Detached paint film due to the action of saline deposits

Formation of saline deposits
This occurs as aggregations of pointed crystals at various stages of development. The survey data was differentiated according to whether the saline deposit was at an early stage, with slight and barely visible blistering of the plaster, or at an advanced stage where the growth of the crystals causes the plaster to swell and break up, leading to detachment and crumbling of the paint film.
From the early 1960s, it became clear that the presence of saline deposits - one of the main causes of deterioration in the Chapel's frescoes - was associated with a process of sulphation in the plaster due to atmospheric pollution. Sulphation generally takes place in the topmost surface layer of the plaster (max. depth of 130 microns), but it often reaches greater depths in areas affected by abrasions and fractures. The negative action of sulphates is accompanied by that of nitrates, detected only recently and in considerable quantities. The presence of chlorides seems to be practically irrelevant.
The formation of saline deposits, at various stages of growth, occurs in circumscribed areas over the whole surface of the Chapel frescoes.

Thin section showing the thrust of the sulphates on the paint film