2012 Theses Master's

The History of Terra-Cotta Glaze-Fit Testing and Artificial Weathering Methodologies and a Comparative Testing Program of Their Impact on Glaze Failure

Rosenblatt, Sarah

Terra cotta failure is often due to glaze defects, including crazing, crawling, and spall. The makeup of both the clay body and the glaze, and manufacturing processes, are of paramount importance in the prevention of these defects. As part of the fabrication process, ceramicists often put terra-cotta wares through a variety of tests to aid in the prediction of a product's durability. Artificial weathering is a valuable technique for gaining such information. Today there are common procedures in place published by the American Society for Testing and Materials (ASTM) and Building Research Establishment (BRE) to mimic the freeze-thaw cycles, moisture, and presence of salts that are typical in real-world conditions. When terra cotta was still a relatively new product in the early 20th century, however, these procedures were not standardized, and neither were techniques for terra cotta production. This thesis explores historic tests that were utilized to improve the longevity of terra cotta as an architectural material. I assembled a chronology, and examined trends seen in testing, focusing on tests that either studied glaze fit or utilized artificial weathering. The unconventionality of some of these tests, when considered in the context of modern procedures, led me to wonder how effective they might have been. I also wondered if there could have been useful information that had been lost on today's industry over the near century since the height of terra cotta's use. Since there was not a clear connection between many of the historic and contemporary tests, it seemed likely that they could produce relatively diverse results and provide different types of information for ceramicists and conservators. In order to better understand and ultimately evaluate these weathering techniques, I put samples from various New York City buildings through both historic and contemporary methods of artificial weathering. These tests were then evaluated for their ability to increase the possibility of glaze failure by performing surface adhesion pull-tests. Although the results of these final tests were numerically scattered, they still provided useful insight into the value of the weathering tests that had been reproduced.