Low temperature sintering of a pottery clay from Burkina Faso
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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