How can rising damp damage a fresco or other wall painting?

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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.

Rising damp, which occurs when moisture travels upward from ground contact through a wall, poses a significant threat to the structural and aesthetic integrity of wall paintings and frescoes. This moisture can cause several forms of damage:

Surface Discoloration: Dampness often leads to blanching and the appearance of drip stains on the painting's surface.1
Efflorescence and Salt Damage: As moisture evaporates, it can leave behind crystallized salts. These salts may form within, above, or below the paint layers, creating a salty "veil" that obscures or disintegrates the image.1
Delamination: The accumulation of salts and moisture can cause the paint layers to lose their bond with the wall, leading to flaking or delamination.1
Secondary Growth: Excessively damp environments frequently foster the growth of mold and mildew, which further degrade the artwork.1

To preserve these works, it is essential to manage moisture levels through building maintenance, such as improving drainage or installing a "damp course" to block water movement. Conservators must avoid using water-impermeable coatings, as these can trap moisture and force it to move laterally, expanding the area of damage.1