Deformation of icon's primer and paint layer

Based on written sources and previous material analyses, primers used for Orthodox icons are quite varied in composition, typically made from locally sourced materials that undergo different treatments. Their vernacular name is "alchia" (Turkish) referring to calcium sulfates, carbonates, kaolin, and other minerals. Their colors can range from white and ochre to red bolus.

Many early and Baroque Serbian icons, for instance, display widespread networks of cracks in the primer, which also affect the paint and protective layers. This is usually a sign that the ground layer contains glue and possibly kaolin, both known to cause such effects.

If the ground layer is elastic enough, it remains compact for a time, and any swelling does not go beyond the wooden support's dimensions. However, changes in humidity and temperature, even without significant damage, can cause this solid layer to detach from its base. As the adhesive and ground materials age, their elasticity decreases, leading to increased rigidity and fragility, which results in cracking. The detached primer (levcas) might also develop fractures.

Various types of swellings—spherical, transverse, vertical, rigid, open or closed, and those with broken edges—can occur due to detachment between the ground and paint layers, accompanied by deformation. These are important signs of damage, often arising from rapid moisture evaporation from the wooden support. Generally, large swellings affect the deeper layers of the artwork, while finer ones impact just the upper ground and paint layers. If swelling is not addressed promptly, it can eventually cause the ground and paint layers to flake off.

Source: M. Stoyanova, T. Lukic (2015). Mapping the Conservations Status of Easel Painting. Craquelure Structure Visualization by Binary Image Segmentation Approach