Shell as calcium carbonate decomposes when heated above about 800°C (depending on rate, duration and atmosphere of firing). If the shell remains as CaO when it cools, hydration results in spalling and subsequent destruction of the vessel. If firing was not controlled to prevent overfiring, the addition of shell to the pottery would have been a risky endeavor not likely to have selective value. Once firing became better controlled, shell became a viable alternative and its technical improvements for pottery more accessible. To test this hypothesis I attempted to measure the degree of firing from sherds collected in Southeast Missouri. Here shell replaced quartz sand as the predominant temper. The objective was to see whether the sand tempered pottery was fired, at least some of the time, above the threshold where lime spalling becomes a problem.