Apr 28 2004
Introduction
Silver is considered to be a soft, malleable metal available with a characteristic sheen. It has the highest electrical and thermal conductivities of all metals. It is usually found uncombined, or in the arsenide or sulfide ores from which it can be recovered as a cyanide complex which is consequently reduced to the metal, in aqueous solution, by using zinc.
The pure metal is stable to oxygen and water but is attacked in air by sulfur bearing compounds in order to form the characteristic black layer of silver sulfide. It is soluble in nitric and sulfuric acids.
Some silver salts are sensitive to light (example, AgI, AgBr and AgCl) and are of ultimate importance to photography. Other industries and applications in which silver is used include the electrical industry (example, in the manufacture of contacts), the manufacture of jewelry (both as a constituent of different alloys and as the pure metal) and for the silvering of glass.
The purity or fineness of silver alloys is now described using the millesimal system in most countries. This system uses a number to represent the purity of the alloy. The number described purity in parts per thousand.
Previous to the millesimal system, the fineness of silver was expressed in carats. While the fineness of silver alloys must be stamped or hallmarked into pieces, the millesimal value is generally compulsory and the carat value now optional.
Table 1. Some of the most common fineness denominations used.
| Fineness |
Common Name |
Purity (wt.%) |
| 999 |
Fine or pure silver |
99.9 |
| 958 |
Brittania |
95.8 |
| 925 |
Sterling silver |
92.5 |
| 800 |
Jewellery silver |
80 |
Pure silver is typically very soft and malleable, hence it is commonly alloyed to increase its hardness and durability for applications such as jewellery. It is typically alloyed with copper in this instance, with sterling silver being one of the most popular alloys, containing 7.5% copper. Copper is used as it is a hardening agent and does not discolour the silver.
Source: AZoM.com