Elsevier

Applied Clay Science

Volume 17, Issues 5–6, November 2000, Pages 279-292
Applied Clay Science

Low temperature sintering of a pottery clay from Burkina Faso

https://doi.org/10.1016/S0169-1317(00)00020-XGet rights and content

Abstract

The sintering at 1000°C of a clay raw material for pottery mixed with calcite was studied to increase the mechanical characteristics of the fired product. The kaolinitic clay used comes from a large pottery production area in Burkina Faso. The experiments of this study were conducted as similar as possible to the traditional practices of potters to ensure a future technological transfer of results. An increase of the flexural strength from 7 to 17 mPa was obtained by adding 15% of calcite. To explain the mechanisms involved, the sintering behaviour was initially observed by dilatometry. Results indicated that at 1000°C, the maximum material densification and properties were obtained after about 15 min of dwell time. Therefore, a quantitative study of crystalline phases nucleated during this period was realised by X-ray diffraction methods. It revealed that anorthite is the most important phase formed during the firing time. Therefore, the nucleation mechanism, mostly involving quartz, but also gehlenite consumption was discussed. At the low firing temperature, it appears that the firing process is characterised by the absence of a liquid phase, drastically limiting the diffusion effect. As a consequence, the sample microstructure, as observed by SEM, shows a network of small dense zones, including quartz grains, interconnected by recrystallized porous phases. The comparison of material containing the natural kaolinitic clay to material obtained from pure reference minerals underlined the important role of iron impurities in anorthite formation.

Introduction

Clay raw materials for ceramic uses have been extensively studied, by e.g. Jouenne (1979), Alliprandi (1979) and Sigg (1995). With respect to raw materials for traditional ceramic applications in west African countries, only a few of studies have been published (Kabré et al. 1998). This is related to a certain extent to the present situation of the ceramic sector mainly producing pottery and small quantities of bricks and roof tiles. Although these products are really necessary for the construction industry and for everyday life, they are not commonly available. Furthermore, heavy clay products cannot be imported due to transportation and production costs with the exception of cement. In any case, in Burkina Faso, 90% of the population cannot afford to buy manufactured clay products. In this country, a limited production of adobe, which is clay mixed with 5–9% cement and dried in the sun, is used as building material. Moreover, the most important ceramic production comes from local potters who apply very traditional techniques to excavate essentially local clays, to process hollow wares by hand and to fire them in traditional wood heated kilns. In general, the quality of the fired pottery or unfired adobe is often poor, in particular mechanical strength is not sufficient.

In this study, we are interested in an important pottery production area named Poa. It is situated between Ouagadougou and Koudougou (Fig. 1). For many years, about 10 000 potters have been working there during summer (mainly women for whom it is an essential activity). The clay raw materials used in this area are mined by hand in open quarries. The geological details about these raw materials have been reported by Kabré et al. (1998) and Kaloga (1987). The clay raw material deposit belongs to an alluvial plain (with an old stream) over a distance of about 10 km. The clay layers are 3.5- to 5-m deep.

The aim of this work is to contribute to the understanding and improvement of the mechanical properties of fired pottery products, using calcite as an additive, taking into account the traditional practices of potters. Particularly, laboratory methods similar to those used by potters for paste mixing and low firing temperature at 1000°C were used. Calcite is a raw material commonly available in some areas in Burkina Faso.

Section snippets

Materials and methods

The P1 clay raw material is a kaolinitic clay. Its qualitative structural characteristics are presented in Fig. 2, and the chemical composition is shown in Table 1. P1 presents the typical X-ray pattern of kaolinite mixed with quartz. Other minor minerals, which could be associated, have not been really detected. From the chemical composition, the semi-quantitative mineralogical composition could be derived and is presented in Table 2, indicating the presence of mainly kaolinite and quartz

Results

In Table 4, the flexural strengths are presented as a function of calcite addition in the raw clay. As can be seen, values were highly increased (which was the primary objective of this study).

In Fig. 5, the dilatometric curves of the three compositions as a function of time are given. When the sintering temperature is reached and maintained at 1000°C, a progressive densification for 15 min and a subsequent volume stabilisation (even after long dwell times) was observed. Finally, P2 is more

Discussion

As it is indicated in Table 4, the strength of the fired material has increased substantially. Particularly, the material strength values are the same for dwell times of 0.1 and 0.3 h. The effect of longer times have not been investigated since they are not applied in traditional firing. The calcite addition in P1 clay leads to a slight additional densification (Fig. 5), while the sample open porosity is reduced from 35 to 34 vol.%. It must be emphasised that the apparent densities of P1 and P2

Conclusion

In this study, the strength of the material appears to be related to the type and quantity of crystalline phases rather than to a decrease of porosity. The microstructure of samples shows Ca-rich regions in which an anorthite phase predominates. These regions form a connecting network between more highly densified zones where larger quartz grains remain. In this case, a particular correlation between Fe concentrations and Ca-rich regions could not be found. Furthermore, the use of reference

Acknowledgements

The authors wish to acknowledge the help of J.P. Laval of the Limoges University in contributing to successful X-ray measurements.

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