Investigation of the Technology of Manufacture of Local LBA Theran Pottery: The Body and Pigment Analysis

The sample included all possible types of design occurring in this period, i.e. white, black, and red decorations and coats. It is found that a calcareous clay is consistently used for the body but the amount of Ca varies considerably. The kilns were designed to sustain rather high firing temperatures since in many cases the temperatures exceed 1050 ^oC. For the white decorations and slips the material consistently used is talc. However, the technique of application is different when it is used as decoration from when it is used as whole body coat. The red is made of a fine clay suspension rich in iron and the same applies to the black, except that the black is more vitrified and adheres better to the body. Both black and red also contain increased different amounts of K, thus indicating the possible use of additives (organics, ochres). In the cases where white, red and black coexist, Mn-Fe minerals are used for the black. The firing temperature used together with the reducing atmosphere cycle, when applied, seems to play an important role in the quality of the slip and decoration. The work includes simulation experiments with natural clays in order to reproduce the paint and slip layers.

INTRODUCTION

A few papers have been published in the past dealing with the technology of manufacture of the Theran pottery concerning either firing temperatures and quality of body (Maniatis and Tite 1978), or the nature of pigments used on the surface (Noll et al. 1975a; Noll 1978). Since then a systematic investigation of local Theran pottery has been started (Marthari 1987). The present work is a preliminary study of local Theran wares of LBA destruction level at Akrotiri based on a complete sequence of samples provided by Mrs M. Marthari. The archaeological and typological groups as well as the questions involved about their technology of manufacture have been defined and discussed in the parallel paper in this volume by Marthari (1990). They contain groups of pottery bearing dark and light paint colours, the variations of dark being black, red and brown. All colours occur either in whole body coats or in decorations including also combinations which provide bichrome or polychrome appearance.

The scientific study of the ancient ceramic technology involves examination of freshly fractured surfaces as well as cut and polished sections of body and pigments under a scanning electron microscope (SEM). Simultaneous qualitative (bulk or point) and semi-quantitative chemical analysis of the sections is obtained using an Energy Dispersive X-ray analysis system (EDAX) attached to the SEM. From the micromorphology of the body observed, the degree of vitrification and hence the firing temperature and the quality of clay can be deduced (Tite and Maniatis 1975a, Maniatis and Tite 1978; 1981).

The examination of the micromorphology of the surface finish provides also information on the degree of sintering and vitrification of the paint layer, its particle nature (fine or coarse) and the degree of adherence to the body. The simultaneous microanalysis of the body and paint layers characterizes the nature of the materials used for the different paints, the type of clay and its refractory properties. Furthermore, selective XRD analysis on scratched paint material allows the identification of the mineral phases present, which to a certain extent can also mark the firing temperatures used.

The above combined effort, together with refirings in the laboratory under controlled conditions, and simulation experiments, provides valuable information on the selection of raw materials, their treatment, their behaviour during the firing and the ways of obtaining the various colours and shades. This throws further light on the utilization or not by the ancient potters of the properties of each material used for body and paint and their compatibility.

MATERIALS AND EXPERIMENTAL TECHNIQUES

A large sequence of local Late Bronze Age pottery from Akrotiri was included in this study. The groups examined so far and presented in this work are listed in Table I. The scanning electron microscope used for the examination of the microstructure was a Philips-515 while for the analysis the energy dispersive system EDAX 9900 attached to the SEM was used.

TABLE I. The groups of pottery examined

*For the definition and description of the groups and pots see parallel paper (Marthari 1990).

EXPERIMENTAL RESULTS

     Body:

The elemental compositions for the major elements obtained with the microprobe on cut sections are shown in Table II for all the sherds studied. It can be seen that no large variations occur in the concentration of any elements between the groups or within each group. Only CaO shows differences which can reach an overall 200%, but its absolute value never falls below 9.6%. It is therefore at first inspection obvious that a calcareous clay, with the meaning and nomenclature suggested by Maniatis and Tite 1981, was used throughout. This is in accordance with previous observations on Theran and Minoan pottery (Maniatis and Tite 1978; Noll et al. 1971; 1975a; Noll 1978).

The microstructure examined under the SEM reveals that sintering and vitrification varies from a slightly sintered, non-vitrified body (Fig. 1) to a very vitrified ceramic body (Fig. 2). This observation holds practically for every group studied. Another feature in the micromorphology is the inhomogeneous distribution of Ca, observed with analysis and Ca-mappings, which results in variable vitrification within the body (Fig. 3).

     Dark paints (black, red and brown):

Concerns the sherds of group B3, A6, A7 and C2. The mean compositions of the paint layers for each group are given in Table III. In relation to the body, the dark paints used for decoration and coats have increased Al, K and particularly Fe content, while the amounts of Si and Ca are reduced. The increased amounts of Al, K and Fe could be explained by natural enrichment when a clay is suspended in water and the fine fraction used. However, the exceptionally high amounts of Fe indicate addition of iron oxides. This is verified by the inhomogeneous distribution of Fe in the paint layers observed with the microprobe and by visual examination under a binocular optical microscope of 40x magnification. Black particles of Fe-oxide aggregates can be easily seen in black, red and brown decoration patterns on sherds of group A6, A7 and C2. They are less easily identified, but certainly exist in the coats of group B3 due to the action of polishing. These particles are attracted strongly by a magnet, indicating Fe-oxides in the form of magnetite.

In the sequence of sherds studied there is only one case, sample A7-39, which bears red and black decoration simultaneously (bichrome effect). The analyses of the two paints are also given in Table III. Although the red on this sample has a composition similar to the dark decorations of  other groups, the black differs substantially, containing a high amount of Mn-oxide. The amounts of the other oxides present in this black layer indicate mixture with a clay, perhaps with the finer fraction of the suspension used for the other paints, as evidenced by the relatively high Fe-oxide content.

Apart from this discrepancy, there are no systematic differences in composition between the various groups, which more or less suggests that the same procedure was adopted in preparing the material for use either as a whole body coat or as decorative rative elements. In addition, the fact that no correlation is observed between final colour and chemical composition, indicates that black, red or brown colours result from differentiation in the kiln atmosphere and temperature.

The microstructure of the paint layers observed under the SEM reveals a variable degree of fineness of the prepared material, as well as a very variable degree of sintering and vitrification. In the best cases the coat or decoration layer looks totally vitrified but contains some bloating pores (Fig. 4). In several cases it looks grainy, containing a lot of particles and irregular pores (Fig. 5), and in the worst cases appears coarse and unvitrified (Fig. 6). The existence of bloating and irregular pores even in the finest layers may be understood by the existence of Fe-oxide aggregates, and in the small amounts of CaO present in the paint material.

Table II: Concentration of major elements expressed as % oxides in ceramic body.

Table III: Average concentrations of major elements expressed as % oxides in the coats and decorations

The average of all colours (black, red and brown) are given together except of group A7 where the averages of red and black are given separately.

Table IV:  Average concentration of major elements expressed as % oxides in the talc white decoration and coat

     White paint:

The average chemical composition of the white paint in the groups studied is shown in Table IV. It can be seen that the composition for the white in the decorative motifs is consistent with a magnesium silicate, the most common of which is talc (Mg₃(OH)₂Si₄O₁₀) or its high temperature phase protoenstatite (MgSiO₃). Its presence has been reported before on Cretan and Theran ware (Noll et al. 1971; 1975a). However, the analysis of the surface of the sherds belonging to C1 group, bearing a whole body white-cream coat is substantially different (Table IV), and apart from the increased Mg content closely resembles that of the clay used for the body (Table I). This does not necessarily imply that talc was mixed with the clay before application, but that the analysis includes unavoidably the underlying body since in most cases the talc is worn off the surface to a great extent. Point analysis on isolated talc flakes (Table IV) on these surfaces gives concentrations similar to those obtained from the white decoration layers, indicating that pure talc has been used at least for the decoration motifs. XRD analysis on selective samples proves the existence of protoenstatite on the white decoration material.

The micromorphology of the white paint examined with the SEM shows the flaky nature of talc (Fig. 5). The flakes aligned all parallel to the surface are of the order of 10 μm and show generally a very low degree of sintering which increases on the higher fired samples (Fig. 7). The low degree of sintering is the reason for the poor preservation of the white decoration and coat and also of the red and black paint in the samples of group C2. In these samples the red and black decoration is applied on top of a whole body talc coat and since the talc flakes peel off easily, the decoration is carried away.

The thickness of the white layer when used as decoration is of the order of 10-15 μm, and can be very easily identified (Fig. 5 and 7). However, when applied as a whole body coat it is much thinner. In some cases it is very difficult to distinguish the coat, but a Mg-mapping clearly shows its existence.

Under the stereoscopic optical microscope one can easily see polishing lines on the surface of sherds of groups C1 and C2. However, the polishing does not seem very elaborate since there are unpolished areas even in the best examples. When talc is used as decoration (groups A6, B3) no polishing is applied. In both cases the unpolished areas seem to have experienced some sintering at the very top layer and generally give the impression they have been applied in the form of a thick liquid with a brush. Simulation experiments in the laboratory proved that this liquid could not have been a simple talc-water mixture because it does not stay at the surface and it is easily removed with a simple finger touch. In no case would it survive polishing.

     Firing temperatures:

The firing temperatures were estimated from the degree of vitrification in the body. They are listed in Table V for all the groups studied, together with a short description of the micromorphology of the existing paint layers. As far as the temperatures are concerned they span the range from 700° C to 1080° C. However, the biggest proportion is above 800° C and as much as 30% are fired above 1050° C. This indicates the use of kilns capable of reaching rather high temperature. The fact that in some of the thicker sherds a gradient of temperature is observed (higher near the surface, lower in the interior) indicates that the firing cycle was not particularly long. No differentiation in firing temperature is seen between groups - only the sherds of group C2 tend to exhibit lower temperatures - which suggests that the firing technique was more or less the same for all kinds of ware. This random scattering of the firing temperatures leads to two possible assumptions: a) the control of the firing temperature in the particular kilns was not so precise, and/or b) the kilns were not uniformly heated, producing temperature gradients inside them of the order of 300°C. This is not entirely impossible since there is evidence (Mayes 1961; 1962) that the temperature inside a Romano-British type kiln can vary by as much as 100° C.

DISCUSSION

It has been recognized by several authors (Tite and Maniatis 1975b; Noll 1978), that the existence of calcite (CaCO₃) in the clay matrix gives certain refractory properties to the ceramic body which can be very useful if utilized accordingly. Firstly, the vitrification is controlled in such a way that the amount of glass (amorphous phase) produced by about 850^o C remains essentially unchanged up to 1050° C, above which the vitrification increases again rapidly. This is in contrast to non-calcareous clays where a substantial increase of vitrification is obtained for every 30° C temperature rise above 800° C. It is obvious that large amounts of glass in a ceramic body are to be avoided if the conditions are not absolutely controlled. Secondly, the CaO which is freed from CaCO₃ above 800° C in the calcareous clays reacts with Fe₂O₃ breaking down the Fe₂O₃ grains and thus bleaching the colour. Pale body colours are therefore produced in the calcareous clays ranging from pink to cream and buff, depending on the amounts of CaO, Fe₂O₃ and the firing temperature. This contrasts with the brown or intense red colours obtained in the non-calcareous clays (Maniatis et al. 1981).

As shown in the results af the LBA Theran pattery (Table II) all pottery is made of calcareous clays. However, as Noll (1978) and Maniatis and Tite (1978) point out one is faced with the question of whether the ancient potters intentionally selected a calcareous clay for its useful properties or whether it was the only one available. Extensive studies have now accumulated data on ancient pottery of the Balkans, Aegean and Near East (Maniatis and Tite 1981) as well as Thessaly (Maniatis et al. 1988) which very clearly shows that calcareous clays have been used systematically since Neolithic times for the production of the best quality fine ware, and non-calcareous clays for the coarse and low-fired pottery.

The bodies of LBA Theran pottery studied in this work contain two interesting features which should be discussed in relation to the above comments: a) the amount of CaO concentration varies dramatically especially in the groups A6 and B3 while it is more consistent in group C1 and perhaps C2 (Table I). b) the body is not very fine, containing a lot of large irregularly shaped inclusions of quartz, mica, iron oxide ores and other rock fragments, even in the thinner and finer looking sherds. In addition the distribution of Ca in the body is very inhomogeneous as discussed earlier (Fig. 3), and large voids rich in Ca suggest that calcite might have been originally present as large particles. This evidence leads one to make two possible assumptions: either a mixture of two clays is used (one calcareous and one coarse non-calcareous) or calcite has been added together with other non-plastic inclusions to an originally non-calcareous or slightly calcareous clay in order to improve its properties and achieve a lighter coloured body, on which a dark decoration produces a more aesthetic appearance. There is further evidence from Neutron Activation Analysis (Kilikoglou 1988) supporting the first assumption, but the addition of a large number of inclusions could produce a mixing effect anyway. If the original clay was sufficiently calcareous there would be no reason for a heavy tempering as seems to be the case for the Theran pottery. Therefore according to these results the second assumption seems more appropriate. This agrees also with Einfalt (1978) at least for unpainted wares in daily use.

For the dark monochrome decoration and whole body coat it is obvious that a non-calcareous clay is used (Table III). Very probably a clay suspension was used which enriches naturally in Al and K, since in the upper lighter fraction the fine illitic particles will predominate. To this raw material an amount of iron oxides was added to intensify the colour. Comparing with the body, where large magnetic crystals exist, one can assume that magnetite ores were available and these could have been used in ground form for the paint, rather than Fe-hydroxides (ochres) which is suggested by Noll (1978). The iron-reduction technique was certainly known and used (Noll et al. 1975b) but the variety of colours - black, red, brown - could suggest different kinds of firing. However, the existence of magnetite aggregates in the red colours and the very magnetic brown colours, indicating the presence of maghemite (γ-Fe₂O₃), observed also by Noll (1978), suggests a reducing atmosphere stage even for the most bright red colours. This leads to the conclusion that the target was to achieve a black colour, and the brown and red can be considered as failures of the reducing atmosphere and/or temperature to a greater or lesser extent. This is further emphasized by Table VI where the statistical distribution of the paint colours among the various firing-temperature ranges is laid out. It can be seen that all the black colours are produced by firing at higher temperatures while the reds are fired at lower ones, the browns representing the intermediate cases. This table reinforces the argument for a 'black target' if one considers the fact that for producing a black colour on a light body two factors have to be right: a) a high temperature for extensive vitrification and b) fully reducing conditions. If these are achieved the reduced iron oxides (black in colour), which give the colour to the paint, remain black during re-oxidation when only the body is re-oxidized.

The nature of the paint material is also responsible for the many failures. It is, in most cases, coarse and grainy (see Table V) and seems to have been made without great care. The variation also in its texture suggests that the potters or artists did not mind if the brush reached the thicker and coarser part of a suspension. It should be borne in mind that a coarser paint could vitrify less easily and would re-oxidize much more easily. In view of the above remarks it is easy to understand the large percentage of sherds fired at rather high temperatures (Table V and VI) which the potters aimed at and succeeded in achieving. However, at these high temperatures it is not an easy task to maintain fully reducing conditions. If the potters had been more persistent and systematic in producing a much finer suspension, they could have a totally vitrified paint layer in reducing conditions at much lower temperatures (850-900° C) as is the case with the Attic pottery (Farnsworth and Wisely 1958; Tite et al. 1982).

The whole body coat made of a finer clay fraction polished and vitrified to a greater or lesser degree produces a non-permeable layer on an otherwise porous body. The fact that this occurs mostly on cups used for liquids (B3 group, Table V) indicates a purpose for its application other than purely aesthetic. However, vessels of the same type, but without a whole body coat, do exist in Akrotiri (Marthari 1987). Therefore, covering the whole body with a less porous and lustrous material on a number of such vessels could suggest an improvement in their suitability for liquids.

For bichrome effect (black-red) the Theran potters seem to have adopted the Mn-black technique where Mn-oxide ores are used for the black decoration; these retain the black colour in the oxidizing atmosphere thus avoiding the oxidizing-reducing-oxidizing cycle. The Mn-black technique is known from Neolithic times in Anatolia (Noll 1982). Mn in an average concentration of 17.4% (Table III) was detected in only one sample (A7-39) of the sequence studied in this paper, and this is associated with high Fe and Si, Al, K, Ca etc. suggesting a mixture of Mn-ores with a clay base. This mixing bas also been observed in other cases of black decoration (Noll et al. 1975b) perhaps in order to provide a better bonding with the ceramic body. It is at the moment premature to discuss the technology involved in producing this Mn-Fe-rich black layer since we believe its production may not be as straightforward as a single oxidizing cycle. This will be further elucidated by the simulation experiments which are under way.

Turning now to the white paint decoration and whole body coat, several aspects about its technology should be discussed. It is clear that the raw material used was talc (Mg₃(OH)₂Si₄O₁₀). As discussed earlier the body was polished after the application of talc as a coat but the polishing was not so elaborate. Talc is a very difficult material to use since it sinters at very high temperatures and would not stay easily on the surface. From Table V one can see that extensive sintering among the talc flakes is achieved mostly in the sherds fired above 1050° C, and this is really not enough to form a well-bonded material adhered to the body. An organic material could have been used as a means to apply talc on the surface of the vase, but even in this case a lot of material would have been removed during polishing and this is perhaps the reason why the polishing was not so elaborate. At first stage it is very difficult to understand the reason why the potters needed to apply a whole body white (or rather cream-grey) coat on a very light coloured body. A possible reason could be that talc helps in polishing the surface and produces a finish with a very smooth and soapy feel. One should bear in mind that the clay used for the body is medium-coarse and simple polishing would not produce such a smooth surface. Reduction in porosity or forming a base for darker coloured decorations should be excluded as reasons, the former because talc layers are very absorbent and porous and the latter because group C1 bears no decoration at all. Coating and polishing with talc would produce a rather nice effect which however would not last long an extensive handling.

Summarizing, one can say that the kind of materials used for the dark-coloured decoration and coat as well as the white decoration and coat would require firing temperatures as high as possible to achieve a good quality and durability, and therefore pottery kilns capable of reaching a maximum temperature of 1080° C, as evidenced by the estimated firing temperatures (Table V), were built. However, the materials used were of such nature that even near this temperature only a limited quality and durability was achieved. Because of this high temperature target, accompanied by reducing conditions, the available clay, which must have been prone to deforming and cracking, was heavily gritted by non-plastic inclusions and calcite.

CONCLUSIONS

The results of this investigation, although still in a preliminary stage, provide interesting information about the level of ceramic technology during the LBA period on Thera.

1. The original available clay was most probably unsuitable for firing at high temperatures and reducing conditions and was therefore tempered with calcite, quartz, magnetite ores and various other rock fragments. Calcite also had an effect in brightening up the colour to a cream-buff body.
2. The monochrome dark decoration was aimed to be always black and the existing red and brown colours almost certainly represent failures.
3. The whole body dark coat, also aimed to be black, was applied mainly on cups perhaps in order to reduce the high porosity of the clay so that these vessels could be used for liquids, and this would represent an improvement to their suitability compared with other uncoated vessels of the same type. These whole body coated wares represent the Theran imitation of the Kamares Cretan ware but their quality is inferior.
4. For either dark decoration or coat a non-calcareous finer clay was used, perhaps produced through a suspension to which iron oxides were added to intensify the colour.
5. The material prepared for the above decoration and coat was not of a very fine nature and therefore it required very high firing temperatures in order to obtain total vitrification which would not allow re-oxidation during cooling.
6. The iron-reduction technique was therefore used with limited success to produce a black decoration on a pale body or a full-coated black body with low permeability.
7. The black decoration on bichrome vessels (black-red) was achieved by enriching the paint material used for the monochrome application with Mn-oxide ores. The firing conditions have not yet been clarified.
8. For the white decoration, talc is always used in a thick liquid form (organic binder?). This material sinters at very high temperatures and makes no firm adhesion to the body or the underlying paint layer, easily peeling off even on pots fired above 1000^o C.
9. Talc is also used as a whole body coat; in this case the surface is polished. The polishing is not very elaborate due to the fear of removing the coat. A nice smooth surface with a soapy feel is produced which was perhaps the only reason for applying this coat on a rather medium-coarse clay body. However, preservation is very poor and the dark decoration applied on top of the white coat in the group C2 wares has practically vanished.
10. In order to improve the quality of the paint materials used for paint they had to fire them at rather high firing temperatures and for this they constructed kilns capable of reaching 1080^o C. The combination of reaching such a temperature and at the same time creating a reducing atmosphere, necessary to produce the dark colour for the decoration, was not a trivial task, as witnessed by many failures. However, the successes in reaching a high temperature were over 50% among the sherds of the sequence studied, though this figure falls to 20% when the successes in the reducing atmosphere are counted. Nevertheless, the products, black red or brown were all attractive, and established the aesthetic standards of the period.

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