One hundred nine majolica sherds from excavations at the Mexico City Cathedral and from Metro construction in Mexico City have been analyzed by direct current plasma-optical emission spectrography. Many of the sherds have been studied petrographically and by x-ray diffraction. Concentrations of calcium (as calcium oxide) of as much as 24% in both imported and locally produced pottery and the presence of calcium silicates diopside and gehlenite, which were produced during firing, were found. This finding suggests that a calcareous clay was involved in majolica production both in Spain and in Mexico. Modern Mexican majolica is produced using two kinds of clay, one calcareous, one of which are mixed together. Firing studies of these clays are reported. For other majolica studies, see AATA 12-1125, 14-377, and 16-751; for complete reference to this text, see 21-1449. -- AATA