What is Copper Patina?
Copper patina is a thin layer that forms on the surface of copper as a result of natural oxidation or chemically induced. Natural oxidation of copper occurs as a result of exposure to atmospheric elements such as acidic rain, carbon dioxide, oxygen and other sulfur bearing compounds. The process by which a patina is formed is known as patination. Formation of copper patina makes a body to lose its original color and texture and forms a bluish or a greenish surface. The nature of the patina formed depends on the environmental conditions surrounding the copper body.
In clean environments such as rural areas, natural patina is formed by a slow chemical reaction that leads to the formation of copper carbonate. In urban areas, where the environment is endowed with sulfurous acid rain, the copper patina formed is mainly composed of sulphide and sulphate compounds. The color of the patina formed can be either green or blue depending on the composition of copper and the chemicals that lead to patination.
Different Types of Patinas
There are two main types of patinas, natural patinas and applied patinas.
Natural Copper Patina
Also known as unintentional patinas are formed as a result of exposure of copper to oxidizing environment. In the natural patination process, copper undergoes several processes until a unique patina is formed. The color changes are also gradual from salmon pink, orange, red, blue, green and purple, until a thick light green or blue green color is finally formed. Natural copper patination process begins immediately copper is exposed to the environment. Copper reacts with atmospheric oxygen to form copper oxide, copper oxide films become noticeable within the first six months of exposure. During the initial stages, the patina film formation is uneven, however, after nine months, the layer thickens and becomes even. After a few years, cupric sulfide conversion occurs which makes the surface to darken and become brownish. As the process continues, the sulfide films are transformed into sulfates which are blue green or gray green. Copper patina is a very thin layer and ranges from 0.05 to 0.07 mm in thickness. However, they adhere to the underlying copper surface.
Applied Copper Patina
The other type of patina is intentional patina. Metalworkers and artists deliberately add patina to the original copper body to reap the benefits of a patinated surface. This manual process of patination usually referred to as finishing, the product gains a particular look, color and feel. Chemically inducing patina is a demanding process which requires a lot of craftsmanship and experience. The techniques used are subject to time, humidity, surface preparation and several other variables. Chemicals used in the patination process include, acid chloride, acid sulfate, ammonium chloride, hydrochloric acid and ammonium sulphate.
Chemical Copper Patination
This process is important especially where old buildings are to be repaired; it produces a close match of color of the already existing patinated copper and the repairing copper. Additionally, it covers any surface markings on the copper body. Another main use of chemical patination is that it is done on surfaces where patination is desired but will not occur. Chemical patination is, however prone to several problems. It lacks adhesion as the natural patina, lack of color uniformity over a large surface and excessive staining of surrounding materials.
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