ABSTRACTTwo bodies differing in composition by a small percentage only of Li2O were found to show marked differences in physical properties, particularly in thermal expansion Petrographic examinations were made of the electric‐furnace products used as the nonplastics and of the fired porcelain to determine what structural or mineralogical differences might be responsible for these variations. The properties of a feldspar body, tested under similar conditions, are given for comparative purpose