The present study investigates the suitability and restrictions of the unreacted shrinking core model to describe the combustion of a fixed organic substance in a clay matrix, as found in the firing process of clay bricks. The model was applied to the experimental data of isothermal measurements and validated by the model prediction of nonisothermal experiments, which were examined by combustion of mixed clay containing several organic additives in a fluidized bed combustor. Besides reactivity measurements, the shrinking core proceeding was also indicated by apparent separated phases within partially reacted clay specimens. The reaction site moves from the surface toward the inner site dividing a reacting solid into two distinct zones with different organic carbon concentrations, which was confirmed by separated analyses. Mass transfer and reactivity parameters of the model were found to be both exponentially related to temperature (Arrhenius law). The apparent chemical activation energies were determined to be 39.8-59.3 kJ/mol and, therefore, relatively small compared to literature data derived from the experimental procedure with low heating rates, but similar to values of experimental data associated with fast warming rates similar to the present investigation. The determined chemical pre-exponential factors seemed to reflect the specific surface of the organic substance incorporated in the clay matrix. The porosity of the clay samples was investigated by means of mercury intrusion porosimetry and found to be dependent on the amount of added organic substance. The porosity of the pure clay sample was 36.4%, while moderately higher values were found for mixed samples (41.0-44.2%). The effective diffusion coefficients appeared to be directly related to the portion of the total porosity generated by the organic additives. The diffusion coefficients found were around 0.002-0.02 cm(2)/s for a temperature range of 350-650 degrees C, which is in the same order of magnitude as reported in comparable studies. The presented model provides an important item for the simulation of the firing process of ceramic goods in oxidizing atmospheres and explains mass transfer mechanisms evolving inside ceramic material blended with organic substances under heat exposure.