How does air pollution and acid rain damage lime-based murals?

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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.

Air pollution and acid rain pose a significant threat to lime-based murals by chemically altering their structural composition. When atmospheric pollutants, such as sulfur dioxide, combine with moisture, they form sulfuric acid. This acid reacts with the calcium carbonate that binds the mural, converting it into calcium sulfate.1

This chemical transformation causes two primary types of damage:

Physical Fracturing: The resulting calcium sulfate crystals occupy nearly twice the volume of the original carbonate. This expansion creates internal pressure within the pores of the wall fabric, leading to cracks and structural failure.1
Increased Moisture Sensitivity: Calcium sulfate has a higher capacity to absorb moisture than the original material, which accelerates the cycle of wetting and drying, further exacerbating the decay.1

Additionally, polluted environments contribute to the accumulation of soot and can cause the discoloration of specific pigments, such as red or white lead, malachite, and azurite. Protecting these works requires a holistic approach to art conservation that addresses both the mural and its architectural environment.1