Patina natural

Another ak pollutant that can have very serious effects is hydrogen sulfide, which is largely responsible for the tarnishing of silver, but also has played a destmctive role in the discoloration of the natural patinas on ancient bronzes through the formation of copper sulfide. Moreover, a special vulnerabihty is created when two metals are in contact. The electromotive force can result in an accelerated corrosion, eg, in bronzes having kon mounting pins. 

FIGURE 40 Patina. Patina is a colored (usually green) layer of corrosion products that frequently develops naturally on the surface of copper and copper alloys exposed to the environment. Since it is sometimes appreciated aesthetically and as a proof of age, patina is also developed artificially, by chemical means, as a simulated product of aging. Copper patina generally includes such compounds as copper oxides, carbonates, and chlorides. In bronze and brass patinas, these compounds are mixed with the oxides of tin and lead resulting from the corrosion of the other components of the alloys. In any particular patina there may be many layers, not necessarily in the order shown in the illustration. 

Metals develop a natural corrosion-resistant film when exposed to the environment. Examples include the rusting of iron, tarnishing of silver, and the formation of the patina on copper. These passive films help prevent further corrosion. However, films do not provide complete resistance to chemical attack and are destroyed by various corrosive agents. 

Robbiola L, Blengino J-M, Eiaud C (1998) Morphology and mechanisms of formation of natural patinas on archaeological Cu-Sn alloys. Corros Sd 40 2083-2 111. 

The chemical properties of substances allow them to transform to new substances. Both natural gas and baking soda transform to carbon dioxide and water. Copper transforms to patina. 

When a piece of jewelry is fabricated, solder is the glue that holds together the individual pieces. After the object is assembled, a surface color may be desired. This surface color, or patina, may develop naturally upon exposure to the atmosphere or may be produced as the result of an anticipated chemical change. Usually, the patina will make the piece of jewelry more attractive than it was originally. 

Patina denotes, like modulation of which it is a type, the expressive potential of the object or substance.32 Patina is a surface appearance that is brought out by chemical, physical and mechanical processes on natural and... 

There is a language of nature to be learnt. There are also new ways of seeing. The final words of Modulation und Patina reveal that vision could be trained and that it was the task of the human to school his eye and his perception for the correct co-experience of this world of appearance .42... 

This is probably the most frequent treatment performed on museum objects. It can range from removal of loosely-deposited surface dust to that of hard, adherent concretions. In general, it is considered as a simple and self-evident task and consequently not much thought is given to it. However, it is not a simple task and, in many cases, difficult decisions have to be made about how it has to be performed. If a metal object has a heavy calcareous incrustation covering most of it, would removal of this incrustation, which may result in the removal of the original patina the object had, be acceptable Loss of the original patina means that part of the information that patina carried (How was that patina formed Was it natural Was it the result of an intentional treatment ) would be lost, and with it, part of the authenticity of the object. 

Examples of anthropogenic phosphate minerals are shown in Figure 9.14 (Harris, 2002 University of Florida). Figure 9.14a shows patina of black, poorly crystalline apatite on the surface of an oyster shell located near the bottom of an early American mound, which once served as a refuse pile (about 5,000 YBP). Bones of fish and other animals were discarded in the pile and became the source of phosphorus that precipitated on the nnderlying shell. Note that oyster shells are composed of calcium carbonate, npon which phosphate readily adsorbs and precipitates (scale bar = 5 mm). Figure 9.14b shows vivianite as precipitate on an aggregate from stream sediment near Lake Okeechobee, Florida, collected a short distance downstream from where a dairy barn floor routinely flushed manure. Soils and stream sediments in the area are naturally low in phosphorus (scale bar = 1 mm). 

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