The Effect of Microbial Contamination upon the Plasticity of Ceramic Clays

John Henrikson, Ted Fleming*, Bradley University

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Abstract: Relatively little evaluation has been conducted on the effects of individual microbes present in ceramic clays. In this study, bacteria present in dry ingredients and also aged clay were surveyed and identified as to the genus level using a combination of culture and microscopic techniques. Isolates of predominant bacteria were individually evaluated by reintroduction into sterile dry ingredients and aging of the wet clays. Subsequently clay samples were subjectively assessed for wet pliability and quantitatively tested for wet plasticity and shear strength after firing. Following a 10-day aging period, experimental clays were found to be 10 percent more plastic than control clays (sterile) and demonstrated 1.5 -13.2 percent lower cured shear strength. p>

Introduction There is anecdotal evidence that the aging of clay improves its plasticity: most artisans who work with the material constantly notice changes in the workability or physiochemical properties. In addition, previous studies have ascertained the improvement in plasticity due to microbial treatment of clay (Naga et. al., 1998). There are many methods used to age clays, ranging from ancient storage in underground pits to modern techniques involving storage in Igloo® coolers; all methods provide time for clays to somehow become more workable. Different theories among ceramic artists related to the aging process include changes to the clay ions, changes in moisture and temperature, or microbial products that may help the clay particles bind more closely together. Previous studies evaluated mixed or indigenous populations of microbes in native clays having varying physiochemical properties (Gaidzinski et al., 2009). This study evaluated the effect of growth of individual microbes under controlled conditions upon specific properties of aged clays. It was hypothesized that specific microbes are predominantly responsible for increased clay plasticity and that there is a positive correlation between the presence of those microbes in aged clay and the working plasticity of that clay. Aged clays were evaluated for plasticity and fired shear strength relative to sterile controls.

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