Amalgam Definition and Uses

An amalgam is a type of alloy found in dentistry, mining, mirrors and other applications. Here is a look at an amalgam's composition, uses, and the risks associated with use.

Amalgam Definition

An amalgam the name given to any alloy of mercury. Mercury forms alloys with almost all other metals, except iron, tungsten, tantalum, and platinum. Amalgams may occur naturally (e.g., arquerite, a natural amalgam of mercury and silver) or may be synthesized. Key uses of amalgams are in dentistry, gold extraction, and chemistry. Amalgamation (the formation of an amalgam) is usually an exothermic process that results in hexagonal or other structural forms.

Amalgam Types and Uses

Because the word "amalgam" already indicates the presence of mercury, amalgams are generally named according to the other metals in the alloy. Examples of important amalgams include:

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Dental Amalgam

Dental amalgam is the name given to any amalgam used in dentistry. Amalgam is used as a restorative material (i.e., for fillings) because it's fairly easy to shape once mixed, but hardens into a tough substance. It's also inexpensive. Most dental amalgam consists of mercury with silver; the presence of this mercury is one disadvantage to using amalgam in dentistry. Other metals that may be used with or in place of silver include indium, copper, tin, and zinc. Traditionally, amalgam was stronger and longer-lasting than composite resins, but modern resins are more durable than they used to be and strong enough for use on teeth subject to wear, such as molars.

Silver and Gold Amalgam

Mercury is used to recover silver and gold from their ores because the precious metals readily amalgamate (form an amalgam). There are different methods of using mercury with gold or silver, depending on the situation. In general, the ore is exposed to mercury and the heavy amalgam is recovered and processed to separate the mercury from the other metal.

The patio process was developed in 1557 in Mexico to process silver ores, although silver amalgam is also used in the Washoe process and in panning for the metal.

To extract gold, a slurry of crushed ore can be mixed with mercury or run across mercury-coated copper plates. A process called retorting separates the metals. Amalgam is heated in a distillation retort. The high vapor pressure of mercury allows for easy separation and recovery for re-use.

Amalgam extraction has largely been replaced by other methods because of environmental concerns. Amalgam slugs may be found downstream of old mining operations to the present day. Retorting also released mercury in the form of vapor.

Other Amalgams

In the mid-19th century, tin amalgam was used as a reflective mirror coating for surfaces. Zinc amalgam is used in the Clemmensen Reduction for organic synthesis and the Jones reductor for analytical chemistry. Sodium amalgam is used as a reducing agent in chemistry. Aluminum amalgam is used to reduce imines to amines. Thallium amalgam is used in low temperature thermometers because it has a lower freezing point than pure mercury.

Although normally considered a combination of metals, other substances may be considered amalgams. For example, ammonium amalgam (H₃N-Hg-H), discovered by Humphry Davy and Jons Jakob Berzelius, is a substance that decomposes when it comes into contact with water or alcohol or in air at room temperature. The decomposition reaction forms ammonia, hydrogen gas, and mercury.

Detecting Amalgam

Because mercury salts dissolve in water to form toxic ions and compounds, it's important to be able to detect the element in the environment. An amalgam probe is a piece of copper foil to which a nitric acid salt solution has been applied. If the probe is dipped in water that contains mercury ions, a copper amalgam forms on the foil and discolors it. Silver also reacts with copper to form spots, but they are easily rinsed away, while amalgam remains.