In ceramic chemistry oxide compounds (minerals in geology) are classified as basic (RO, R2O group) or acidic (RO2). The greater the proportion of oxygen the more acidic a mix is. Since SiO2 contains the most oxygen, silica rich rocks are the most acidic. Feldspars contain alot of RO2 and R2O, thus they are among the most acidic rocks. However most people simply refer to the acidics as the glass formers in a glaze.