The chemistry of dental amalgam fillings

Dental amalgam is a cavity-filling material made by combining elemental mercury with a powdered silver–tin–copper alloy. It looks silver-grey and is used mainly for load-bearing back teeth. Its value lies in strength and tolerance of a slightly wet field, although every placement also creates a small, measurable release of mercury vapour.

Understanding its chemistry explains why an old filling can remain serviceable for years yet still corrode at its margins. The account below describes conventional high-copper amalgam; formulations and waste practices differ slightly between countries.

Typical composition and structure

A fresh amalgam typically contains about 40–60% mercury by mass, with most of the balance being silver, tin and copper, plus small amounts of zinc or other alloying elements. Typical final proportions are roughly 50% mercury, 25–35% silver, 10–15% tin and 2–6% copper. The powder is not chemically identical to the placed filling: trituration and setting redistribute the elements into microscopic phases.

Silver provides much of the strength and corrosion resistance; tin binds mercury and later forms protective corrosion products; copper strengthens the alloy and promotes a more coherent oxide layer. Amalgam does not chemically bond to enamel or dentine, so the surrounding tooth must provide mechanical retention. For broader historical context on metallurgy and laboratory practice, historical science readings can accompany a modern chemistry text.

The setting reaction

The dentist or dental team opens a capsule, triturates the alloy powder and mercury into a plastic mass, then condenses it into the prepared cavity. Mercury wets the alloy particles, some silver and tin enter the mercury phase, and intermetallic compounds begin forming. The original γ-Ag₃Sn₄ phase is depleted, while mercury-rich phases such as Ag₂Hg₃ and HgSn₇₋₈ appear beside new silver–tin phases. Copper-rich Cu₃Sn and Cu₆Sn₅ particles also remain within the matrix.

These formulas identify phases rather than one balanced equation. Setting continues for hours, and the material gains strength over time. Tin is especially important because it reacts with mercury and oxygen to form stable tin–mercury and tin-oxide products, although much elemental mercury remains distributed through the restoration. This mixture of reacted and unreacted material is why a set amalgam is not simply a solid lump of mercury.

Corrosion and mercury vapour

In saliva, electrochemical corrosion gradually alters the surface and microscopic margins. Oxygen, chloride and sulphide species enter the structure; tin compounds accumulate near the surface, while copper forms oxides, sulphides and soluble ions. Elemental mercury can volatilise from unreacted regions, so release is modest in ordinary room air but higher around a newly placed or freshly cut filling, during trituration and in a fine aerosol.

Amalgam can also create a galvanic potential against a dissimilar metal filling, producing a momentary sharp pain or metallic taste when the two restorations touch. Measurable oral currents are real, but broad claims that ordinary galvanic currents poison the body are not supported. Rare local reactions, including contact allergy or a lichenoid patch near the restoration, are possible; most wearers have no such reaction.

Strength, wear, and failure

Amalgam has high compressive strength, resists heavy chewing and often lasts 10–20 years or longer. Its corrosion products can partly seal microscopic gaps, a useful self-sealing property. It also conducts temperature and wears differently from natural tooth structure, so sensitivity to cold food or a newly placed filling is common during settling. These properties explain its long record in large, load-bearing cavities.

It is not indestructible. Recurrent decay at a margin, fracture, wear against opposing teeth and corrosion can end useful service. Unlike some glass-ionomer formulations that accept repair material, amalgam cannot be repaired in place. Replacement requires removal of the old restoration and sound tooth, so preserving healthy tooth structure remains an important clinical goal.

Safe handling in Australian clinics

Occupational exposure deserves particular attention because dental workers handle concentrated mercury and aerosolised filings. Effective controls include gloves, suitable respiratory protection, high-volume suction, local extraction, sealed capsules, spill procedures and an amalgam separator connected to the waste system. Patients should avoid unnecessary removal of a sound filling, since cutting it temporarily raises vapour release and can weaken the tooth.

In Australia, the Therapeutic Goods Administration regulates dental amalgam as a medical device, while state and territory dental boards oversee professional practice. Clinical waste is handled under relevant state requirements and Australian Standards such as AS 5148; practices use approved separator and disposal pathways because mercury-bearing waste must not reach ordinary drainage. Guidance is not identical in every state, so local workplace and environmental rules govern.

Material choice and dental access

Amalgam remains chemically useful where a durable, relatively economical restoration is wanted, particularly for a large posterior cavity. Tooth-coloured composite resin bonds to prepared tooth and is preferred for visible surfaces, but it needs careful moisture control and adequate curing. Glass ionomer releases fluoride and bonds chemically, although some formulations wear sooner. Australian dentists weigh cavity size, isolation, caries risk, tooth structure and cost rather than treating one material as universal.

Access shapes choice. Medicare generally does not cover routine fillings, while state and territory public dental services have eligibility rules and may involve waiting periods. Private clinics in Melbourne and Sydney may provide several material choices, while public services and outreach programmes in Darwin, Alice Springs and remote communities follow local referral pathways. In everyday Australian speech, these restorations are simply “fillings,” and patients may be told to “bite down gently” while the material sets. The next concrete step is a dental review in which the dentist records the filling material and checks its margins.