The whitening of calcareous ceramics was studied in Mössbauer spectroscopy and X-Rays diffraction. This phenomenon is similar to that which occurs in firing calcareous clays without chloride, but it appears at lower temperature : NaCl accelerates the formation of calcium aluminosilicates susceptible of Fe3+ -trapping. The alkali metals are lost, during firing, in the form of chlorides. Their loss percentage increases from Li to Rb ; it augments also in function of CaCO3 and NaCl proportions.