Ceramic Technology in the Aegean World During the Bronze Age

It is shown that the temperatures at which vitrification occurs and the vitrification structures develop, depend on the chemical composition and texture of the clay and the atmosphere in which it was fired.

Two fairly distinct ceramic technologies are identified. The first, which is associated with Thera, Phylakopi, and the Peloponnese, is based on the use of calcareous clays fired in an oxidising atmosphere at temperatures in excess of 800^o C. The second, which is associated with Servia and the Balkans, is based on the use of non-calcareous clays fired in a reducing atmosphere at a temperature below 750^o C.

INTRODUCTION

Pottery sherds from Thera, from Phylakopi in Melos, from Servia in northern Greece and from various sites in the Peloponnese and the Balkans were examined, using a scanning electron microscope (SEM) in order to obtain information on the ceramic technologies employed in the Aegean and surrounding areas during the Bronze Age. This programme of work forms part of a more comprehensive study of ancient ceramic technologies, which includes pottery from both Europe and the Near East, spanning the period from 6000 B.C. to 1500 A.D. (Tite & Maniatis 1975a).

Fresh fracture surfaces of the pottery were examined in the SEM, both in the as-received state and after re-firing at known temperatures in oxidising or reducing atmospheres in a laboratory furnace. In addition, semi-quantitive analysis for major and minor elements (Si, Al, K, Fe, Mg, Ca and Ti) was undertaken using the energy-dispersive X-ray fluorescence spectometer attachment to the SEM, and the fracture surfaces were examined under an optical microscope (X10 magnification) in order to obtain an assessment of the texture of the pottery fabric.

The SEM examination of the pottery in the as-received state provided information on the internal morphology developed during the original firing in antiquity, and in particular on the extent of vitrification (the glassy phase) and pore structure. From the study of the pottery after re-firing at known temperatures, it was possible to follow the subsequent development of the vitrification and to assign approximate firing temperature ranges to the various stages in this development. In addition, on the basis of the X-ray fluorescence analysis and the optical microscopy, the relationship between the chemical composition and texture of the clay and the vitrification structures developed was established. It was thus possible to define the ceramic technologies used in the manufacture of Bronze Age pottery from the Aegean in terms of the types of clay and firing temperatures and atmospheres employed.

DEVELOPMENT OF VITRIFICATION

The first stage in the development of vitrification in fired clay is the appearance of isolated smooth-surfaced areas or filaments of glass in the fracture surface examined under the SEM (Plate 1), this structure being referred to subsequently as the initial vitrification stage (IV). With increasing firing temperature, the extent of the glassy phase increases until ultimately the total vitrification stage (TV), in which a continuous smooth vitrified layer is formed over the entire fracture surface, is reached (Plate 2). The structures associated with the initial and total vitrification stages are essentially the same for all types of fired clay. However, the firing temperatures at which these stages are reached and the intermediate vitrification structures which are formed depend critically on the chemical composition and texture of the clay (Tite & Maniatis 1975b), and the atmosphere in which it is fired (Maniatis & Tite 1975).

The concentration of silica (SiO₂), alumina (Al₂O₃) and the fluxes (K₂O, Fe₂O₃, CaO, MgO and TiO₂) for the Aegean pottery are plotted on a ternary diagram (fig. 1). The boundaries defining the concentration ranges associated with the various types of clays and vitrification structures discussed below are also drawn, these being based on data obtained from the analysis of 200 pottery sherds, included in the more comprehensive study.

The clays used in the manufacture of the Aegean pottery reach the initial vitrification stage (IV) after the firing to 800 - 850^o C in an oxidising atmosphere, and are of a low refractory type, containing more than 9 per cent of fluxes. On the basis of the subsequent development of vitrification the clays can be divided in two main types. Since the concentration of calcite in the raw clay types are referred to as non-calcareous (NC) and calcareous (C). In the case of the non-calcareous clays (NC), which normally contain less than 5 per cent of calcium oxide, the amount of glass increases progressively with increasing firing temperature, a temperature range of about 150^o C separating the initial and total vitrification stages. In contrast with the calcareous clays (C), which normally contain more than 5 per cent of calcium oxide, an intermediate extensive vitrification structure (V) is formed at about 850^o C and this structure remains essentially unchanged for 200^o C until a firing temperature of about 1050^o C is reached. This stable vitrification structure is probably associated with the formation of crystalline phases (i.e. calcium silicates) through the reaction between the calcium oxide and clay minerals. When fired above 1050^o C, the amount of glass in the calcareous clays increase rapidly, and the total vitrification stage (TV) is reached by about 1100 - 1150^o C. Included within the calcareous group are a few clays which exhibit an essentially stable vitrification structure but which contain less than 5 per cent of calcium oxide. It appears that, in these clays, the lower calcium oxide concentration is compensated for by a higher than normal magnesium oxide concentration, that is 5 - 8 per cent of magnesium oxide as compared with less than 4 per cent for the non-calcareous clays.

The structure associated with the intermediate extensive vitrification stage (V) depends on both the composition (i.e. non-calcareous or calcareous) and the texture of the clay. In the case of fine-textured pottery in which few non-plastic inclusions are visible under the optical microscope but alignment of the pores can be seen, a regular pattern of aligned smooth-surfaced glass filaments are observed under the SEM. This structural anisotropy presumably reflects the alignment of the clay particles achieved during the forming process in the absence of inclusions. In the non-calcareous clays, the filaments coalesce at the extensive vitrification stage, whereas in the calcareous clays, the filaments remain separate (Plate 3). This more open structure is probably the result of the disturbance of the clay particles by the carbon dioxide liberated when the calcite is dissociated at an earlier stage in the firing (600 - 750^o C). In the case of coarse-textured pottery in which many inclusions are visible under the optical microscope, and no alignment of pores or inclusions can be seen, the areas of glass observed under the SEM show no preferred orientation and the vitrification structure is more disturbed. In the non-calcareous clays, continuous areas of glass with an uneven topography are formed (Plate 4), whereas in the calcareous clays, an open and essentially isotropic network of glass separated by pores is observed (Plate 5). Since it has been found that, in practice, fine-textured pottery is made almost exclusively from calcareous clays, the distinction between aligned or anisotropic (A) and non-aligned isotropic (I) extensive vitrification structures is used only in the case of calcareous clays when plotting the analytical data on the ternary diagram (fig. 1).

The development of vitrification also depends on the atmosphere in which the clay is fired. When non-calcareous clays are fired in a reducing atmosphere, the temperatures at which the various stages in the development of vitrification are formed are lower by between 50 and 100^o C than those for firing in an oxidising atmosphere. In addition, firing in a reducing atmosphere results in the formation of a distinctive pattern of bloating pores (Plate 6), which are associated with the release of gas after vitrification has begun (Maniatis & Tite 1975), this effect being particularly pronounced when a fast heating rate such as 800^o C/hr, is employed. In contrast, the effect of a reducing atmosphere on calcareous clays is less severe, the temperature ranges and structures associated with the various vitrification stages being similar for both oxidising and reducing atmospheres.

The approximate firing temperature ranges associated with the various vitrification stages observed in non-calcareous clays (NC) and calcareous clays (C) are summarized in Table 1, the data for firing in both oxidising and reducing atmospheres being included in the case of the non-calcareous clays. These temperature ranges assume a heating rate during firing of 200^o C/hr and a soaking time of 60 minutes at the peak temperature. If significantly faster heating rates (say 800^o C/hr) and shorter soaking times (1 minute) were employed, then firing temperatures higher by about 50^o C would be required in order to reach the same vitrification stage.

Table 1: "Firing temperature ranges"

RESULTS

The data for the types of clays used and the vitrification stages reached in the manufacture of the Bronze Age pottery from Thera, Phylakopi (Melos) and Servia (N. Greece) are summarized in Table 2, the pottery being divided into coarse, medium and fine fabrics on the basis of the density of non-plastic inclusions observed under the optical microscope. The associated firing temperature ranges are given in Table 1, and the analytical data for these three groups of pottery are plotted on the ternary diagram (fig. 1).

Table 2: "Clay type and vitrification data for Bronze Age pottery from the Aegean area"

The sherds from Thera which were examined were mainly from the Late Bronze Age destruction levels, but a few sherds of Early and Middle Bronze Age data were included. The results indicated that the ceramic technology was based on the use of calcareous clays which were fired in an oxidising atmosphere at temperatures normally in excess of 800^o C such that initial or extensive vitrification was produced. All the medium and fine pottery was made from calcareous clays, the majority of the latter exhibiting the anisotropic vitrification structures. However, both non-calcareous and calcareous clays were used for the coarse pottery. Further, on the basis of somewhat limited evidence, it appears that the ceramic technology remained essentially unchanged throughout the Bronze Age.

The sherds from Phylakopi were divided fairly equally between the Early, Middle and Late Bronze Age periods, and included several sherds of fine pottery which were believed to be important. The results indicated that the ceramic technology was essentially the same as that employed for the pottery from Thera, and that no obvious technological changes occurred during the Bronze Age. Again, the medium and fine pottery was made from calcareous clays which were normally fired at temperatures in excess of 800^o C, all the fine pottery exhibiting the anisotropic vitrification structure. Similarly the non-calcareous clays were used only for the manufacture of coarse pottery, and this was fired at temperatures below 800^o C such that no vitrification was produced.

The sherds from Servia were all of Early Bronze Age date and included two sherds of fine pottery which were thought to be imported. The results indicated that the ceramic technology differed significantly from that employed for the pottery from Thera and Phylakopi, the majority of the Servia pottery being made from non-calcareous clays which were fired in a reducing atmosphere at temperatures normally below 750^o C such that no vitrification was produced. The one sherd which was fired to a sufficiently high temperature to produce total vitrification also contained bloating pores, and since bloating is indicative of a fast reducing firing, it seems probable that the total vitrification resulted from the chance positioning during firing rather than from any deliberate firing procedure. In contrast, the two sherds of imported fine pottery were technologically comparable to those from Thera and Phylakopi, being again manufactured from calcareous clays which were fired at temperatures in excess of 850^o C such that extensive vitrification with the anisotropic structure was produced.

On the basis of the examination of a more limited selection of pottery, the results for which are not included in this paper, it has further been established that the ceramic technology employed in the Peloponnese during the Bronze Age was comparable with that at Thera and Phylakopi, while the technology employed in the Balkans (i.e. Yugoslavia, Hungary, Bulgaria and Romania) during the Early Bronze Age was comparable with that at Servia.

CONCLUSIONS

The results from this study indicate that two fairly distinct ceramic technologies were being employed in the Aegean and surrounding areas during the Bronze Age.

The first, which is associated with pottery from Thera, Phylakopi (Melos) and the Peloponnese, was based on the use of calcareous clays fired in an oxidising atmosphere to temperatures in excess of 800^o C such that initial or extensive vitrification was produced. The second, which is associated with the pottery from Servia (N. Greece) and the Balkans, was based on the use of non-calcareous clays fired in a reducing atmosphere to temperatures below 750^o C such that no vitrification was produced. In the former case, it is possible that kilns were employed whereas in the latter case, since the presence of bloating in the few sherds exhibiting total vitrification suggests a fast heating rate, as well as a reducing atmosphere, it is probable that the firing was carried out in a clamp or bonfire.

In considering the reasons for these different ceramic technologies, one factor is clearly the availability of the different types of clay. However, the results for the pottery from Thera and Phylakopi indicate that non-calcareous clays were available but were only used for making coarse pottery. It therefore seems probable that deliberate selection from among the available clays was being undertaken.

One major advantage of using calcareous clays is that the extensive vitrification structure remains essentially unchanged over a temperature range of about 200^o C (from 850 to 1050^o C), whereas with non-calcareous clays the extent of vitrification increases progressively with increasing temperature. Therefore, if the stronger and more durable pottery associated with an extensive vitrification structure is being produced, the control of the firing temperature needed to achieve a consistent quality is much less critical in the case of calcareous as opposed to non-calcareous clays. Since the non-calcareous clays were normally fired at temperatures below 800^o C such that no vitrification was produced, whereas the calcareous clays were normally fired at temperatures above 800^o C such that initial or extensive vitrification was produced, it is possible that the ancient potters were aware of the different firing characteristics for these two types of clay.

The fact that the majority of the pottery defined as fine-textured on the basis of examination with the optical microscope exhibits the anisotropic vitrification structure is probably merely a reflection of the alignment during the forming of the pottery from specially selected or prepared clays containing few non-plastic inclusions. However, the correlation does serve to confirm the less porous and probably stronger fabric associated with the fine pottery.

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