Amalgam
An alloy of mercury with another metal, liquid or pasty when fresh — how gold was extracted for two thousand years, and what most fillings were made of.
Mercury dissolves other metals at room temperature. That single fact — unremarkable in a furnace, extraordinary in a bowl — is the basis of amalgam, and it made mercury one of the most consequential elements in industrial history.
An amalgam is soft or liquid when made and hardens as intermetallic compounds crystallise out of it. Dental amalgam exploits that directly: a powder of silver, tin and copper is mixed with mercury, packed into a cavity as a paste, and sets hard within the hour.
Processing
Gold amalgamation is the older use and by far the larger one. Crushed ore is contacted with mercury, which takes up the gold and leaves the rock; the amalgam is then heated until the mercury boils off and the gold remains. In a properly equipped plant the vapour is condensed and reused.
In artisanal mining it is not. The amalgam is burned in the open, often over a cooking fire, and the mercury goes into the air and then into the water and the fish. Artisanal gold mining is now the largest single source of mercury emissions in the world, ahead of coal.
The Spanish colonial silver industry ran on the same chemistry at enormous scale. The patio process amalgamated silver ore in open courtyards, and the mercury for it came from a single mine at Almadén — which is why control of a mercury deposit in Spain determined the output of silver mines in Peru and Mexico.
Economic significance
Dental amalgam's retreat has been gradual and is not really about the patients. The evidence that a set filling harms the person carrying it has not materialised; the Minamata Convention's case is about the mercury's whole life — mined, handled by dental staff, and released at cremation.
What replaces it is composite resin, which is more expensive, more technique-sensitive, and does not last as long in a large posterior cavity. That trade-off is why amalgam persists in exactly the places least able to absorb the cost of the alternative.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is composed of
- Mercury — element · 40–55% · the defining constituent — an amalgam is by definition mercury with something dissolved in it
- Silver — element · 20–35% · the main component of the dental powder, and what makes the set filling strong
- Tin — element · 8–15% · in the dental alloy, controlling how fast the mixture sets
- Gold — element · in the amalgamation process rather than in a made alloy — the gold is what is being recovered, and the mercury is driven off afterwards
is produced by
- Alloying and melting — process · the one alloy made without heat: mercury dissolves the other metal at room temperature, which is exactly why it was used in fillings and exactly why it stopped being
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing