Fire assay by cupellation is an early form of quantitative chemical analysis that required the use of specialized vessels typically made with ashes. Its use became particularly prominent in Europe since the Middle Ages, as reflected in technical treatises and archaeological remains that record a variety of recipes for their manufacture and use. Here we present chemical and microstructural analyses of cupels from the Porto Mint in Portugal, recovered from two distinct contexts spanning from the 14th to the 16th century CE. Based on the data, we argue that one set of cupels was likely used for the analysis of bulk metals as they entered the mint, while the other was used for the assay of bullion coins for quality control. We also show that most cupels were made of a mixture of bone ash and wood ash. In addition, our work illustrates strategies to identify and study severely corroded cupels. We recommend further studies of cupellation contexts, which will allow us to map knowledge transfer and technological traditions in this crucial strand of early science.