6 Clay Selection and Procurement

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1 6 Clay Selection and Procurement As previously noted in the Chapter 1, the archaeometric study of ceramic fabrics provides information on the type of raw materials used in the manufacture of the vessels. In this sense, it is considered that the composition of the pottery is a signature of the provenance of the raw materials involved in the production. However, we have seen how fabric composition is only partially related to the provenance of the raw materials, since can be also related to other stages of the life cycle of ceramics. Thus, changes occurring in the life cycle can greatly alter clays of the same provenance. It is often extremely difficult to identify and locate the specific deposits used to make pottery. Hence, it is more feasible to search for the use of certain types of clay deposits in regional territories and then try to determine their distribution in the areas under study. Therefore, a proper starting point regarding the use of raw materials could be the relation of the fabric s features with broad source areas. In a second stage, it may be feasible to focus on the peculiarities of the deposits located in the territory in order to best detect the clay sources of interest. Nevertheless, it should be remembered that usually the main purpose in provenance studies of ceramics through chemical and petrographic analysis is to establish different reference groups or petrogroups related to specific sites and periods. The main purpose of these studies is by no means to determine the use of specific clay sources located in well-defined areas of the territory. There are geographic areas, such as the Mediterranean basin, where it is common to find a close geological composition between distant zones and deposits. These areas may overlap in their geomorphological features because they respond to the same formation and depositional processes. In these cases, it can be very difficult to pinpoint the specific provenance of the raw materials involved in pottery production, even if petrographic and chemical analyses are conducted (Ortega et al., 2005; Peterson, 2009). Hence, it is often common to refer to a generic provenance compatible with the mineralogical and geochemical characteristics observed in the ceramics. From the compositional features of the fabrics we can determine the areas within the local geology that best match with the results and distinguish those pottery vessels that do not belong to these areas. One of the difficulties that can arise from the study of the provenance of the raw materials is the absence of optical properties that allow us to suggest accurately the use of particular clays (Gibson and Woods, 1990). Sometimes the mineral assemblages documented by petrography and XRD can be fully consistent with the geological features of the areas under study, but may be also present in other areas or regions. This fact introduces some uncertainty, as it cannot be absolutely ensured that the vessels do not belong to an external or distant source with a composition very similar to the local clay deposits (Cuomo Di Caprio, 1985; Muntoni et al., 2009; Nesse, 1991; Velde and Druc, 1999) Daniel Albero Santacreu This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License.

2 Technology and Ceramics 61 This uncertainty derives from the limited mineralogical and textural variation that may exist between certain clay deposits. Thus, these cases may require the characterization of the chemical composition of the vessels to deepen in the provenance of the raw materials. Therefore, from the combination of petrographic, textural, mineralogical and chemical data, as well as through the characterisation of local clay sources, we can determine the provenance of the raw materials used in pottery production and even relate them to specific sources in the territory (Howard, 1982; Riley, 1982). Moreover, this analytical strategy reveals if deposits with specific characteristics and properties were preferred and triggers hypotheses about the reasons for their preferential selection. As seen, special attention should be paid to the geomorphology and lithology of the territory under study in order to establish the features that characterize the local sources potentially attributable to the provenance of the artefacts. The particular characteristics and regional variations that take place in the frequency, size, distribution, roundness and sorting of detrital minerals such as quartz and feldspars, in conjunction with the chemical composition, refer to differences in the depositional environment and processes of formation of the sources (Tucker, 1991). Most clay minerals and other detrital minerals that are present in the raw materials derive from the erosion of the Earth s crust and the transportation of suspended particles by water until their deposition in low-activity environments as deltas, lagoons, lakes, marisms or coastal areas, etc. These processes determine the features and properties of the raw materials such as their colour and sedimentary structure as well as their mineralogical and geochemical composition. Hence, the study of all these parameters in the pottery allows us to approach the type of raw materials used in the production of ceramics. Besides the comparison of the mineralogical, petrological, chemical and textural composition of clays and vessels we can also search for the presence of distinctive components, for example clay pellets or argillaceous rock fragments (Fig. 6.1). These particles are similar to crushed ceramics but may differ in their morphology and the orientation of the nonplastic components, as well as in a composition more or less similar to the clay matrix (Cuomo di Caprio and Vaughn, 1993; Whitbread, 1986). These components should not be confused with crushed ceramics, since normally clay pellets and argillaceous rock fragments are particles naturally occurring in the clays. However, they can also be related to clay pellets which are dried in the potter s hands and thereafter are incorporated into the paste forming isolated clay granules. Thus, in both cases their presence may provide evidence of the textural characteristics of the raw materials, being especially useful to study the original features of the clay when tempers are added to the paste (Kretier et al., 2007; Livingstone-Smith, 2007; Potter et al., 2005; Vince, 2009).

3 62 Clay Selection and Procurement Figure 6.1: Thin section microphotographs taken in cross polarised light of fine-textured and wellrounded clay pellets that clearly differ from the surrounding clay matrix (A: Image width= 2.7 mm; B: Image width = 1.75 mm). Also the provenance of the raw materials can be approached through the characterisation of the microfossils existing in the clay sources collected by the potters (Quinn and Day, 2007a). As stated before, micropalaeontological analysis provides relevant information regarding both the clay used and its depositional environment. Thus, sediments placed in marine environments are rich in pelagic fauna, such as planktonic foraminifera (e.g., Globigerinidae), which is deposited in the deep ocean (Fig. 6.2 A). In contrast, benthic foraminifera (Fig. 6.2 B) are characteristic of warmer depositional environments such as shallow, coastal saline waters containing abundant fauna that tolerates hypersaline environments like brachiopods, arthropods, molluscs and bivalves (Tucker, 1991). Figure 6.2: Thin section microphotographs taken in cross polarised light showing foraminifera identified in archaeological ceramics (Globogerinidae and Rupertina) (Image width= 215 µm).

4 Technology and Ceramics 63 Clays can be roughly classified regarding their calcareous and non-calcareous nature. Calcareous clays have more than 5% of CaO in their composition while the clays that have up to this concentration are so-called border calcareous. In contrast, if the clay lacks CaO, it is classified as non-calcareous. This main distinction is important, since non-calcareous and calcareous clays have different properties and behave differently. These physical differences determine, for instance, different coefficients of thermal expansion (Tite and Kilikoglou, 2002). On the one hand, calcium carbonate (CaCO 3 ; e.g., calcite) is a very soft material that is quite soluble, being common to find it cementing other mineral components such as clays. Through this process a type of sedimentary rock called marl is formed. Marls are mainly composed of calcite and clay and have whitish colours as well as tones that can vary according to the proportions and diverse composition of the major minerals (Blatt, 1982). Marly clays are characterized by containing up to one third of its weight in calcium carbonate. Clay sediments with high lime content are quite widespread in certain regions, so that marls are raw materials commonly used in pottery production. For example, in the Mediterranean basin many potters have been using sources related to Cretaceous, Miocene, Pliocene or Pleistocene marly clays since ancient times. Thus, the use of calcareous clays is well-known, among many other places and times, such as the Neolithic in Southeast Italy (Muntoni et al., 2009), the Bronze and Iron Age in Mallorca (Albero, 2011a) or the Iron Age in Galilee (Shoval et al., 2006). Typically, this type of raw material is fine-grained and has, due to its high contact surface area, a great plasticity. In addition, marly clays frequently contain a certain amount of inclusions and organic matter, being usually fairly consistent and workable (Cau et al., 2002; Cuomo Di Caprio, 1985; Echallier, 1984; Gibson and Woods, 1990; Rice, 1987; Velde and Druc, 1999). On the other hand, non-calcareous clays involve raw materials without calcium carbonate but of higher siliceous nature. In ceramics made from these sources and without temper the presence of calcite is linked to secondary processes that took place during the burial of the vessel (Cau et al., 2002). Anyway, the use of this type of refractory clays, such as terra rossa (Fig. 6.3 A), is also quite widespread in the manufacture of pottery, for instance in the Mediterranean basin. This kind of raw material has been used since the Neolithic in the manufacture of coarse-wares, for example in the production of the whole Cretan ceramic repertoire during the Bronze Age (Hein et al., 2004), in the Cyrenaica area in the Roman period (Swift, 2005) or to manufacture cooking pots during the Iron Age in Galilee (Shoval et al., 2006). Its use is also documented in the production of building materials in the Bronze Age in the Balearic Islands (Albero and García Amengual, 2010). Currently, many communities of potters continue to use these kinds of raw materials in the Mediterranean (Albero and Puerta, 2011; Hein et al., 2004). Finally, we can also distinguish the more or less ferruginous nature of the calcareous and non-calcareous clays. The use of ferruginous clays for making pottery is relatively common in ceramic production (Cuomo Di Caprio, 1985; Morales, 2005).

64 Clay Selection and Procurement Iron-rich calcareous clays were used, for example, in the manufacture of pottery in Syria since Hellenistic times to the Islamic period (Schneider et al., 2007).

5 64 Clay Selection and Procurement Iron-rich calcareous clays were used, for example, in the manufacture of pottery in Syria since Hellenistic times to the Islamic period (Schneider et al., 2007). In this sense, heavy minerals and iron oxides naturally occurring in the sediments may be characteristic of a particular source (Fig. 6.3 B). Since these components have some resistance to erosion they are usually common in the clays used to manufacture the ceramics. In this way, in the Mediterranean basin the presence of certain lateritic clays which can exceed 10-15% in iron content is well documented (Cuomo di Caprio, 1985). Thus, differences in the ceramics regarding the concentrations of Fe 2 O 3 or the presence of certain opaque inclusions associated with iron oxides can be used as evidence to identify vessels made from different clay deposits (Albero, 2011a; Krause, 1984; Riederer, 2004; Sauer and Gassner, 2009; Shoval et al., 2006; Tucker, 1991). Pottery classifications based on the iron content of the fabrics have therefore proved very efficient, for instance in Bronze Age ceramic assemblages from several archaeological sites in Mesopotamia. Through re-firing tests carried out at 1050 C along with the optical analysis of the pottery the researchers were able to establish several groups based on the more or less ferruginous nature of the clay (Van As, 1984). Figure 6.3: Thin section microphotographs taken in cross polarised light (Image width = 4.6 mm) showing A) terra rossa clay used as building material and B) a well-rounded iron nodule of pedogenic origin containing some quartz grains in an archaeological ceramic.

Chapter 6 Pages of Earth s Past: Sedimentary Rocks

Chapter 6 Pages of Earth s Past: Sedimentary Rocks Introduction! Drilling into the bottom of the North Sea, we encounter: " Soft mud and loose sand, silt, pebbles, and shells. Then: " Similar materials