Silver artefacts

Tissot I, Abrantes LM (2007) Electrochemical techniques to produce and characterize protective systems on cultural silver artefacts, Metal 07, Book 3 - Use of Electrochemical Techniques in Metal Conservation, Rijksmuseum Amsterdam 62-67. 

The minerals from which the metals are extracted, existed for millions of years in the earth s crust and are the most stable form of the metal. A considerable amount of energy is required to convert this mineral into the metal. Once this pure metal comes into contact with the natural environment such as sea-water or soils, the metal slowly converts back to its original starting material. Iron, for example, is obtained from the mineral, haematite, an oxide of iron. Once the pure iron comes into contact with water and air (oxygen), it slowly converts back to the oxide. This is called corrosion and the product is familiar to everyone as red rust. Nearly all metals will corrode in natural environments although the rates of corrosion will vary from metal to metal and alloy to alloy. In addition, the rates of corrosion will vary from one natural environment to another. Iron will corrode at approximately 50 pun per year in freshwater but at 120 pm per year in seawater. The reason for this is due to the difference in chemical composition between freshwater and seawater. The latter contains salt (sodium chloride) and this is very deleterious to the corrosion behaviour of the metal. Silver artefacts may be excavated after several hundred years buried in soils with only minimal amounts of corrosion. Those recovered from marine sites after a similar period of burial, have completely corroded and have reverted back to 100% mineral. This is entirely due to the presence of chlorides in seawater. 

Silver items recovered from marine sites are often completely mineralised due to the non-protective nature of the corrosion products formed in both aerobic and anaerobic sites. The corrosion product is either silver chloride (AgCl) or silver sulfide (Ag2S). All silver artefacts recovered from the Mary Rose were found to be in very poor condition. 

The history of lead is also inextricably linked with the mining and recovery of silver, which was produced for its value as a currency of trade, as well as a precious metal for the manufacture of jewellery and artefacts. Because of lead s association with silver and its potential use for degrading silver coinage, lead mining and smelting operations were often closely controlled by the application of strict laws. 

Artefacts may also be produced by chemical interaction with the photographic emulsion by substances in the tissue section. These substances may be normal constituents of the tissue or substances which have been applied to the section such as fixatives or stains. Haematoxylin, for example, has been shown to cause a loss of silver grains [119]. Artefacts may also be due to pressure and are most commonly observed in the dry mounting procedure where pressure is applied to achieve close contact between the section and the... 

A gold-silver sulfide, Ag-Au-S, has been reported by a number of authors on artefacts from dynastic Egypt (Frantz and Schorsch, 1990 Hatchfield and Newman, 1991 Colinart, 2001). Also known as Egyptian red gold. 

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