a combination of these, suitable for mixing with liquid mercury to produce the dental amalgam. Amalgam is used most commonly for direct, permanent, posterior restorations and for large foundation restorations, or cores, which are precursors to placing crowns. The reaction between mercury and alloy when mixed together is termed an amalgamation reaction. It will result in the formation of a silver-grey workable mass which can be condensed into cavities. After condensing, the dental amalgam is carved to generate the required anatomical features and then hardens with time. The standard composition of alloy prior to 1986 is referred to as conventional amalgam alloy. More recently (post-1986), there has been a change in the compositional standard of the alloy due to better understanding of structure-property relationships for the materials. Conventional amalgam alloy commonly consists of silver (~65% ), tin (~29%), copper (~8%) and other trace metals; current amalgam alloy consists of silver (40%), tin (32%), copper (30%) and other metals. Metallurgy of amalgam To fabricate an amalgam filling, the dentist uses a mixing device to blend roughly equal parts (by mass) of shavings of a silver-base alloy with mercury until the shavings are thoroughly wetted. The silver alloy is typically 40–70% Ag, 25-29% Sn, 2–40% Cu and 0–2% Zn (when the alloy is formulated Zn is a scavenger and is mostly consumed during melting and lost as oxide). The dentist packs the plastic mass, before it sets, into the cavity. The amalgam expands ≈0.1% over 6–8 hours on setting. The final structure is a metal matrix composite, where γ1, η and γ2 phases, are a matrix for unreacted original alloy, minus the fast-reacting β-phase and excess Sn. Properties of amalgam Amalgam is a mixture of two or more metals (alloy) with mercury which has been purified first by distillation to remove impurities. Currently, major components of the alloy are silver, tin, and copper. The composition of the alloy powder is controlled by ISO standard for dental amalgam alloy (ISO 1559) to control the properties of amalgam. Plastic deformation (creep) Creep or plastic deformation happens when subjected to intra-oral stresses such as chewing or grinding. Creep causes the amalgam to flow and protrudes from the margin of the cavity forming unsupported edges. "Ditch" is formed around the margins of the amalgam restoration after fracture due to amalgam creep at the occlusal margins. The γ2 phase of amalgam is primarily responsible for high values