Book 27, Question 119
Why does rain slowly eat away a limestone statue?
Limestone and marble are made of calcite crystals.
Rain is slightly acidic, because it picks up carbon dioxide from the air, and polluted rain is even more acidic. The acid slowly dissolves calcite, so sharp carved details turn soft and blurry over many years. Some stone also gets a crusty layer of new crystals, called gypsum. Caretakers clean old statues gently and sometimes cover them to protect them.