The whitening of calcareous ceramics was studied with Mössbauer spectroscopy and x-ray diffraction. This phenomenon is similar to that which occurs in firing calcareous clays without chloride, but it takes place at lower temperature; NaCl accelerates the formation of calcium aluminosilicates able to trap Fe³⁺. The alkali metals are lost, during firing, in the form of chlorides. Their loss percentage increases from lithium to rubidium; it augments also as a function of CaCO 3</sub> and NaCl proportions.