The rising water table has saved some of the marble decorations in the ancient sanctuary of Saint-Martin-au-Val. It has also preserved materials of organic origin. Yet a grey to black material has appeared associated with large stone shims visible on the plaster surface. These shims are known from Roman plasters. They are traditionally understood to be fitting elements, but this remains hypothetical. The material used is unprecedented: a kind of mortar made from pitch, beeswax and marble grains. It was obviously used to glue the shims to the underside of the marble slabs. These shims therefore had a completely different function: they were actually used to hold the marble parietal decoration in place, adding points of contact between the decoration and its plaster.