GEOLOGICAL AND GEOCHEMICAL VARIABLES THAT AFFECT THE QUALITY OF CLAYS USED BY CERAMIC ARTISTS AND POTTERS

Most ceramic artists use commercially produced clay bodies as they are manufactured to provide consistency with respect to working properties (e.g., plasticity, strength, shrinkage and texture) and firing characteristics (e.g., cracking etc.). However, some potters desire a more “localized” flavor as far as their ceramics are concerned. Locally-sourced clay that is atmospherically fired in wood-burning anagama-style kilns produces unique coloration and patterning that is highly desirable in the art world The Alabama Gulf Coast is enriched with modern clay-dominated depositional environments and clay-bearing Cenozoic geological units, and we have been investigating the ceramic potential of some of these materials. Our study sediment includes: 1) modern organic-rich clay from a tidal marsh on Dauphin Island, 2) modern estuarine clayey-silt from Weeks Bay, 3) modern-Holocene? clayey-silt (dredge material) from Gaillard Island in Mobile Bay, 4) a variety of clays and silty-clays from the Pliocene Citronelle Formation sampled from across southern Alabama, 5) calcareous clay from the Oligocene Bucatunna Clay Member of the Byram Formation and 6) clay-rich sediment from other undifferentiated Miocene units, all within 160 km of Mobile, Alabama. TOC, carbonate content, and grain size variation were determined for each target material in our study. Coarse-grained contaminants in clay bodies can negatively impact pottery quality. Consequently, we sieved a portion of each target material to remove sediment coarser than 125 µm. Clay bodies were made from refined fractions and raw fractions for each of the target materials and from them, we produced a series of test plates, bowls and cylinders. In addition to assessing mechanical behavior during the working/throwing stages, we assessed percentage shrinkage of the clay bodies as they passed from “leather hard” through “bone dry” stages and after bisque and high firing in kilns. Organic-rich clays were the hardest to work with when wet and also shrunk and cracked the most during the drying stage. The quality of the other target materials was variable.