Investigations of the Black Core and Swelling in Firing Porcelain Stoneware

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One of the reasons for the comparatively low quality of dense ceramic materials (porcelain tiles, facade tiles, and floor tiles) resulting from the appearance of a black core is the presence in them of unburned carbonaceous particles remaining in the articles after firing because of poorly selected compositions and heat-treatment regime. Studies have shown that the highest weight content of C (carbon) with respect to other layers of porcelain stoneware is found in the black porous core (9.68%). The appearance of a black core is explained by the fact that in the heating process organic impurities are charred primarily in the inner layers of the porcelain tile which are isolated from the air oxygen. Moreover, the formation of the black core will also be promoted by sintering with the participation of a liquid phase, which will isolate from the air the not yet fully oxidized carbonaceous material.