amalgam

An amalgam is a metal alloy in which mercury serves as the primary solvent. Unlike most alloys that form solid solutions, amalgams remain plastic or semi-fluid at room temperature, allowing the constituent metals to be mixed and shaped before hardening. Mercury dissolves most metals readily, which is why it has been the basis for amalgamation processes for centuries. The mercury content typically ranges from 40 to 70 percent by weight, depending on the intended application and the metals being combined.

Dental amalgam remains the most widespread industrial use. Composed of mercury combined with silver, tin, copper, and sometimes zinc, dental amalgam sets through a reaction between the mercury and the powder metals rather than through cooling. The material hardens over 24 hours and achieves its final strength through microstructural changes in the alloy matrix. A typical dental amalgam restoration contains roughly 50 percent mercury by weight, bound into the set solid matrix, which limits leaching under normal conditions.

Preparation and hazards

Amalgams are prepared by trituration, grinding the powdered metal into liquid mercury in a mechanical mixer. The process must be controlled carefully because excess mercury produces a weak, paste-like product, while insufficient mercury prevents proper setting and packing. Heat accelerates the amalgamation reaction, so both temperature and trituration time are critical variables. Historical processes used hand-grinding with mortar and pestle, a notoriously hazardous practice due to mercury vapor exposure.

Mercury is volatile and toxic. Inhalation of mercury vapor causes chronic poisoning affecting the nervous system, kidneys, and lungs. Occupational exposure limits in most jurisdictions are set at 0.05 mg/m³ as a time-weighted average. Proper ventilation and personal protective equipment are mandatory during preparation. Spent amalgam waste, including extracted dental amalgams, is classified as hazardous waste in many countries and must be handled through approved disposal routes rather than standard waste streams.

The term amalgam derives from the Arabic al-malgama, meaning soft mass or poultice. It entered European metallurgical vocabulary during the medieval period when amalgamation became central to gold and silver extraction from ore. While industrial amalgamation for precious metal extraction has largely been replaced by modern refining methods, dental amalgam persists because no single substitute combines its durability, ease of placement, cost-effectiveness, and clinical track record across multiple decades of continuous research.