Britannica Sources used in this answer

Why can acid rain cause plaster and paint layers to crack?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Inside the ongoing preservation of the Vatican — Art conservation and restoration Learn about the restoration of tiles in St. Peter's Basilica, Vatican City.

Acid rain causes plaster and paint layers to crack primarily through a chemical reaction that alters the physical structure of the wall material. Many wall paintings are executed on lime- or cement-based plaster, which contains calcium carbonate. When sulfur dioxide from air pollution reacts with moisture, it forms sulfuric acid, which attacks this calcium carbonate and converts it into calcium sulfate.1

This chemical transformation is destructive because the resulting calcium sulfate crystals occupy nearly twice the volume of the original carbonate. This expansion creates significant internal pressure within the pores of the wall fabric, leading to fracturing and cracking. Furthermore, because calcium sulfate has a higher capacity to absorb moisture than the original material, it exacerbates the cycle of wetting and drying, which further destabilizes the plaster and causes paint layers to flake or delaminate.1

Preserving these works requires careful environmental management, as the conservation community emphasizes that wall paintings are physically and aesthetically dependent on their architectural context.1