This week, we had two objectives: to present our work at the first annual JHU Undergraduate Research Day, and to analyze the results of our firing. At URD, everyone in the class took turns manning our class’s poster and answering questions about our project. It was exciting that so many people were interested in our work! In class, we returned to the museum after several weeks away at Clayworks to inspect our fired cups and tiles. Some pieces showed the signature red and black of Athenian pottery we were aiming for, while others did not turn black where they were supposed to, and others turned black almost everywhere. We observed a variety of defects, including scorching, warping, overfiring, underfiring, cracks, and discolorations from debris. These problems are reflective of the production process—from its very earliest stages all the way up to the firing process—and we have a lot to learn from each pot!
A major factor in the variation in color of our pots was the approximately 250°C temperature difference between the top and bottom shelves of the kiln. Our kiln had a total of four shelves that stacked one on top of another. In order to track the temperatures at different levels in the kiln, we had placed pyrometric cones at the lowest shelf (the one just above the firebox) and also the third shelf up from the firebox, in addition to a digital pyrometer at the very top of the kiln’s firing chamber. Pyrometric cones are small, pyramid-shaped clay objects that bend when subjected to a certain amount of heat. Matt and Cami created two “cone packs” for our kiln shelves by grouping cones designed to melt at 896°C, 929°C, 949°C and 991°C. Based on our measurements, the pyrometer at the top of the kiln recorded approximately 950°C, the cone pack on the next shelf down suggest we got to 991°C, and the fully melted conepack on the lowest shelf in the kiln suggests that we got close to 1200°C! The result of these temperature differences was obvious. The slip on the pots on the top shelf was barely vitrified, whereas the slipped areas on the pots near the bottom of the kiln which was the hottest, turned black but were likely overfired.