Brown and black patina is created in water solutions of sulphurated potash and ammonium sulfide (amorphous mixture of polysulfides).Ī thin layer of copper sulfide is formed on bronze surfaces. Mixtures of different chemical substances are used for patination, but the creation of the aesthetically-desirable even surface layers of artificial patina is overly complicated. This technique is mainly used when a desired surface finish needs to be reached quickly. (Related reading: Using Pickling and Passivation Chemical Treatments to Prevent Corrosion.)Īrtificial patination is not suitable for big outdoor surfaces such as copper roofs or copper parts of building cladding because this process takes too long. Chemical cleaning includes drawing-off (use of chelation solutions) and pickling (use of a pre-treatment and application of various types of protective coatings). Physical techniques include water blast cleaning to eliminating soluble parts of corrosion crust and layers, and mechanical abrasive cleaning to remove crusts, deposits and growth from the surface while retaining a thin layer of patina. (For more on this topic, read Protecting Public Art Against Corrosion.) Patination and properties of artificial patinas including their transformation after atmospheric exposure and their conservation. The cleaning and preservation of copper and its alloys can be done using various techniques, both physical and chemical. The depth of pits (if they occur) on surfaces exposed for long periods of time range from 50 to 70 µm, but also from 100 to 105 µm in some cases. The thickness of each sublayer and the ratio between brochantite/cuprite sublayers can vary depending on the purity of the copper in the material and the size of inclusions found in the layers. Some investigations have found the average thickness for the copper patina to range from 1 to 10 µm for cuprite and 10 to 40 µm for brochantite/antlerite. These alloys influence the rate of formation and characteristics of the patina layer. (Related reading: 11 Uses for Cupronickel and Why You Should Be Using It Now.) High-copper alloys, silicon bronze and tin bronze corrode at a moderate rate, while brass, aluminum bronze, nickel silver and copper nickel corrode at a slower rate. In general, copper alloys are very suitable for atmospheric exposure.
The corrosion rate of copper and its alloys usually decreases with time. Gerhardtite is a rare copper nitrate mineral and has been detected in vein deposits with a long-time accumulation. Malachite, one of the first ores used to produce copper metal, is often found with azurite, goethite and calcite, and is typically associated with copper deposits around limestone. Tenorite occurs in the oxidized zone associated with deeper primary copper sulfide ore bodies and may be found as a sublimation product on lava. Occasionally, other copper minerals can be observed in the copper patina constituents, including tenorite (CuO), malachite (Cu 2CO 3(OH) 2) and gerhardtite (Cu 2NO 3(OH) 3).
Copper patina, light green in an unsheltered area (wash-off effect).
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