Tempered strength: A controlled experiment assessing opportunity costs of adding temper to clay

Abstract

The addition of pottery additives (temper) provides both production-based benefits gained during the initial vessel formation phase, and performance-based benefits associated with post-firing vessel daily use. This paper presents the results of a controlled archaeological experiment designed to assess the opportunity costs associated with the addition of temper to clay during prehistoric pottery production sequences. Specifically, this study builds upon earlier research using material science methods to more broadly assess whether vessel strength is sacrificed by the addition of temper into the clay body. Standardized experimental ceramic test specimens, based directly upon petrographic analysis of archaeological samples from a regional context (South Central Ohio, USA) and produced using glacially-deposited illite-based clay, were subjected to mechanical strength tests using an Instron Series IX universal testing machine. The results demonstrate that there are indeed opportunity costs associated with temper addition: lost potential strength and reduced vessel use-life. Overall, untempered samples were significantly stronger than samples tempered with the most commonly used regional tempers—grit, limestone, and burnt shell—in terms of peak load and modulus of rupture. In other words, the results presented here suggest that prehistoric potters were losing the opportunity to create significantly stronger vessels in favor of the benefits that come with the addition of temper. Understanding of the existence, kind, and degree of opportunity costs that come with the addition of temper to clay emphasizes just how important the benefits of tempering must have been for the technology to be invented, experimented with, and ultimately so widely adopted.