The interfaces between different materials of the multilayered wall painting
system may be heavily attacked by microbial growth. Microbial biomass devel-
oping beneath the paint layer will at least partly destroy the connection between
paint layer and the plaster below and thereby induce flaking, especially when
larger parts are attacked and when climatic changes take place. Similar situa-
tions may occur beneath applications made over paintings, resulting in uncon-
trolled exfoliation. The problem is worse in cases where microbial subsurface
growth is accompanied by salt crystallisation in the same layer (Figures 4 and 5),
as frequently observed.
Metabolism of organic bindings as substrate. It is well known that natural
organic bindings, like animal or plant glue, lose their binding effect when
Wall Paintings: Aspects of Deterioration and Restoration 251
Figure 4Flaking of paintlayer, sample from Gnoiden, Germany
Figure 5SEM of subsurface layer shows microbial growth and salt efflorescences