How do soluble salts damage ceramic objects?

Soluble salts, such as chlorides, nitrates, and phosphates, damage ceramic objects through a process of migration and crystallization. These salts enter the ceramic's pore structure when combined with water from the environment. As the water evaporates, the salts effloresce, forming crystals that occupy a greater volume than the salt in solution.1

This expansion exerts significant stress within the ceramic structure, leading to microfracturing and structural degradation. The damage is particularly severe when salts accumulate beneath a glaze, which is less permeable to water vapor and salt crystals. Because the crystals cannot grow outward, they form at the body-glaze interface, causing the ceramic to become powdery and the glaze to flake away.1

It is important to note that this destructive process is distinct from the intentional creation of a salt glaze, which is a deliberate decorative technique applied during the firing process to create a protective, glassy coating on stoneware. In conservation, the presence of harmful soluble salts requires careful removal, typically through desalination methods like soaking in deionized water or using water-based poultices.21