Lime-Barium and Lime-Zinc Raw Glazes with Raw Materials from Tanzania and Cameroon
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Abstract
In the paper, the lime-barium and lime-zinc glazes used in Southern and Eastern Africa Mineral Center (SEAMIC) ceramic unit in Tanzania were investigated for the production of high temperature raw glazes in Cameroon. The base Seger formula used was 0.3KNaO, 0.3CaO, 0.4ZnO or BaO; xAl<sub>2</sub>O<sub>3</sub> and ySiO<sub>2</sub>. Different Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> formula ratios for glossy and matt glazes were tested with raw materials from Cameroon and Tanzania. In the Cameroonian glazes, a local pegmatite (quartz, microcline, albite) with low coloured oxides replaced an albitic feldspar in the Tanzanian glazes. The other materials were kaolin, limestone, silica and barium carbonate or zinc oxide. The glazes were tested at Seger Cone 7 (1240℃) on stoneware slabs (made up of the same feldspathic glaze material: 18%, silica sand or quartz 27%, ball clay 40% and kaolin 15%) from both Tanzania and Cameroon materials. From results obtained, zinc oxide acted as a more vigorous flux than barium carbonate at cone 7, which is obvious in the glazes with pegmatite (with high quartz content). At a Seger formula of Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub>1:10, transparent glazes were obtained. In these glazes the alumina content was lower and when increased to higher Seger ratios 1:4.7 - 8.7, the gloss was lowered to matt-luster due to fine crystal formation.
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