Generic term to indicate a phenomenon of discohesion within the pictorial layers (or between these and the support) that is manifested in the formation of an empty space in the form of a semisphere. In mural painting, for example, the term may indicate both a swelling, as well as caused by the application of a protecting layer (final varnish) with an insufficient transparency. In paintings on wood panel or canvas, bubbles may develop due to various reasons: paintings placed on altars, for example, may easily be subject to this phenomenon due to the presence of a heat source (candles) placed near the pictorial surface.