Element	Function
Silver	Main constituent of alloy, combines with tin
Tin	Combines with silver
Copper	Increases mechanical properties, decreases creep, increases corrosion resistance, decreases the amount of the γ2 phase formation
Zinc	Acts as a scavenger of oxygen
Mercury	Sometimes added to increase the rate of reaction (pre-amalgamation)
Indium, palladium, selenium, platinum, gold	All increase corrosion resistance and improve certain mechanical properties of the final product. Decrease creep

Amalgamation reaction of a conventional alloy

The mercury reacts with the outer layer (3–5 μm) of the silver–tin alloy particle in a reaction shown in Figure 6.3A. This means the bulk of the particle is left unreacted. These unreacted cores sit within a matrix of the silver–mercury and tin–mercury phases. The structure of the set material is shown in Figure 6.3B.

Fig. 6.3 (A) The setting reaction of dental amalgam. (B) The three major phases of set amalgam.

The role of zinc

Zinc was traditionally included in the alloy because it acts as an oxygen scavenger during the alloy’s initial production. It preferentially reacts with any oxygen present to produce clean castings of the ingot. Alloys containing more than 0.01% zinc are called zinc-containing alloys and those containing less than this are termed zinc-free alloys.

If a zinc-containing dental amalgam is contaminated with water from saliva during the condensation process, it produces hydrogen gas, which becomes incorporated within the setting amalgam mass and leads to excessive delayed expansion. This is clearly undesirable and may cause pain and even tooth fracture. Good moisture control is therefore highly desirable when placing dental amalgam restorations (as with any other restorative material) and, in particular, those alloys containing zinc. Most modern alloys are now manufactured in an inert atmosphere so the need for zinc has been obviated.

The gamma (γ)2 phase

The γ2 (tin–mercury, see Figure 6.3) phase is the most chemically and electrically active component of the set amalgam. Its physical properties are inferior than the γ (silver–tin) and γ1 (silver–mercury) phases and its presence in conventional amalgam alloys results in:

• Reduced strength

• Increased susceptibility to corrosion

• Increased creep (a conventional amalgam restoration shows about 6% creep).

Creep: gradual dimensional change under load. The clinical consequence of creep is ditching of the amalgam margin altering the marginal contour. This increases the potential for plaque retention (Figure 6.4).

Fig. 6.4 Amalgam restoration in tooth 15 showing long-term guttering and loss of contour.

High copper amalgam alloys (Figure 6.5)

More recently, attempts have been made to reduce or even eliminate the γ2 phase by increasing the copper content in the alloy to above 13%. This modification of the setting reaction has resulted in some important beneficial changes in the properties of the amalgam:

• Early full strength achieved within 1 hour of placement

• Increased mechanical properties, e.g. compressive strength

• Reduction in creep and therefore ditching (creep values of these alloys are between 0.09% and 1.8%)

• Reduced corrosion potential.

Fig. 6.5 Structure of a high copper amalgam alloy.

Setting reaction of high copper amalgam

The setting reaction of these alloys is the same as the reaction for conventional alloys (see Figure 6.3A). After the formation of the γ2 phase, there is a reaction between this and the silver–copper component, leading to the formation of a copper–tin phase and γ1. Although silver is present in the alloy particles, there is a preferential reaction between the copper and tin, thus forming a silver–tin–copper complex. The result is that little or no γ2 is left in the final amalgam.

Types of high copper amalgam alloy

There are two types of high copper amalgam alloy:

• Dispersed phase alloys – in these alloys, the copper is dispersed throughout the alloy and has the effect of increasing its strength. The alloy particles consist of either silver–tin or silver–copper. This means that the total silver content may be kept the same as in conventional alloys.

• Ternary alloys – ternary means a compound containing three elements, usually silver, tin and copper in this case. Ternary alloys are sometimes called unicompositional alloys as they have all three components in one particle.

Many manufacturers reduce the silver and tin content when increasing the copper content. A small amount of zinc is sometimes added to high copper amalgam to improve its clinical performance, in particular by decreasing marginal breakdown. Table 6.2 gives the general composition of the three types of alloy.

Table 6.2 Typical percentage composition of amalgam alloys by weight

Implications of the composition of the alloy for corrosion

The susceptibility of the γ2 phase to corrosion offers a clinical advantage. The corrosion products fill the microgap (marginal gap) at the tooth–amalgam interface, which helps to decrease microleakage. One drawback of the high copper amalgam is that the microgap persists due to lack of the γ2 phase, and so these alloys have been associated with greater microleakage. Clinically, this may manifest as increased postoperative sensitivity with a risk of recurrent caries. It has been suggested that an intermediate bonding agent should be used in combination with these alloys to seal the dentine at the microgap (see p. 61).

Microleakage: leakage of minute amounts of fluids, debris, and microorganisms through the microscopic space between a dental restoration or its cement and the adjacent surface of the cavity preparation (adapted from Mosby’s Medical Dictionary, 8th edition. © 2009, Elsevier).

Types of alloy

Currently, three types of dental amalgam alloys are available: lathe cut, spherical and admixed (Figure 6.6). Their handling characteristics are all very different and it is important that they are manipulated correctly when used in the clinic for optimal performance of the set product (see pp. 63–65).