The many patinas of bronze are created by different chemical reactions to the metals in a bronze alloy. Artists often manipulate these reactions to create the desired effect -- from gold to green to chocolate -- because the rich patina that develops naturally as bronze ages takes a long time and isn't easily predicted.
Bronze -- a Natural Blond
The color of bronze is relatively gold in hue. The mix of copper and tin balance to give its color. Many people don't expect this to be bronze's natural color, since the most popular color of bronze is brown.
The rich chocolate brown color of bronze is the most widely recognized. Potassium sulphide is applied to the surface of bronze, where it reacts to the copper in the alloy, turning the color a an extravagant, deep, creamy brown. Often heat is applied with a torch to hasten the coloration.
Naturally occurring oxidation turns unfinished bronze green or verdigris. A good example of this would be the Statue of Liberty -- the vibrant green hue of the bronze statue in New York Harbor is caused by decades of exposure to the air and water. Verdigris is actually a crystalline layer that protects a bronze from further deterioration -- preserving a natural verdigris finish preserves the piece it covers. Artists create instant green patinas by applying chlorides to an unfinished bronze surface.
Yellow-brown as well as red finishes are produced using ferric nitrate. The color is softer than the deepest bronze browns and is valued because it highlights details of a sculpture or carving.
Experimentation with chemicals can lead artists to develop interesting patina results. Auguste Rodin was known for such experiments, often layering a green patina over a coat of brown to create a marbled effect to his sculptures.