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Why are murals and wall paintings vulnerable to moisture damage?
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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.
Murals and wall paintings are highly vulnerable to moisture because they are integral to architectural structures, often applied directly onto porous materials like plaster, masonry, or stone. Moisture acts as a primary catalyst for decay through several mechanisms:
- Structural Damage: Dampness can rise from the ground through the wall or infiltrate from the exterior, causing materials to expand and contract. This cyclic wet-dry process often leads to blanching, staining, and the delamination of paint layers.1
- Efflorescence: As moisture evaporates at the surface, it leaves behind crystallized salts. These salts can form within or beneath the paint, creating a "salty veil" that disintegrates the image or causes the paint to flake away.1
- Chemical Erosion: In polluted environments, moisture reacts with atmospheric sulfur dioxide to form sulfuric acid. This acid converts the calcium carbonate in lime-based paintings into calcium sulfate. Because these sulfate crystals occupy more volume than the original material, they create internal pressure that fractures the wall fabric.1
- Biological Growth: Excessively damp environments provide ideal conditions for the development of mold and mildew, which further degrade the artwork.1