Patina is a thin layer of tarnish that forms on the surface of copper and similar metals, changing in color with time and exposure. The tarnished look is produced by oxidation or chemical exposure that occurs naturally with age or wear. The process can happen faster by polishing or applying chemicals. Patina provides a protective covering to copper that would otherwise be damaged by corrosion or weathering, and many people find it aesthetically appealing.
Color plays a large role in design aesthetics, and copper is a metal that transitions through many colors during the patina process. If you place copper next to any chemical, it will react by changing color. This is why copper is utilized in many ways, from kitchen appliances to jewelry, and in backyard and outdoor designs like lighting. Copper takes to patina especially well. Either by hand or over time, you can achieve the tarnished look that will actually strengthen and protect the metal rather than weaken it.
Like every metal, copper has its own reaction to the patina process, and no two pieces will ever look the same. You can look online and find hundreds of chemical patina recipes if you would like to bring the tarnished look to copper quickly. However, if you want your copper to stay that rust gold color, you will need to coat it with sealant. This way, the metal will not naturally begin the patina process whether it is located inside or outside your home or business.
A good example of patina on copper is the Statute of Liberty. The green color covering the statute is from the patina process caused by continuous moisture and salt water exposure. Just like the Statute of Liberty, copper will last hundreds of years and is a very durable metal. The coloring process will depend on the chemicals being used. If you let copper go through the natural weathering process, then the coloring will turn a rich red bronze color and eventually progress to a green, blue, or turquoise color.
If you live near the ocean, then the typical patina timeframe is generally five to seven years. In rural atmospheres where salt in the air is low, patina formation may not reach a dominant stage for ten to fourteen years. In arid environments, patina may never form due to the lack of sufficient moisture. In all instances, it is the amount of time and exposure that determines the patina process on each piece of copper.