Which chemical reaction causes de-colourisation of monuments?

The discoloration or color change of monuments, such as the Statue of Liberty, is primarily due to a series of complex chemical reactions involving oxidation and the interaction of the monument's material with environmental factors.

Here are the key chemical reactions and processes involved:

Oxidation of Copper

The Statue of Liberty is made of copper, which undergoes oxidation when exposed to air. The initial reaction involves the formation of copper oxide (Cuâ‚‚O), a red or pink mineral, when copper reacts with oxygen molecules23.

Formation of Tenorite

The copper oxide (Cuâ‚‚O) further oxidizes to form copper(II) oxide (CuO), which is black in color. This black mineral is known as tenorite23.

Sulfur Reactions

Sulfur dioxide (SO₂) in the air, often from industrial activities and natural sources like volcanic eruptions, reacts with water to form sulfuric acid (H₂SO₄). This acid can react with the copper oxides to form green minerals such as brochantite (Cu₄SO₄(OH)₆) and antlerite. When sea spray containing chloride ions hits the statue and isn't washed away by rain, the sulfate in brochantite can be replaced by chloride, forming olive-green atacamite23.

Carbonate Reactions

Carbon dioxide (CO₂) and water in the air also react with the copper oxides to form carbonate minerals like malachite (Cu₂CO₃(OH)₂) and azurite (Cu₃(CO₃)₂(OH)₂), which contribute to the green patina3.

These reactions collectively result in the formation of a green patina, known as verdigris, which is the characteristic blue-green color of the Statue of Liberty.

In summary, the discoloration is not a simple de-colorisation but rather a complex color change due to the formation of various minerals through oxidation and reactions with sulfuric acid, carbon dioxide, and other environmental factors.