Modification of clay properties by aging: Role of indigenous microbiota and implications for ceramic processing

Storing clays for a period of time before their use in ceramic processing is recognized to improve their technological properties as compared with the corresponding freshly-mined materials. Although the mechanisms underlying this ‘aging’ process are not well understood, the improvements in quality have often been attributed to biological factors. The paper investigates the role of indigenous microorganisms and their activity in clay aging. Two clays from Brazil were sterilized by autoclaving and gamma-ray irradiation, and their chemical, physical, and technological properties were measured after storing for 6 months. For comparison, the same measurements were carried out on non-sterilized samples that were subjected to the same conditions. The clays did not present the same response to sterilization, since aging was beneficial for the ceramic properties of one clay and detrimental to the other. A comparison of data from sterilized and non-sterilized aged samples showed that the changes in chemical properties could not be directly related to the presence of microorganisms, since the clays responded to aging independently of sterilization. Further, the changes in the technological properties of the clays with aging were probably related to their initial physical properties and not to the presence of microorganisms.