Salt crystallization in cultural heritage From fundamentals to deterioration

Climate change is a global issue that threatens our cultural heritage. Many historical artifacts and structures, such as sculptures, tiles, and masonry, are made from porous materials that absorb water, including groundwater and rainwater, which contain dissolved salts. When the water evaporates during warmer or drier weather, the salts remain inside the material. Repeated cycles of wetting and drying, common with changing weather, cause these salts to accumulate, leading to the growth of salt crystals. The pressure from these growing crystals can cause significant damage, such as the detachment of glaze from antique tiles or the flaking of plaster and stone surfaces.
Despite advances in understanding how salt crystallization damages simple porous materials, much remains unclear, particularly regarding why salts prefer to crystallize between layers of different materials, such as the interface between clay and glaze in a tile. This thesis addresses the need for further investigation into the behavior of composite materials in cultural heritage and the crystallization dynamics of layered systems.
The research presented in this thesis focuses on the relationship between salt crystallization and material properties. For example, Chapter 2 explains how sodium chloride and sodium sulfate cause different types of damage to antique Dutch tiles.
Further chapters explore various scenarios in which salt crystallization affects cultural heritage. Chapter 3 examines how the drying process through small defects leads to different crystallization patterns in comparison to larger defects, while Chapter 4 investigates how sodium chloride crystals can form complex self-similar structures. Chapter 5 addresses how hydrophobic treatments, often applied to protect heritage materials from moisture, can inadvertently cause salts to accumulate in unintended areas.
The final chapters delve into the behavior of sodium sulfate and sodium chloride under high humidity, and their impact on gypsum, an important building material. These findings offer essential insights into the preservation of cultural heritage and highlight the complexities of salt crystallization, underscoring the need for careful restoration and conservation practices to protect these valuable artifacts for future generations.