How do salts and efflorescence harm painted plaster walls?

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Charles Sprague Pearce: Religion — Mural Detail of *Religion*, mural in lunette from the Family and Education series by Charles Sprague Pearce, 1897; in the Library of Congress, Thomas Jefferson Building, Washington, D.C.

Salts and efflorescence pose a significant threat to painted plaster walls, primarily through the physical damage caused by crystallization. When moisture within a wall evaporates at or near the surface, it leaves behind crystallized salts. This process, known as efflorescence, can manifest in several harmful ways:

Surface Disintegration: Crystallized salts may form above, below, or within the painted image, leading to the physical breakdown of the surface or the obfuscation of the artwork by creating a salty "veil."1
Structural Damage: The formation of these crystals can cause the delamination of paint layers, where the paint separates from the plaster substrate.1
Internal Pressure: In cases where pollutants like sulfur dioxide react with moisture to form sulfates, the resulting crystals occupy significantly more volume than the original wall material. This expansion creates internal pressure within the pores of the wall fabric, which can lead to fracturing and spalling.1

To prevent these issues, conservators emphasize controlling moisture levels, as rapid drying of ambient air can trigger efflorescence, and avoiding water-impermeable coatings that force moisture to move laterally and expand areas of damage.1