Poor cohesion is related to crystallisation of soluble salts

Poor adhesion and cohesion of the paint film

This is mainly due to processes of crystallisation of soluble salts in the superficial layers of the plaster or immediately below the painted surfaces "a secco". In some areas, the situation is aggravated by the fact that the paint film is impregnated with consolidating agents, both natural and synthetic, applied during previous restoration work, in rather high concentration or quantity (see residues of consolidating agents). There are also clear signs of de-cohesion of the dark blue pigments and sometimes the red ones, applied "a secco", mainly due to the denaturation of the organic binder, and often associated with a thinning of the painted layers (see abrasions on the paint film).

Poor adhesion of the preparatory layers
While checking the loss of adhesion between the preparatory layers and the masonry, some serious and quite serious defects were recorded, ignoring those of a low level which were not in a precarious situation in terms of conserving the plaster, and whose consolidation was not particularly urgent.

Poor cohesion in the preparatory layers
The problem of rainwater infiltration was solved by consolidating and repairing the Chapel walls during the 20th century. Subsequently, problems of cohesion seem to be wholly related to the crystallisation of soluble salts (see formation of saline deposits) and only occur in the surface layers of the plaster.

Pre-existing stucco work
These differ according to the various materials used, the granular composition, the depth of the layer and the finishing of the surface. Such diversity depends not only on the period when they were carried out but also on the size of the area of missing plaster to be filled. There are three main types of stucco work: those done with soft mortar, those done with hydraulic mortar (which also includes the very large number of filled nail holes) and those done with a type of putty based on synthetic resin. The majority of the stucco work was finished with a neutral tint, though there are several examples of pictorial imitation on the large-scale lacunas especially in the areas of the dark-blue sky, the "Cosmati-style" decorations on the cornices, and parts of the imitation marble along the foot of the walls.

Poor adhesion and lacunas in the metal laminas
As for the painted surfaces, degradation is related, though to a lesser extent, to the presence of saline deposits, as well as to previous damage caused by infiltration of moisture.