The use of Dynamic Evolved Gas Analysis (DEGA) to resolve ceramic defects

Two out of four samples of porous bodies from Tunisian clays fired up to 1070 °C under a fast 45 min cycle showed “black core” defects, while the remaining two samples had craters and gas bubbles in the glaze layer. The “black core” is caused by incomplete oxidation of organic matter, while glaze defects are mainly caused by the late release of gas from decarbonation reactions when the glaze is melted. Dynamic Evolved Gas Analysis (DEGA) has been used to avoid these defects. DEGA shows that CO related with organic substances escapes at low temperature, while NO and NO2 require further heating. SO2 and CO2 escape at relatively higher temperatures and are respectively associated to sulphides and/or sulphates, and carbonates. An increase of the firing cycle from 45 to 60 min is therefore suggested and the use of two low-rate heating stages: 1) 11 min from 520 °C to 590 °C to avoid “black core” defects; and 2) 8 min from 800 °C to 900 °C in the decarbonation zone, thereby eliminating glaze craters and bubbles. In addition, enamel fusion was delayed by increasing the proportion of refractory components in the glaze.