# Amalgam Restorations > Amalgam restorations for BDS Operative Dentistry — composition, phases, trituration, condensation, carving and mercury hygiene explained. - Canonical URL: https://prepelephant.com/topics/bds/operative-dentistry/amalgam-restorations - Exam / course: BDS · Subject: Operative Dentistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Amalgam Restorations", PrepElephant, https://prepelephant.com/topics/bds/operative-dentistry/amalgam-restorations ## Direct answer Gamma, gamma-1 and gamma-2 are the three phases of set amalgam: the silver–tin gamma (Ag3Sn) particles react with mercury to form the gamma-1 (Ag2Hg3) matrix binding unreacted particles, while in low-copper alloys a weak tin–mercury gamma-2 phase forms and corrodes. High-copper alloys — admixed (Dispersalloy type) and single-composition (Tytin type) — eliminate most gamma-2 by reacting tin with copper, giving higher early strength, creep below 1% and better margins, which is why they replaced low-copper alloys. Clinical success rests on proportioned capsule trituration, incremental condensation beginning in the proximal box with firm lateral force, precarve burnish, anatomical carving and delayed polishing at 24 to 48 hours. ## What you must remember - Composition: low-copper alloys are silver–tin with small copper and zinc; high-copper alloys contain 10–30% copper as silver–copper eutectic particles (admixed) or as a ternary particle (single composition). - Setting reaction: gamma Ag3Sn + mercury forms gamma-1 Ag2Hg3 plus gamma-2 (tin–mercury) in low-copper alloys; copper reacts preferentially with tin in high-copper alloys so gamma-2 is nearly eliminated. - ADA/ISO benchmark: minimum compressive strength 80 MPa at one hour; high-copper alloys commonly exceed 400 MPa at 24 hours; creep below 3% by specification, typically under 1% for high-copper. - Trituration: mechanical amalgamator with timed capsule mixing; under-triturated mix is dull, crumbly and weak; over-triturated mix is wet, sticky and also weakened. - Zinc-containing alloys contaminated by moisture expand late (delayed expansion, hydrogen from zinc–water reaction); non-zinc alloys suit moisture-exposed fields such as subgingival margins. - Condensation: carry small increments, the first into the gingival angle of the proximal box with the smallest condenser nib that fits; spherical alloys need lighter force with a larger nib, admixed alloys need heavier force with a smaller nib. - Precarve burnishing with heavy strokes improves marginal adaptation and marginal integrity by pressing fresh amalgam against walls. - Carving reproduces anatomy immediately; polishing is deferred 24–48 hours so the set is complete — polishing a fresh amalgam weakens it and draws mercury-rich material to the surface. - Mercury hygiene: capped capsules, no squeeze-in-hand technique (it adds free mercury), high-volume evacuation, scrap stored under fixer or water in a sealed container, and amalgam separators for chairside effluent. ## Placing a Class II amalgam, stepwise Matrix and wedge seated, cavity lined or based as depth demands, the capsule is triturated for its specified time and tapped to gather the mix. Carry the first small increment to the proximal box and condense it into the gingival angle with the smallest condenser face that fits, using firm lateral strokes against the axial wall and gingival floor — the margins no instrument reaches directly are adapted by the pressure transmitted through the mass. Build the box in small increments, each condensed over the whole previous layer, then move to the isthmus and occlusal step before the mix loses plasticity: a mix that has stiffened cannot be condensed, only crammed. Overfill slightly, run the precarve burnisher heavily from tooth to amalgam across the margins, then carve with a discoid–cleoid or Hollenback: marginal ridge height matched to the neighbour, occlusal anatomy defined but shallow, margins cleared of flash. Check the occlusion with articulating paper once carving is complete, and recall at 24 to 48 hours for polishing with finishing burs, fine stones and rubber cups charged with polishing paste. ## Where students slip The exam trap is phase chemistry: asked "why are high-copper alloys superior", the complete answer is copper's preferential reaction with tin, eliminating the corrosion-prone gamma-2 phase — improving strength, creep and marginal integrity — not merely "more copper is stronger". The second error is condensation timing and force: waiting beyond the working time or using large increments traps mercury-rich matrix and laminates the mass, and examiners spot the fault in carved restorations that crumble at the isthmus. Third, polishing early: students polish at placement to finish the practical exam quickly; the correct statement is that polishing before full set (under 24 hours) weakens margins, hence the 24–48 hour rule. Finally, the free-mercury question in viva — the old squeeze-the-amalgam-in-a-gauze technique raises mercury content and is abandoned in favour of pre-proportioned capsules. ## Frequently asked questions ### Which phase of amalgam is weakest and why? The tin–mercury gamma-2 phase of low-copper alloys — it is the most corrosion-prone and weakest, so high-copper alloys are formulated to eliminate it. ### What is the minimum one-hour compressive strength by specification? 80 MPa, which allows same-day carving and function; high-copper alloys comfortably exceed this and reach 400 MPa or more by 24 hours. ### How do under- and over-trituration affect the mix? Both weaken the set amalgam: an under-triturated mix is dull and crumbly with unmixed particles, an over-triturated mix is sticky and wet, sets rapidly and is likewise weakened. ### Why is polishing delayed for 24 to 48 hours? The alloy continues gaining strength as setting proceeds; polishing early drags mercury-rich weak material across margins and can fracture them. ### How is a deep cavity managed under amalgam? A calcium hydroxide liner over the deepest dentine near the pulp, then a base (glass ionomer or zinc phosphate type) thick enough — about 1–2 mm — to insulate against thermal conduction.