TRUE PATINA OF COPPER
Patina is the final result, extending over years and often centuries, of highly varied external influences on copper and copper alloys such as bronze and brass. In addition to the external chemical influences, patina formation depends upon the alloy involved and thickness of the metal surface. Patina therefore varies in color and has different forms, surfaces and thickness.
Dug coins can be preserved in the soil extensively due to the absence of soluble salts, or because an equilibrium has been attained between the corrosion and the neighboring soil.
Vesicular, nodular, lumpy and wartlike forms of patina are considered unsightly, otherwise, natural patina is attractive, and since it is an indication of age, it is generally allowed to remain on coins and other objects.
Patina is formed essentially from the following influences:
• Green patina, leafy green to bluish green, can occur in the form of basic copper sulfate, CuSO4Cu(OH)2, from the sulfur dioxide content of urban air. In the vicinity of the coast it can also contain copper chloride (CuCl) from salt dissolved in fog droplets or copper iodide (CuI, CuI2). It further results from air containing carbon dioxide, in combination with dirt and moisture, or from carbon dioxide in the soil in connection with organic acids and salts, and is then in the form of basic copper carbonate (CuCO3). Finally, green patina is a natural result of verdigris.
• Red patina, bright red through dark red to violet, consists of Cuprous Oxide (Cu2O) produced by decomposition (reduction) of the cupric oxide formed initially. For example, this is caused by organic constituents of the soil.
• Brown patina is either a very thin film or light brown to red and black copper oxide formed through the action of oxygen, or it may be a thick mixture of green and red patina.
Dirt of the most varied form often is a constituent of patina, as are inclusions of sand, rust and similar materials.
All coatings on copper and copper alloys such as bronze and brass possessing a patina-like appearance, but which in contrast to true patina, are white or saltlike or resemble clay or grease can be called false patina.