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Organic Inclusions

Abstract and Keywords

Organic inclusions in ceramics may occur naturally in clay deposits or be intentionally added to the paste as temper. In the first case, the inclusions are composed of entire microscopic organisms and/or parts of microscopic and macroscopic plants and animals found in the local environment. In the second case, the plant or animal tempers are specifically selected, used alone or mixed with other organic or inorganic tempers, and come from a wide variety of geographic and ecological contexts. During firing, organic compounds undergo partial to complete destruction; charred organic materials or their heat-resistant remnants are nevertheless useful for the identification of their origin. The use of different tempers provides valuable information about ceramic technologies and regional potting traditions. In addition, organic inclusions may demarcate the geographical area of ceramic manufacture, the depositional environment of the clay, and/or ancient agricultural practices in the area of production.

Keywords: organic inclusions, ceramic technologies, organic temper, inorganic temper, plant temper, animal temper, ceramic manufacture

Introduction

Ceramic artifacts are made with clay hardened by kiln firing. The general term “clay” indicates a mixture of one or more clay minerals (hydrous aluminum phyllosilicates) containing small amounts of other mineral phases and organic compounds. Clay deposits may also contain microfossils, such as pollen, diatoms, and other animal and plant remains.

One of the important considerations for ceramic manufacture is the performance of the clay during firing. If the right balance between clay minerals and inclusions is not maintained, ceramics can slump or fail during firing. To avoid this, temper is often added to the clay. Tempers may consist of inorganic, non-plastic materials, such as minerals, rock fragments, and grog (crushed pottery and firebricks), and/or animal and plant matter. The mixture of clay and temper improves both the plasticity of the paste and the thermal properties of the vessel. The type and amount of temper used depend upon the manufacture technology used to produce the vessel and its ultimate use/function. The selection of a specific organic temper may depend upon local availability of resources, ease of access to recycled materials, and/or be dictated by cultural factors and intended usage of the resulting artifact.

In this text, organic inclusions are regarded as remains of biological matter; that is, portions of organisms or even entire small organisms, which are trapped in ceramic paste.

The Origin and Nature of the Organic Inclusions

Organic inclusions may occur naturally in the raw materials used to manufacture ceramics or they may be intentionally added to the clay paste by potters. During the firing, organic compounds undergo a partial or complete destruction, creating pores in the ceramic fabric. These pores make the vessel permeable and resistant to temperature changes and therefore suitable for cooking and as food storage containers (Maggetti, 1994). Minute charred organic (p. 566) materials or their heat-resistant mineral components are often found in these pores and can be extremely useful in the identification of the type of organic material included in the paste.

Biological remains, when present, can be detected in the paste of ceramic artifacts and sometimes even on their surface: they appear as small spots or pits and their size is usually so small that they are hardly visible to the naked eye. In most cases, only magnifying lenses and microscopes can detect the presence of biological remains and reveal their shape. When the observation of the morphological features of organic inclusions in ceramic paste does not permit identification, microanalytical investigations can determine chemical composition and reveal important information about the type of temper employed. The study of organic inclusions in ceramics is, therefore, a promising source of information about ancient environments, sourcing of raw materials, and ceramic manufacturing techniques.

When we study organic inclusion in ceramic wares, one of the first logical question is whether the inclusions were present in the raw material; that is, the natural deposit from which the clay was taken. Were the inclusions accidentally trapped in the paste during the kneading process and, therefore, present in the environment where the pottery was produced? Or rather, were they deliberately added to the paste in the form of temper to improve the plasticity of the clay? In some instances, the high concentration and type of inclusion leave us in no doubt, but it is not always easy to find a definite answer to these questions.

Organic Inclusions Occurring in Raw Materials

Organic inclusions that naturally occur in clay deposits and therefore may be found in ceramic paste can be entire microscopic organisms or fragments and/or parts of macroscopic plants or animals. A good example is the pottery of Thailand’s Ban Chiang region (c.3600 bc–200 ad), where freshwater diatoms and sponge spicules were detected in ceramic wares. Tempers of different composition were intentionally added to Ban Chiang pottery throughout its history: since freshwater diatoms and sponge spicules are always found in the ceramic paste of wares from all periods in spite of diverse temper combinations, this suggests that they were present in the clay deposits of local lakes that supplied the raw material (McGovern, 1989).

Diatoms are unicellular algae belonging to Bacillariophyta, widely distributed in damp and aquatic habitats. Their cell wall (called frustule) is composed of two overlapping thecae or valves: the larger one is called epitheca and the smaller hypotheca. The frustule is mostly made of silica (hydrated amorphous silicon dioxide, SiO2.nH2O); it has a peculiar shape with sculptures, that is, surface relief, and perforation patterns, which are valuable in systematics. When the algae die, the frustules sink and are preserved in sediments at the bottom of water bodies. Some types of diatoms only survive within a restricted range of ecological parameters, making diatom analysis useful for the environmental reconstruction of ancient artifacts. Beside diatoms, many other microscopic organisms may be preserved in clay sediments and survive the firing process, such as, for example, foraminifera, ostracods, radiolaria, and silicoflagellates (for references, see Quinn and Day, 2007).

Sponge spicules are siliceous or calcareous structural elements produced by sponges in freshwater, brackish, and marine environments. Siliceous spicules, which occur in a wider morphological variety than the calcareous ones, are divided in megascleres and microscleres depending upon their size. Once sponges are dead, their organic elements decompose and (p. 567) their spicules accumulate at the bottom of water bodies: chemical composition, symmetrical axes, shape, and dimensions of these spicules are useful tools for the identification of the sponges.

Organic Inclusions

Figure 31.1 Fan-shaped phytolith from modern rice (1 mm).

Phytoliths are another type of organic inclusion commonly found in ceramic paste: the term refers to all mineral (generally siliceous) plant deposits and often disarticulated remains surviving the decomposition of organic matter (Figure 31.1). In growing plants, silica may be deposited inside the epidermal cell walls, in intercellular spaces, or inside the cell lumen: the latter deposits are more properly termed silica bodies. Silica deposits have distinctive shapes useful for plant identification. An International Code for Phytolith Nomenclature was published for standardizing phytolith classification and descriptive parameters (Madella et al., 2005). Phytoliths may occur naturally in clay sediments used for pottery manufacture, or their presence in ceramic wares may result from the intentional use of vegetable temper. The quantity of phytoliths in the artifact generally provides clues as to their origin: when phytoliths appear in negligible amounts they are usually found in clay deposits. Conversely, if they reach significant quantities they were most likely added together with the temper.

Unusual organic findings in ceramic are the well-preserved palynomorphs that were discovered in half-burnt potsherds from a Chalcolithic–Early Historic site in West Bengal, India (Ghosh et al., 2006). The palynomorphs include algal remains, spores, and pollen, presumably coming from the surrounding area. It is interesting to note the presence of fossil disaccate pollen grains (cf. Striatopodocarpites) of Permian origin. The preservation of the pollen grains is thought to be due to the short duration of the firing process.

Organic inclusions may also be accidentally trapped in the clay paste during the kneading process: in this case their presence is infrequent. An example of accidental inclusion is the barley caryopsis (Hordeum vulgare) found in a potsherd from the Caverna dell’Aquila at Finale Ligure, Italy (Arobba and Caramiello, 2006). The caryopsis was completely burnt out during the firing, leaving a void in the paste. It is proposed that the barley was present at or around the location of ceramic manufacture.

(p. 568)

Organic Inclusions

Figure 31.2 Thin section of rice-tempered ceramic from Sumhuram, Sultanate of Oman: dark (above) and bright (below) rice lemmas/paleas. The bright one was more severely affected by oxidation during firing (1 mm).

Intentionally Added Organic Materials

A large variety of organic materials were used as tempering agents in archaeological ceramics. During firing, some types of organic tempers are preserved in a carbonized state (Figure 31.2), while others are completely destroyed, leaving only impressions in the paste and pits on the surface of the artifacts. These impressions—the negative imprints of the organic inclusions—result in tiny holes that increase porosity of the ceramic body and make the artifacts lighter and more permeable than those where mineral tempers were employed. When organic temper contains mineral components, such as cereal chaff or mollusk shells, these are more easily preserved during firing than organic compounds such as mollusk bodies.

The deliberate addition of a specific temper often introduces a small amount of “contamination” into the paste. For example, since weeds are harvested together with crops, cereal chaff normally incorporates weed seeds; their quantity depends on the degrees of weed infestation in the fields (Harvey and Fuller, 2005).

Organic tempers may be divided into two main groups: plant temper and animal temper. They may be used alone or mixed with other organic or inorganic tempers.

Plant Tempers

Plants used as tempering agents include numerous species, but generally consist of byproducts of the cereal crop processing waste and include different portions of the plant: stalks, husks, and ear bristles. The use of this tempering technique appears to have been quite common in the Prehistoric Old World, and survived to recent times in restricted areas: cattail fuzz was still used as ceramic temper in Afghanistan, near Kandahar, during the second half of the twentieth century (Matson, 1974); in Palestinian pottery, the tempering ingredients, threshing-floor straw, and cattail fuzz together with animal dung, shells, or sand, were added (p. 569) during wedging and worked into the clay to improve workability and reduce shrinkage (Johnston, 1974).

The use of cereal chaff as ceramic temper has been attested in many Asian archaeological sites. It is related to cereal farming and, consequently, availability of cereal processing byproducts. On the other hand, since pottery was widely used in trade, the presence of cereal chaff in wares does not necessarily indicate local cultivation, but may indicate the geographical area of production of the ceramic artifacts. The local provenance of the wares may be verified by comparing the mineral composition of the ceramic paste with those of the clay deposits occurring in the area. Otherwise, local cereal cultivation may be identified by means of pollen analysis or detection of cereal phytoliths in the soil.

The use of cultivated cereal chaff for tempering ceramic is first documented in southern China in 10,000 cal BP, where rice husks (Oryza sativa) were used as temper in Neolithic pottery of the lower Yangtze River region. Among other plant remains the ceramic contained fan-shaped rice phytoliths (Jiang and Liu, 2006). In Northern India, rice chaff-tempered pottery dating between c.2500–1000 bc was recovered in several Neolithic sites (Bellwood et al., 1992; Fuller, 2006). At Khok Phanom Di in Thailand, pottery from the second half of the second millennium bc was found to contain foreign rice temper: although rice was naturally available in the area, apparently the local pottery industry did not exploit it. Indeed, petrographic analysis indicates that clay was sourced north of the area, in the upper Bang Pakong Valley (Vincent, 2003). A similar situation also occurred in Bali, where rice husk-tempered pottery appears to have been imported from India, although rice was grown locally and is attested by phytoliths in the soil (Bellwood et al., 1992).

Rice-tempered ceramics were discovered at several locations outside the area of ancient rice cultivation, such as the Red Sea and coastal region of the Arabian Peninsula (Figures 31.2 and 31.4a); these findings indicate the existence of trade routes throughout the area around the Indian Ocean (Mariotti Lippi et al., 2011; Tomber et al., 2011). In the Arabian Peninsula, cereal chaff was found in ceramic fragments from the first millennium bc at Qala’at al-Bahrain: since three grains of barley were also found impressed in the pottery, the chaff was tentatively identified as two-row hulled barley (Hordeum distichon). The provenance of the object is unknown (Willcox, 1994).

Cereal chaff-tempered ceramics were also found in Sahelian Africa. Listed in chronological order from Neolithic to Iron Age, the dominant taxa are Sorghum, Setaria, Panicum, and Pennisetum (Fuller, 2013). Most interesting, ceramics tempered with Sorghum husks were recovered at Essouk-Tadmakka in Mali (c.1100–1300 ad); as the site bore no evidence of sorghum processing or consumption, this confirms the hypothesis that the wares were imported to Essouk-Tadmakka from the Niger Bend, where stylistically similar pottery was produced (Nixon et al., 2011). In Europe, the use of cereal chaff seems to have been be uncommon; plant fragments, possibly Triticum sp. chaff, were however found in Early Neolithic clay figurines, in Hungary (Kreiter et al., 2014), while unidentified chaff dating to 500/1 bc, was found in pre-Roman Iron Age vessels in Sweden (Stilborg, 2001).

In addition to cereal byproducts, other plants were intentionally collected for tempering ceramics. A few meaningful examples are listed here by geographical area. For centuries in far East Russia (Asia), coarsely chopped grass was added to the clay for ceramic vessels (Miermon, 2006; Ponkratova, 2006); conifer needles were used with similar purposes in far East Russia during the Neolithic (Ponkratova, 2006). Sizeable fragments of stems and leaves of sedge (Cyperaceae), horsetail (Equisetum), and burdock (Arctium lappa, Asteraceae family) (p. 570) were found in Neolithic ceramics from South Sakhalin (Zhushchikhovskaya and Shubina, 2006). In Africa, leaf fragments of sedges (Cyperaceae) and impressions of other wild plant materials were observed in pre-Pastoral and Pastoral sherds at Gobero (south Sahara). The use of these vegetable fragments suggests the intentional gathering of wild plants for ceramic production and indirectly the absence of crop byproducts (Fuller, 2013). In Europe, a mixture of mosses, mostly Neckera crispa, was commonly used for tempering ceramics in France and Belgium during the Neolithic (Constantin and Kuijper, 2002). In South America, particularly in Amazonia, one of the most common organic tempers consisted of ashes and microcharcoals made by burning the bark of trees belonging to several genera of the Chrysobalanaceae family, particularly Licania spp. The barks of these plants contain silica bodies in parenchymatous and epidermal cells, and often their cell walls are also silicified (Cronquist, 1981; De Walt et al., 1999). Native populations living in the Amazon have used this material as temper for generations (Evans and Meggers, 1962; Costa et al., 2009, 2011).

It is important to mention that most plants used as temper contain phytoliths and are therefore so-called Si-accumulating plants: in other words, they are capable of absorbing silicon from soil solutions (more precisely mono-silicic acid) and accumulating it as silica. Si-accumulating plants most commonly used for temper are Equisetum; Poaceae cereals and grasses; sedges (Cyperaceae family); Asteraceae. Silica is also present in the bark of Chrysobalanaceae and the needles of gymnosperms (Hodson et al., 2005). In Poaceae, silicon dioxide (silica) can reach 15% of the plant’s dry weight (Neethirajan et al., 2009). When these plant tempers are used in pottery making and burn out during firing, their phytoliths may be preserved in the voids, thus allowing the identification of the plant.

Finally, organic inclusions in ceramics may also be due to the use of biochemical sedimentary rocks as tempering agent. For example, the addition of diatomite to the paste introduces algal microfossils (i.e. diatoms) in the ceramic.

Animal Tempers

Many organic tempers are animal in origin: bones and shells (Figure 31.3a) are the most common. Bone-tempered ceramics were spread through Europe during the Neolithic and Bronze (p. 571) Age (Stilborg, 2001; Freudiger-Bonzon, 2005). Before being used in ceramic manufacture, bones were subjected to a heat treatment, a necessary precursor to crushing or grinding them into minute fragments. In the ceramic artifact, these fragments generally have an angular outline and a smooth surface: at high magnification, their histological features and canals may be visible. When these diagnostic characteristics are not present, the tiny fragments are often insufficient to determine the type and origin of the bone used. Bone-tempered ceramics have a high phosphorus content, much higher than the low values of natural clay (P2O5 = 0.1–0.5 wt.%); however, the high phosphorus pentoxide content is not sufficient to attest the use of bones as temper, more generically indicating the employment of animal temper (Freudiger-Bonzon, 2005).

Organic Inclusions

Figure 31.3 Shell-tempered ceramic from Sumhuram, Sultanate of Oman: (a) fracture surface showing shell fragments (stereomicroscope); (b) EDS spectrum indicating the presence of calcium in the animal material.

An unknown and unusual temper of animal origin was found in the Neolithic ceramics at Quadrato di Torre Spaccata, near Rome (Italy). Petrographic analysis revealed voids in the shape of parallelepipeds and energy dispersive X-ray spectrometry (EDS) analysis indicated high phosphorus values, both suggestive of animal temper. According to Pallecchi (1995), these voids likely result from the charring of organic matter, possibly meat, which was cut or processed to produce a regular pattern.

Crushed shells of freshwater and saltwater mollusks are often found in pottery from the Neolithic period onward. For example, large surface pits with a peculiar shape were found on the exterior and on the breaks of Neolithic pottery from Sakhalin Island (northern Pacific). The shape of the pits recalls mollusk shell fragments: indeed, small shell fragments are sometimes found in the pits. Morphometric analysis of the pits enabled the identification of several types of mollusks still growing in rivers, estuaries, and coastal waters of the area (Zhushchikhovskaya and Shubina, 2006). Among them, the most frequent species is Corbicula japonica, which has a very fragile shell. The consistently high phosphorus content of these wares suggests that both mollusk shells and bodies were used as temper: the shells prevented the clay from cracking during drying and firing, while the bodies were used to increase the porosity of the artifact (Zhushchikhovskaya and Shubina, 2006).

Prehistoric Oceanic pottery was tempered with a wide variety of organic materials. Among them were calcareous sands composed primarily of biogenic reef debris, such as the remains of corals, mollusk shells, foraminifera, and so on (Dickinson, 2006), and burnt coral, used to temper pottery in the Yap Islands, Micronesia (Intoh, 1988).

Organic Inclusions

Figure 31.4 Fracture surface of a potsherd from Sumhuram, Sultanate of Oman: (a) rice chaff fragment (stereomicroscope); (b) EDS spectrum indicating the presence of silicon in the plant material.

(p. 572)

Organic Inclusions

Figure 31.5 Renaissance ceramic tempered with wool (polarized 1 mm).

Sponge spicules have been found in Iron Age ceramics from Africa (inland Niger delta) and South America, especially in the Amazon Basin (Evans and Meggers, 1962; Costa et al., 2009; Nixon, 2009; Costa, 2011). In these ceramics, spicules often have parallel orientation and occur in clusters. This particular orientation and grouping, rather than the overall number of spicules in a vessel or sherd, suggests that sponge fragments were intentionally added to the paste, and excludes the possibility that the spicules were originally in the deposits where the clay was collected.

Many other animal tempers are occasionally found in clay wares worldwide: feathers, fish scales, dung, fibrous materials such as baleen, wool (Figure 31.5), and animal fur; for example, deer and horse hair. (London, 1981; Miermon, 2006; Ponkratova, 2006, Kiryushin et al., 2012). A remarkable application of animal material in ceramic manufacture consists of using animal hair as a framework during the shaping of vessels with the coiling technique: bunched hairs are arranged as filler in the seams between the coils and added to the paste, coating the vessel before baking. An example of this technique is the use of horsehairs in comb-patterned ceramics recovered at Tytkesken-2 in Russia. This tradition was also widespread in western Siberia from the early to the last Neolithic period (Kiryushin et al., 2012).

Laboratory Methodologies

In order to identify organic inclusions in archaeological ceramics it is necessary to observe and identify their structure and micromorphology with instruments of sufficient magnification to detect diagnostic features. Generally, an initial screening of the material at low magnification is the first step in order to identify the potsherds potentially containing organic inclusions. A simple magnifying lens (magnification 6–15x) held close to the eye might be sufficient to notice discontinuities in color and reflectivity or pitting in the vessel walls, all potential indicators of temper. However, owing to their small size, the identification of organic (p. 573) inclusions requires greater magnification, achieved by using a stereomicroscope (magnification commonly up to 60x) or even a compound light microscope (LM) (magnification up to 1000x). Both these microscopes utilize the interaction of the objects with visible light; however, they provide different information about the material: the former is useful for identifying the general tridimensional shape and micromorphology of the organic inclusions, while the latter allows us to see their inner structure and micromorphology.

The stereomicroscope is used for a variety of analyses over the course of the analysis because it allows a rapid surface screening of the specimen. The instrument is easy to use (even though many researchers utilize it under suboptimal conditions), does not require a previous preparation of the specimen to be examined, and allows direct observation of objects of rather large size, such as potsherds. Observation under LM, on the other hand, always requires laboratory preparation of the sample or specimen. Since light must pass through the sample in LM, the thickness of the specimen must be minimal, usually ranging from 1 to 10 μm for biological specimens and 30 μm for minerals. When the thickness of the sample exceeds these dimensions it is necessary to cut it in sections prior to viewing. The sections are then placed on a slide and covered with a coverslip, using aqueous or non-aqueous solutions as mounting medium. When analyzing ceramics, the thin sample sections are attached to a flat slide with epoxy resin, abraded to reach optimal thickness (25–30 μm). At about 30 μm of thickness, all the sample components are translucent and can be observed under LM. The sample section is covered with a coverslip, again using epoxy resin as mounting medium. The presence of the coverslip unfortunately excludes any successive microanalysis. Although polarizing light microscopy is successfully employed in petrographic analysis, it is generally not useful in the study of organic inclusions, which lose their original optical properties with charring. Moreover, organic inclusion might turn into cavities during combustion and later appear simply as dark areas in the thin sections.

When we observe material cut into thin sections all the components appear two-dimensional and their shape depends on the inclination of the cutting plane. Only by analyzing the same object sectioned along different cutting planes and collating the information can the object’s original three-dimensional shape be reconstructed. Finally, the sample material must be recorded as photographs; it is important to keep in mind that a reference scale is a fundamental element of any photograph or reproduction of the materials analyzed under a microscope.

Scanning electron microscopy (SEM).

A three-dimensional view of organic inclusions, at a higher magnification than can be achieved under a stereomicroscope, is possible by SEM. In addition, SEM analysis facilitates a more detailed surface analysis of inclusions because of its very high resolution (Figure 31.6). SEM observation requires mounting the specimens on metal stubs, usually 12.7–32 mm in diameter, using conducting double-sided adhesive tape or other conductive adhesives. Some samples can be observed directly with SEM, although generally they need to be covered by a thin layer of conducting material to present a suitable image. This is usually achieved using a sputter coater, which covers the specimens with a thin layer of gold or carbon (usually ranging from 15 to 30 nm thick) inside a vacuum chamber.

When the organic inclusions are burnt out in the kiln, cavities with well-defined shapes can be seen in place of the charred biological materials: the negative surface of these pits can be examined by SEM after cleaning, using compressed air or gentle brushing to remove dust and carbon from the surface, and coating with a conductive material. Some of these impressions may be cast with silicone elastomers at low viscosity such as polyvinyl siloxane (Fuller et al., 2007). The benefit of these casts of ceramic pits is that they are three-dimensional (p. 574) replicas of the original organic inclusions incinerated in the firing of the pottery. Once morphological observations of the organic inclusions are finished, they need to be compared to modern reference materials. This comparison will depend upon the geographic and ecological region of ceramic manufacture and is often best and most accurately conducted by a specialist. When fresh reference material is not easily available, specimens kept in museum collections can be used. The laboratory procedures depend on the nature of the material and are provided in the manuals of laboratory techniques and specific literature.

Organic Inclusions

Figure 31.6 Rice-tempered ceramic from Sumhuram, Sultanate of Oman: epidermis of the rice husk with evident silicized tubercles (SEM).

EDS analysis.

Many SEMs are equipped for EDS analysis, a technique used to identify the elemental composition of a sample. This information is rarely conclusive, but often indicates the nature of the inclusion when it is unidentifiable on a morphological basis. As mentioned, a high phosphorus content usually indicates animal material while silicon is more easily found in plants. Siliceous sponge spicules are a well-known exception. Calcium may occur in animals and plants (Figures 31.3b and 31.4b).

Radiography.

X-radiography may be used to obtain quantitative measurements of size, density, and orientation of temper particles. In fact, when organic tempers are burnt out, the voids appear as darker areas in X-radiographies (Braun, 1982). Radiography allows all useful data to be simultaneously detectable even when analyzing a large area. It may be directly performed without any previous preparation of the sample, but may render the ceramic unsuitable for further analysis, such as thermoluminescence analysis and radiocarbon analysis.

Interpretation of Results and Identification of the Inclusions

Interpretation of the microscopic and microanalytical data is a fundamental part of the analysis of organic inclusions. We must, first of all, keep in mind that in microscopy we are not observing real objects, but their images. The structure of the original object must therefore (p. 575) be deduced with care from the images we collect. The appearance of a specimen depends in large part on sample preparation techniques and the characteristics and operational conditions of the instruments. In other words, the same object produces different images when subjected to different laboratory procedures or observed under different instruments and conditions. Consequently, interpretation of the images must take into consideration the different procedures and instruments employed in the analysis of the specimen.

The interpretation of scanning electron micrographs, for example, is not easy despite their high resolution and magnification. Multiple images of the same specimen must be taken from different angles and orientations (in relation to the detector) or with different magnifications in order to accurately characterize its shape, size, and structure. Imaging specimens at high voltages may result in deceptive phenomena such as transparency or artificial brilliance of spiky elements such as spines. A non-corrected astigmatism of the instrument may produce incorrect shapes of the sample’s minute features and decrease resolution. Thus, researchers need to have good knowledge of technical procedures used to prepare the specimens and understand the functioning of the instruments to maximize performance.

The identification of organic inclusions involves naming organisms or parts thereof through comparative analysis of existing classification. Numerous micromorphological features have taxonomic value and permit the identification of the organisms. The same morphological characteristic can have or not have taxonomic value depending upon the group of organism under consideration, therefore it is critical to refer to and/or have knowledge of the specific literature in order to understand the significance of a particular feature. In the case of chaff, for example, a detailed SEM analysis of the glumes’ surface allows us to observe whether it is smooth or wrinkled, as well as to examine the morphology of epidermal cells, the presence or absence of trichomes and papillae, and their morphology; all of these features are useful for identification of the species of cereal. On the other hand, by analyzing thin sections of these same samples, we may detect other taxonomically relevant features, such as the number, arrangement, and form of the vascular bundles, the disposition of sclerenchyma, and the section outline of the glumes.

As indicated, direct comparative analysis of the organic inclusion with reference materials and exemplars is critical. When the comparison is performed by means of photographs or micrographs, it is essential that magnification and preparation techniques be explicitly stated.

Sometimes, scientists avail themselves of scientific texts that include diagnostic keys. The keys indicate which characteristics must be investigated in order to attribute a specimen to a precise taxon (i.e. group of organisms). Diagnostic keys are extremely useful but not always applicable: organic inclusions in samples are very fragmentary and rarely exhibit all the characters necessary for identification. Furthermore, keys presuppose a detailed knowledge of the group under examination and a command of specific and specialized terminology. In addition, because identification of an organism is accomplished by referring to an existing classification, it is important to use classification systems that are widely accepted and commonly used, in order to make the identification unambiguous and unequivocally comprehensible.

During analysis of a sample it is important not to disregard those characteristics that are not immediately useful for identification because they may provide supplementary information about the organism and, indirectly, the ceramic. For example, when analyzing thin sections, the occurrence of transversal sections of small stems with large intercellular spaces (p. 576) suggests that the plants were growing in an aquatic environment: the anatomical and histological characteristics of the plant are in fact adapted to the environmental conditions in which the species grow. The recognition of aquatic environments provides information about the production area of the archaeological ceramics of interest, and/or indicates raw material selection.

What Do Organic Inclusions Tell Us?

Tempers are added to a ceramic paste so that the resulting vessels and artifacts will posses specific qualities and characteristics. The selection of a particular temper may depend upon local manufacturing traditions, abundance or availability of the material, or technological strategy. Different pastes are often associated with specific wares and/or functional classifications and easily identifiable macroscopically. Tempers, therefore, can tell us not only about technology and tradition of ceramic manufacture in a given culture area or time period but also potentially provide information about the geographical/ecological origin and depositional environment of the raw material, agricultural practices, and so on.

Geographical Area and Depositional Environment

Organic inclusions may be derived from remains naturally contained in the raw material used for pottery manufacture. Alternatively, organic matter may be intentionally added to the clay paste by the potter. In either case tempers indicate a geographical area and/or environment, but there are significant differences in the archaeological interpretation of naturally included and intentionally added organic material. It is reasonable to assume that raw materials were collected relatively close to the place where the potter worked, and therefore clay itself may contain organic remains that are characteristic of the environment where it was deposited. Examples are the sponge spicules or diatoms that have been found in ceramic vessels from Thailand (McGovern, 1989). In cases such as this, where these organic materials were not intentionally added to the paste, inclusions provide information about the type of sedimentary depositional environment from which the clay was sourced. Information can be more or less accurate depending on the nature of the remains: diatoms, for example, are generally good indicators of environmental parameters and may also reveal the origin of the clay. However, some diatoms can live in a wide range of environmental conditions, while others are extremely sensitive to one or more environmental parameters, such as pH, salinity, water depth, and so on. In these cases, diatoms are more precise indicators of sedimentary depositional conditions.

When organic materials are deliberately added to the clay as tempers, they often provide information about the environment where the vessels were manufactured because, like the raw clay materials, tempering materials are generally collected close to the site of pottery production. Therefore, these inclusions are indicative of the geographical area where those species were present. For example, a particular type of cereal chaff included in a ceramic paste, clearly points to countries/regions of manufacture where that cereal was grown. On the other hand, the finding of ceramics tempered with chaff of cereals that were not (p. 577) locally grown suggests that those ceramics were produced in a different place and traded. Additionally, environmental information related to location of manufacture may also come from crop processing byproducts and local “contaminants” occasionally incorporated into the paste during ceramic manufacture, such as weed seeds appearing together with the chaff temper. These seeds provide evidence of surrounding wildlife, climatic condition, and sometimes eating habits of ancient populations.

Organic Inclusions

Figure 31.7 Karnataka rice winnowing in South India.

Agricultural Practices

Plant inclusions in ceramic fabrics have played an important role in studies of prehistoric plant use, cultivation, and domestication. As a matter of fact, analyses of crop remains or impressions (grains and chaff) in pottery have provided important data about the diffusion of cereal cultivation in prehistoric times and allowed us to detect morphological characteristics that reveal stages of domestication. For example, pottery tempered with rice husks that were shorter and wider than those of wild rice demonstrated that the beginning of rice domestication was associated with the origins of sedentism in southern China (Jiang and Liu, 1994).

In cereal processing, chaff is the result of winnowing, which follows either threshing (separating the spikelets from the straw) or milling and pounding (separating the husk from the grains) (Figure 31.7). The waste resulting from the winnowing after threshing and after milling and pounding is different: the first contains light seeds and fragments of stems and leaves; the second mainly husks and heavy seeds (Harvey and Fuller, 2005). It is generally the second waste, a result of dehusking, which is used for tempering ceramics. In addition to the husk, other byproducts of winnowing/crop processing, such as plant fragments, seeds, leaves, or stems may occur as “contaminants” in ceramics.

Regarding animal remains, the small bone fragments used as temper are difficult to identify, and thus generally offer little information about livestock. On the other hand, the occurrence of other remains, such as fish scales and bones, wool, or animal hair, may improve our knowledge of ancient populations’ economic strategy and exploitation of resources.

(p. 578) Chronological Data

Organic inclusions in ceramics may provide chronological data about the artifacts by means of accelerator mass spectrometry (AMS) dating. Carbons of different origin may be present in archaeological ceramics: carbon from the organic compounds of clay deposits relate to the age of sediment formation, and can be used to identify potential raw material sources/clay beds; carbon inclusions deliberately added as temper, however, provide information about the date of manufacture. The presence of carbons from two different sources and time frames in the same sample can skew AMS dating of a ceramic, reporting dates older than the real manufacture period.

Before preparing samples for radiocarbon dating it is convenient to evaluate the abundance of organic inclusions. This information may be gathered by observing the thin sections or using other methods capable of furnishing quantitative data on inclusion density and distribution. The size of the sample to be dated is dictated by the amount and the nature of the inclusions and also original firing temperature and condition, which can cause carbon oxidization.

Conclusion

The study of ceramic wares means coming into contact with ancient cultures, local traditions, manufacturing techniques, and trade history. Modern research methodologies supply interesting information about natural and cultural factors that influenced the production of ceramics. With regard to the specifics of organic inclusions, these can occur naturally in the clay or be added intentionally to the ceramic paste. Inclusions naturally occurring in the raw material offer information about the depositional environment of the basin from where the clay was extracted and, in some instances, they represent a sort of fingerprint of a specific clay deposit. When organic inclusions were intentionally added to the clay paste as tempers, they provide clues about potting technologies and traditions which evolved in different geographical areas. Moreover, since tempers are usually selected among available materials near the site of pottery production, they may allow us to identify geographical areas of ceramic manufacture, possibly even giving evidence of economic strategies, agricultural practices, and resource exploitation.

Finally, as ceramic “ingredients” are linked to the natural environment from which they came and maintain its traces, the interpretation of these traces can add considerably to our understanding of the relationship between humans and their environment in the historical past.

Acknowledgments

The authors wish to express their gratitude to Prof. A. Avanzini (University of Pisa), the Archaeologists of IMTO (Italian Mission To Oman), and the Adviser for Cultural Affairs, Sultanate of Oman, for the opportunity they all provided to study pottery recovered in (p. 579) Sumhuram. The authors are also grateful to Dr. Roberta Panzanelli for her critical reading, comments, and improving of this text, and to Dr. Alice Hunt for the improvement of the original English text.

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