The Statue of Liberty was originally a shiny copper brown, not green.

Copper reacts slowly with oxygen, water and pollutants in the air. First it darkens to brown, then over years a crust of copper compounds forms. On the statue this layer is mostly copper carbonates and sulfates, which give it its pale blue green colour. The process is natural and well understood, and you can see the same effect on old copper roofs and church spires.

Photographs and records show the statue changing through the late 1800s and early 1900s. The patina was well established within a few decades of the dedication. In 1906, Congress considered painting the statue, but the idea met public objection, and engineers argued that the patina actually protected the copper.

The green coating is only a fraction of a millimeter thick, but it shields the copper beneath from further corrosion.

The statue, officially Liberty Enlightening the World, was a gift from the people of France to the United States. Sculptor Frédéric Auguste Bartholdi designed it, and Gustave Eiffel's company engineered its internal iron framework. The copper sheets were hammered by hand over shaped forms in a technique called repoussé, and they are only about 2.4 millimeters thick, roughly the thickness of two pennies stacked together.

It was shipped to New York in pieces and assembled on a pedestal paid for largely by public donations, helped by a fundraising campaign in Joseph Pulitzer's newspaper. It was dedicated on 28 October 1886. A major restoration for its centennial in the 1980s replaced the original torch and much of the iron framework, but the copper skin and its patina were preserved.

Bartholdi intended the statue to celebrate liberty and friendship between France and the United States. For millions of immigrants arriving by ship into New York Harbor, it became a first sight of a new home.

Emma Lazarus's poem The New Colossus, with its famous lines about huddled masses, was later placed on a plaque inside the pedestal and shaped how people saw the statue.

Copper has been used for roofs, domes and statues for centuries precisely because it protects itself. The patina that forms is stable and slows further corrosion, so copper roofs can last a century or more. Architects sometimes even speed up the process with chemical treatments to get the green look on new buildings.

The statue's colour has become part of its identity, so much so that souvenir models, posters and logos almost always show it green, not as Bartholdi first saw it.