This article presents a thorough study and investigation of lime mortar, which serves as bed for wall mosaics, through the application of physico-chemical methods.

The methods employed are three: a. the XRD. which examines the mortar surface, b. the SEM, which analyses the chemical components of the materials and c. the 14C, a method of absolute dating. Samples for this study come from two churches in Thessaloniki, dating from the Early Christian and Early Byzantine period, also from the katholikon of Hosios Loukas Monastery in Boeotia and that of Daphni Monastery in Attica, both of the Middle Byzantine period. The conclusions of the study are the following: First, the striking difference in craftmanship and technique of the mosaic bed between the earlier and later period is more than obvious. Second, the application of the 14C method leads to a precise dating of lime mortar, thus solving dating problems concerning those Byzantine monuments, which were decorated with wall mosaics more than once over a period of time.