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Development of engobe compositions for local roof tiles
Engobes can be described as a coating layer on ceramic surfaces to produce the desired color. In this thesis a commercial roof tile, which was taken from a company, was used. For good fitting between the engobe and the roof tile their coefficients of thermal expansion (CTE) should be almost the same. One way of adjusting the CTE values, the chemical analysis of the roof tile was analyzed and based on these different engobe recipes with high purity raw materials and frit that was made in the laboratory with high purity raw material were prepared. The roof tiles were rich in SiO2, Al2O3, (Na+K)2O. Therefore, by changing the percentage of these oxides new engobe recipes were prepared to find the most suitable composition for roof tiles. After application on surfaces of roof tiles, the samples were fired at 1000oC for 75 min. 70% SiO2, 20% Al2O3, 10% (Na+K)2O with 10% laboratory frit and 70% SiO2, 20% Al2O3, 10% (Na+K)2O with 30% laboratory frit were found to be two of the most proper engobe compositions for the roof tile. Later these engobe recipes were prepared with lower purity industrial grade raw materials and industrial frit because the cost of producing frit is high for a roof tile manufacturer. The low purity engobes were applied successfully on the surface. However, lower purity raw materials containing little Fe and Ti led to a slightly darker color compared to higher purity engobes. The life cycle of the products were tested with different kinds of acids and temperature tests. And the results showed that the engobe samples were durable against the environmental effects. Finally, metal oxides were used up to 5% in the engobe compositions to give different colors to the roof tiles.