, all these samples of pottery come from the same kaolinite-serpentine mineralogical group and do not have any degrading agents or additional substances, and therefore it is inferred that the raw material was not processed but was used directly for the creation of the pottery. The raw material fits into the three subgroups of the kaolinite-serpentine group.

5. Conclusions

The pottery samples analyzed for this work show compositional differences but also belong to the same mineralogical group of raw material (the kaolinite-serpentine group). The results obtained come from the geochemical characterization of the samples taken from the Los Postes deposits. The origins of these samples appear to be the same based on our analysis. The analyzed samples do not show any particular surface treatment or decoration, except for the samples POST-5 and POST-9. Therefore, there was no different surface treatment related to the possible function of the pottery forms. Different varieties of clay were used to produce the analyzed pottery. The analyzed pottery was made from three subgroups of the kaolinite-serpentine mineralogical group, which include kaolinite, meta-kaolinite, and serpentine. The analyzed pottery have no degrading agents and no additional substances, and therefore it is clear that the raw material was not treated but used directly for the creation of the pottery. The organics present in a few samples can be traced back to post depositional activity. The analyzed pottery samples were compared with sediment samples taken at the Cueva de Los Postes and at other sites considered to be of interest for their concentration of clay. The obtained results suggest that the clay used to make the analyzed pottery was composed of raw material taken from the regional mineralogical formation. The analyzed pottery and sediments do not show a strong correlation, but all show that the various clays belong to the same mineralogical group. These clays are present in the geomorphological formation of the region but are not completely identical. It is therefore not possible to identify a precise origin of the supply of the raw material for the production of pottery found in the Los Postes cave. However, it can be hypothesized that the supply was found in regions close to the cave. The clay used in the pottery production is similar to some of the natural clays (sediments) collected near the cave [41].

Since the various clay samples show the same composition, this suggests that there were few differences (if any) in the manufacturing process. There are no decorations or different treatments on any these pottery sherds, except for POST-5 and POST-9. The clays are very homogeneous and therefore there is no evidence that suggests the presence of allochthonous materials.

The analyzed pottery samples come from different stratigraphic units but do not present compositional differences. This element, considering the fact that there is no change in the composition of the raw material utilized for the creation of the pottery during the use and frequentation of the Cueva de Los Postes, confirms the idea that the raw material is of regional origin and that there was a continuous use of local raw materials over time.