Dental Amalgam Composition, Mercury Hygiene and Handling

Dental amalgam is the silver-coloured filling material that dentists have used for more than 150 years to restore teeth damaged by decay. It is an alloy made by mixing a powder of silver, tin and copper with liquid mercury, which binds the particles into a plastic mass that hardens inside the cavity.

This topic matters because amalgam is still widely used for back-tooth fillings, and every student must understand its composition and mercury safety. Strict mercury-hygiene rules govern its storage, mixing and disposal, because mercury vapour is toxic.

Key Concepts

• Alloy powder: roughly 65–70% silver, 25–29% tin, 2–6% copper, up to 2% zinc.
• High-copper alloy: 12–30% copper; eliminates the weak gamma-2 (tin-mercury) phase for higher strength and corrosion resistance.
• Mercury: liquid metal forming about 50% of set amalgam by weight; it dissolves the alloy particles (amalgamation).
• Condensation: packing the plastic amalgam into the cavity in small increments with a condenser, which squeezes excess mercury to the surface and removes porosity.
• Mercury vapour limit: 0.1 mg per cubic metre (OSHA ceiling) and 0.05 mg per cubic metre time-weighted average (NIOSH) — so spills and open handling are never acceptable.

How It Works

1. The dentist selects a pre-proportioned disposable capsule containing alloy powder and mercury in the correct ratio.
2. The capsule is triturated in the amalgamator. Mercury dissolves the surfaces of the alloy particles and the mixture becomes a cohesive, workable mass.
3. The mass is packed into the cavity in layers (condensation), pressing excess mercury to the surface.
4. Before the material sets (working time is about 3 to 4 minutes), it is carved to reproduce the natural anatomy of the tooth — cusps, grooves and contact with the neighbouring tooth.
5. After full crystallisation the filling is polished, and all scrap amalgam and used capsules go into sealed containers for mercury recycling.

Worked Example

A clinic uses single-spill capsules that each contain 400 mg of alloy powder and 400 mg of mercury. A dentist places two such fillings in one sitting.

1. Total alloy used = 400 mg × 2 = 800 mg.
2. Total mercury used = 400 mg × 2 = 800 mg.
3. Total amalgam mass = 800 mg + 800 mg = 1600 mg, which is 1.6 g.
4. Mercury percentage = (800 ÷ 1600) × 100 = 50 percent, matching the expected composition of set amalgam.

Common Misconceptions

• "Amalgam fillings release so much mercury that they poison the patient." Corrected: set amalgam releases only trace amounts of mercury vapour and is considered safe for the general population, though it is avoided in pregnancy and young children as a precaution.
• "High-copper amalgam is just a marketing name." Corrected: the extra copper removes the corrosion-prone gamma-2 (tin-mercury) phase, measurably improving strength and lifespan.
• "Amalgam scrap can go with other biomedical waste for burning." Corrected: incinerating amalgam releases mercury vapour. Amalgam waste must be collected in sealed containers for authorised recycling, and chairs should use amalgam separators.

Real-World Applications

• Posterior restorations: amalgam remains a durable, low-cost choice for molars in patients with heavy chewing loads, especially where moisture control makes composite bonding difficult.
• Core build-ups: amalgam cores under crowns use the material's strength to rebuild badly broken-down teeth before crown preparation.
• Public health dentistry: in government and rural clinics, amalgam offers an affordable long-lasting option, which is why mercury-hygiene training is part of the BDS curriculum.

In short, dental amalgam is an alloy of silver, tin, copper and about 50 percent mercury; high-copper versions are stronger and more corrosion-resistant. Safe practice means pre-proportioned capsules, good ventilation and sealed waste collection every time amalgam is used.