JOURNAL OF MATERIALS SCIENCE LETTERS 18 (1 9 9 9 ) 1653 – 1655 Improving the green and fired fracture strength of a kaolinte ceramic using some vegetable syrup B. O. ADUDA, F. W. NYONGESA, G. OBADO Department of Physics, University of Nairobi, P.O. Box 30197, Nairobi, Kenya E-mail: physics@healthnet.org.ken Potters (in certain parts of Kenya’s Lake Victoria re- gion) have noted that when they use a vegetable (known locally as apoth, but whose botanic name is Corchirus olitorius) syrup extract and water to plasticize the clays, apparently stronger green and fired wares are obtained. The syrup is simply extracted by boiling the fresh green leaves of the plant in water for a short period of time un- til the water becomes viscous and slimy. This study aims at quantifying and elucidating the apparent increase in strength. This phenomenon, if properly understood, may help in the development of a cheap, stronger and long lasting ceramic stove lining for the energy saving and efficient stoves for rural and urban area households in many developing countries where wood/charcoal is the main source of energy [1]. The ceramic lining is daily subjected to cyclic thermal stresses and, at times large thermal shocks during the heat-up and cool-down in the fairly predictable cooking schedules, resulting in rapid strength degradation and consequent reduction of the life span. A high fracture strength and hence ther- mal shock resistant lining would entail a longer life span and wider acceptance and usage of the ceramic stove than currently hitherto. The ramification of this will be an improvement on the economical use of the biomass based fuels and consequently less deforesta- tion and environmental degradation. In the present study kaolin, a widely available and well characterized clay (supplied by the Athi River Mining Company, Athi River) was used. The plant syr- up was extracted by boiling a known mass (500 g) of the edible vegetable plant (Corchirous olitorius) leaves, purchased from local markets, in a half liter of water for 10 min. The resulting syrup was dark green in col- or, highly viscous and slimy- or mucous-like, which upon cooling to room temperature was filtered, bottled and stored in a domestic fridge before use. This process, each time with fresh leaves, was repeated until an ad- equate amount of the syrup for specimens’ fabrication was obtained. Four different sets of clay were prepared. The first set was plasticized with a known volume (V 1 ) of water only (0% syrup concentration) while the sec- ond set of clay was plasticized with 0.5V 1 water and 0.5V 1 syrup (50% syrup concentration). The third set of clay was plasticized with 0.25V 1 water and 0.75V 1 syrup (75% syrup concentration) and the fourth set of clay was plasticized with V 1 syrup (100% syrup concen- tration). The test specimens will henceforth be referred to by their syrup concentration given in parentheses. Before forming the test specimens, the different sets of clay were left to age overnight, after which specimens, 1.5 cm in diameter and 15.0 cm length, were extruded using a mold fabricated in our laboratory. The extruded samples were left to dry at room temperature for 7 days at the end of which they were further oven-dried for 24 h at a temperature of 110 ◦ C and finally stored in moisture-free chambers. All the specimens for fired- strength measurements were sintered at 1150 ◦ C. The bulk density of the samples with different syrup concentrations, both when green and fired, were deter- mined from the dimensions and mass measurements to an accuracy of 0.001 g cm −3 . The strength of the green and fired samples were measured in the three point bend test mode using a universal tensometer at cross head speed of 3 mm min −1 . The strength values reported here are an average (mean) of at least five similarly prepared specimens. The fractured surfaces were observed in a JOEL scanning electron microscope (SEM). In addition to the above tests, chemical analysis was also performed on the as-received kaolin, green and fired samples. Table I shows the chemical analysis results for the kaolin, the green, and the fired samples. It is noted that the amounts of the various oxides (except Si 2 O) show a small increase in both the green and fired states at 0% concentration relative to those of pure kaolin. This trend of increasing percentage of the oxides may be attributed to the ordinary tap water used in fabricating the specimens. It is noteworthy that with increasing syrup concentration in the clay (kaolin) the amounts of the various oxides in both the green and fired samples, except those of MgO and CaO which show a consistent decreasing (downward) trend, remained fairly constant. The variation of the bulk density of the fired sam- ples with the syrup concentration is shown in Fig. 1. It is observed that the bulk density increases, reaches a maximum about 50% syrup concentration, and there- after decreases slightly. This decrease in bulk density could be due to the increased amount of the volatilized organic matter since the higher concentrations imply more organic content in the initial green sample. Fig. 2 shows the variation of strength with concentra- tion of the syrup, for both the green and the fired sam- ples. It is observed that the green strength increases nearly linearly with increase in the syrup concentra- tion. Between 0% and 100% concentration the strength changes from 17.25 to 22.32 MPa, an increase of about 29%, and for the fired samples the change is nonlinear and has increased from 29.94 to 72.17 MPa, an increase of nearly 141%. At zero syrup concentration, the effect 0261–8028 C  1999 Kluwer Academic Publishers 1653