Introduction
Dental amalgam is mercury alloyed with a silver–tin–copper powder, condensed into the cavity while plastic and setting in the mouth. Its phase chemistry explains why modern high-copper alloys last longer, and its handling decides whether a restoration fails early.
Composition and types of alloy
Alloy powder is mixed with liquid mercury (Hg), today in pre-dosed capsules at a mercury: alloy ratio of about 1:1; the set filling contains about 40–50% Hg. Above about 55% Hg, strength falls and creep rises.
- Silver (Ag, 40–70%): strength, less creep and corrosion; with tin forms Ag₃Sn.
- Tin (Sn): reacts readily with mercury and slows setting, but forms the weak tin–mercury phase.
- Copper (Cu): strength and hardness; in high-copper alloys (usually 12–30%) it binds tin and prevents γ₂.
- Zinc (Zn, 0–2%): deoxidiser in alloy manufacture. Moisture (saliva, blood) in a zinc-containing mix causes delayed expansion (Zn + H₂O → ZnO + H₂), starting after 3–5 days and may exceed 4%: pain, protrusion, fracture.
Particles are lathe-cut (irregular filings) or spherical; spherical ones need less mercury.