Studies in ground layers of paintings on wooden support from XV and XVI centuries have shown based on SEM observations and elemental chemical identification by EDS performed on samples that had been taken from plaster preparations layers, that the common presence of strontium is a strontium sulfate (celestine). In this context, some questions have been asked, either related by the origin of celestine, particularly in the context of evaporative formations rich in gypsum, or related with technological issues associated to the higher occurrence of this mineral in the "gesso grosso” preparations (coarse gypsum plaster). Thus, the aim of this work is to try to explain the genesis and presence of celestine based on it is properties, the geological settings in gypsum outcrops, particularly in Portugal, and on preparation procedures of "gesso grosso" and "gesso mate” (fine gypsum plaster) known through coeval treatises.