The green patina that forms naturally on copper and bronze is known as verdigris and consists of copper carbonate. Patina also refers to accumulated changes in surface texture and colour that result from normal use of an object such as a coin or a piece of furniture. Artists and metalworkers often deliberately add patinas as a part of the original design and decoration of art and furniture, or to simulate antiquity in newly-made objects.
Patinas are restricted to exposed surfaces and can flake off. One reason bronze is so highly valued in statuary is that its patina protects or passivates it against further corrosion. This natural patina seldom shows a tendency to flake. Brass is also resistant to corrosion, but it is, in the long run, not as attractive since local pitting shows against the shiny background.
Figuratively, patina can refer to any fading, darkening or other signs of age, which are felt to be natural and/or unavoidable.
The chemical process by which a patina forms is called patination, and a work of art coated by a patina is said to be patinated.
One example of a patina is a green surface texture created by slow chemical alteration of copper, producing a basic carbonate. It can form on pure copper objects as well as alloys which contain copper, such as bronze or brass.
A wide range of chemicals, both household and commercial, can give a variety of patinas. They are often used by artists as surface embellishments either for color, texture, or both. Patination composition varies with the reacted elements and these will determine the color of the patina. Exposure to chlorides leads to green, while sulfur compounds (such as "liver of sulfur") tend to brown. For artworks patination is deliberately accelerated by heat. Colors range from matte sandstone yellow to deep blues, reds and various blacks, sometimes with the surface sheen enhanced by waxing for artwork displayed indoors.
Patina is also found on slip rings and commutators. This type of patina is formed by corrosion, what elements the air might hold, residue from the wear of the carbon brush and moisture; thus, the patina need special conditions to work as intended.
Patinas can also be found in woks, which form when properly seasoned. The patina on a wok is a dark coating of oils that have been burned onto it to prevent food sticking and to enhance the flavor of the foods cooked in it. Steaming foods or using soap on a wok could damage the patina and possibly allow the wok to rust.
In terms of antiques, "Patina is everything that happens to an object over the course of time. The nick in the leg of a table, a scratch on a table top, the loss of moisture in the paint, the crackling of a finish or a glaze in ceramics, the gentle wear patterns on the edge of a plate. All these things add up to create a softer look, subtle color changes, a character. Patina is built from all the effects, natural and man-made, that create a true antique." - Israel Sack
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