Deterioration of fresco paintings results from the open, porous nature of their support (walls and ceilings of buildings or other carriers) and their interaction with the surrounding microclimates.
The porous mortar backing provides an easy route for the movement of dilute salt solutions. Salts contained in the building materials or the surrounding area can be readily transported to the plaster underlying the painting. Old leaky roof, clogged gutters or subterranean walls and/or semi-buried walls – anything that would cause water to soak and remain in the wall will eventually bring salts from adjacent areas into the plaster. The wall and fresco can get wet from the rains and not be affected as long as it is exposed to freely moving air that would dry it naturally.
Expansion in volume associated with crystallization of these salts disrupts the plaster-pigment adhesion and leads to disintegration of the surface. Such crystallization depends on the identity of the salts and the moisture content of masonry which is subject to seasonal variations in atmospheric humidity and the amount of rain.