One of the important assumptions of compositional analysis is that the elemental composition of an artifact reflects the source of the materials used to make it. Thus, pottery from a particular source will be chemically similar to the raw materials from that source. This “commonsense” assumption seems beyond dispute, but the fact that pottery is a mixture of clay, water, and often temper added by the potter, complicates the interpretation of compositional data from ceramics. This article examines the relationship between potters' behavior in obtaining and using raw materials, on the one hand, and the chemical composition of their finished pottery, on the other, by comparing the elemental composition of ethnographic pottery and raw materials from contemporary pottery-making communities in the Valley of Guatemala. The results of this research show that the relationship between pottery and its constituent raw materials is not as obvious as was first supposed. The article concludes with an alternative approach to compositional analysis that is more in line with the realities of real-world pottery production.