We continue to review some of the most important materials in heat treatment and metallurgy.
Silver (chemical symbol: Ag)
Silver is a bright, shiny, precious metal located below copper and above gold on the periodic table. It shares the high electrical and thermal conductivity of both. In fact, it is the most electrically and thermally conductive of the known elements. It also has the highest emissivity of any element, which means it is the best reflector of light.
Silver exists in the Earth's crust in its pure elemental state as well as alloyed with other metals like gold. Silver can also be found in minerals such as argentite (silver sulfide; Fig. 1) and chlorargyrite (silver chloride). Most silver is obtained during the refining of other metals, such as copper, gold, lead and zinc. Its chemical symbol, Ag, is derived from the Latin word for silver, “argentum,” which originates from argunas, a Sanskrit word for shining.
Humans have used silver since prehistoric times. In fact, it is one of the seven metals of antiquity – metals that had been identified and used since the beginning of recorded history. The others are gold, silver, copper, tin, lead, iron and mercury. The three metals of group 11 on the periodic table (copper, silver and gold) all occur in their pure form in nature and are thought to be the first forms of money. Despite its unique thermal and electrical properties, silver was not instrumental to the development of metallurgy due to its lack of structural strength. In ancient times, silver was less plentiful than gold due to its chemical reactivity and relative rarity in its native form. As a result, silver was more expensive than gold in Egypt until around the 15th century BC.
Another property silver shares with gold and copper is its malleability. Silver is extremely ductile, and it can be formed into a wire just 1 atom wide (Fig. 2). Silver-based alloys are commonly used in brazing applications to join metals such as nickel, cobalt, copper-based alloys, tool steels and other metals. Brazing alloys consist of silver and copper with manganese, phosphorous, tin, palladium, cadmium and zinc elements added, depending on the specific application. Silver increases workability and corrosion resistance during use.
Silver has important uses in the medical field. The silver ion is bioactive and interferes with enzymes critical to the survival of bacteria. As a result, it is used in antibiotic coatings to prevent bacterial growth in urinary catheters and endotracheal breathing tubes, where it is thought to reduce infections. For the same reason, silver compounds are used in wound dressings and infused into clothing to help prevent body odor caused by bacterial growth. Bacteria do not develop resistance to silver as they do to antibiotics. As a result, silver is seeing increased use in industrial, healthcare and household applications. Silver has numerous other uses in electronics, nanomaterials, jewelry and chemistry.
Here are a few important facts about silver.
- Atomic number: 47
- Atomic weight: 107.8682
- Melting point: 1234.93 K (961.78°C or 1763.20°F)
- Boiling point: 2435 K (2162°C or 3924°F)
- Density: 10.501 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 5
- Group number: 11
- Group name: none