The word "verdigris" in English comes from the French "verte grez," which is a deviation from the phrase "vert-de-Grèce," or "green from Greece." While the reason for the term is not officially known, it is thought that Greek artists used the green-blue tint in paintings and sculpture. It may have been made by suspending copper enclosed in a container over boiling vinegar, and then scraping the resulting characteristically green crust to mix into a paste.
Verdigris results from a process called oxidation. Oxidation is the process by which atoms lose electrons due to the presence and effect of an oxidizing agent. This loss of electrons is said to increase the atom's oxidation state. In the case of verdigris, the exposure of copper, brass or bronze to acidic elements over time, such as rain or vinegar, causes a green-blue patina to form on the metal's surface. The chemical composition of verdigris varies, but it can consist of copper carbonate, copper chloride or copper acetate.
Before more stable pigments were readily available for visual artists to use in paintings and other art forms, verdigris was manufactured by creating different chemical reactions. In the Middle Ages, artists attached copper strips to a wooden block, added acetic acid and then buried the block in the ground for a few weeks. In the early 19th century, artists used more controlled manufacture processes, in which copper sulfates were reacted with barium, lead or calcium acetate. Because the sulfate solutions were insoluble, a precipitate would form, and leave the copper acetate---verdigris---in the solution.
Verdigris was produced for use as a pigment in visual art. Verdigris is lightfast in oil paint---it provides a stable green pigment when mixed with oil that will not fade when exposed to light. For this reason, verdigris remained the most stable green pigment for centuries, before a wide variety of more stable pigments could be manufactured commercially. Verdigris is highly toxic, which posed a health hazard for artists. The patina also occurs naturally on such famous structures as the Statue of Liberty in New York. When the green-blue hue first appeared on the original dull copper surface of the statue, it was thought that the surface of the statue was being eroded. Once it was determined that the verdigris lay on the surface of the copper, local authorities decided to leave it alone as a protective skin for Lady Liberty, acknowledging that it added a certain unique charm to the well-known emblem.