Amalgam

Definition: Dental amalgam is a metallic restorative material formed by the chemical reaction of mercury (Hg) with an amalgam alloy powder (mainly silver–tin, plus copper, sometimes zinc). Strictly, the term amalgam refers to the set product after mercury has reacted with the alloy. It is non-adhesive, sets by metallurgical reactions (not polymerisation), and is primarily used for durable posterior restorations.

Clinical Case

A 47-year-old patient presents with recurrent marginal breakdown around an old large occluso-proximal restoration on a lower first molar. The tooth is symptom-free, the patient has moderate caries risk and bruxism, and isolation is achievable. You replace the restoration with a high-copper amalgam, using a properly adapted matrix, incremental condensation with firm pressure to reduce residual mercury, careful carving to avoid thin margins, and delayed finishing/polishing to optimise marginal integrity.

Uses

• Direct posterior restorations where durability is a priority

• Core build-ups (less common now)

• Complex posterior restorations where moisture control is challenging
  

How to use amalgam in clinical practice

1. Case selection: posterior load, moisture control, aesthetics

2. Cavity preparation: mechanical regention (since amalgam is non-adhesive), avoid undermined enamel, ideally 90° cavosurface angle where possible

3. Matrix and wedge

4. Trituration

5. Condensation → to remove excess Hg

6. Carving and burnishing

7. Finishing and polishing: often delayed 1 week until amalgam is strong enough

8. Post-op: check occlusion, advise on early loading, review contact and margins

Tips:

• Bulk matters: amalgam is brittle and weak in tension → avoid thin sections and acute margin angles

• Mercury content matters: higher residual mercury → worse strength/creep/corrosion

• Condense properly

Constituents

Amalgam alloy powder:

• Silver (Ag): main strengthening constituent; present largely as Ag₃Sn (gamma, γ)

• Tin (Sn): required for the Ag-Sn intermetallic compound; influences reaction and corrosion behaviour

• Copper (Cu): improves strength and reduces/eliminates the Sn₇Hg (gamma₂, γ₂) phase

• Zinc (Zn) (optional): not essential to setting but historically included; can be associated with delayed expansion due to a reaction of Zn with H₂O producing H₂ gas, especially if moisture contamination occurs

Forming the powder:

The amalgam alloy powder is available as either lathe-cut particles, or spherical particles.

1. Lathe cut: produced by maching a solid ingot of alloy

   • Produces irregular shaving

   • Generally need higher Hg for a workable mix

   • Can be harder to condense

2. Spherical: alloy ingredients melted together then sprayed into an inert atomsphere causing droplets to form as spherical pellets of various sizes.

   • Easier condensation

   • Often allows lower Hg in final restoration

3. Admixed: blend of lathe-cut and spherical particles (common in high-copper amalgams)

Liquid:

• Mercury (Hg): must be very pure (classically triple distilled) because surface contaminants interfere with the reaction and handling

Chemistry

Key phases:

• Gamma (γ): Ag₃Sn (unreacted alloy core)

• Gamma₁ (γ₁): Ag₂Hg₃ (silver-mercury matrix; relatively stronger than gamma₂)

• Gamma₂ (γ₂): Sn₇Hg (tin-mercury phase; weak and corrosion prone)
  

Setting reaction (traditional amalgams):

• Ag₃Sn + Hg → Ag₂Hg₃ + Sn₇Hg + Ag₃Sn

  • (γ + Hg → γ₁ + γ₂ + γ)

What it means structurally: the surface of γ (Ag₃Sn) dissolved into mercury during trituration; γ₁ and γ₂ precipitate, binding unreacted γ cores into a solid mass. The more residual mercury, the more matrix phases (especially problematic γ₂) and the weaker and more corrosion-susceptible the restoration is.

In traditional (low-copper) amalgams, setting can be understood as a staged dissolution–precipitation process. Immediately after trituration, there is:

1. Initial dissolution: the outer surface of the Ag–Sn (γ, Ag₃Sn) alloy particles dissolves into the liquid mercury, liberating silver and tin into solution (Ag₃Sn + Hg → Ag + Sn + Hg).

2. Gamma₁ (γ₁) formation happens first and fast → the released silver reacts rapidly with mercury to precipitate Ag₂Hg₃, which forms preferentially along and around the alloy particles, effectively “cementing” them together.

3. Gamma₂ (γ₂) forms later and more slowly → the free tin released during dissolution gradually reacts with mercury to form Sn₇Hg, which ends up more randomly distributed within the developing γ₁ matrix.

4. Set amalgam: the amalgam is considered set once essentially all available mercury has been consumed into γ₁ and γ₂, leaving a solid matrix binding unreacted γ (Ag₃Sn) cores; with no free mercury remaining, there is no further reaction with γ beyond this point.

Setting reaction (high-copper amalgams):

1. Step 1 (same initial reaction as traditional)

   • Ag₃Sn + Hg → Ag₂Hg₃ + Sn₇Hg + Ag₃Sn

     • (γ + Hg → γ₁ + γ₂ + γ)

2. Step 2 (high-copper "γ₂-elimination" reaction

   • Sn₇Hg + Ag-Cu → Cu₆Sn₅ + Ag₂Hg₃

     • (γ_{2 +} ε(Ag-Cu) → η(Cu₆Sn₅) ₊ γ₁)