Eventual leaking and tendency toward infection by moss and lichen have reduced the usefulness of traditional adhesives. The Laboratoire des Recherches des Monuments Historiques has undertaken a study of 11 different adhesives. These have, as a rule, higher viscosity than the traditional types, and therefore cannot be applied without first being diluted. A table is given with commercial names, compositions, method of application and behavior after aging. The results show that silicone-based mastics are best as they can be diluted for brush application without affecting their quality.