Fired and Fractured: An Analysis of Broken Ceramics

How does the construction of a ceramic object affect how it breaks? How does the force applied affect this break? These questions are based on my first-hand observations of the multitude of ceramic fragments found in different states of decay at archaeological sites. To answer these questions, I created samples that imitate ancient ceramics and then broke them in different ways. I then analyzed the fracture surfaces and looked for evidence of weak points in the objects. Pieces that were hit by a blunt force, such as a hammer, had radial fractures that formed along paths that had clusters of inclusions, especially shell inclusions. Seashells, which are primarily made of Calcium Carbonate (CaCO3), have a low tensile strength compared to the clay matrix that makes up most of the ceramic. The breaks nucleate from the weakest part, being these inclusions. Samples that were dropped broke in a linear pattern with a main break running through the sample and secondary breaks running perpendicular to the main break. These breaks are caused by compression fractures as the sample hits the ground, and both the force of the ground and gravity push the objects in opposite directions. Another outcome came from the decomposition of the seashells that turned into powder due to being heated over 825°C. This led to the tiles flaking and breaking down due to the structural integrity being compromised by the decayed seashells. Moisture also can influence the break. Samples that were soaked in water had cleaner breaks as the clay could absorb more of the impact before breaking, while the samples that were dry had cracks spreading out from the inclusions that could lead to more breaks.