Historically, the silver alloy used in dental amalgams had low (2 to 4 wt %) amounts of copper. These alloys were combined with mercury to form “low-copper amalgam.” However, the development of silver alloys with higher (13% to 30%, generally about 20%) copper (Figure 5-3) has replaced low-copper alloys because high-copper alloys produce amalgams with higher strength, less corrosion, less creep, and better longevity at the margins (edges of the cavity preparation). Other elements such as palladium and indium are sometimes included to reduce corrosion. Today’s high-copper silver alloys are essentially free of zinc (<0.01 wt % per ADA Specification No. 1) because zinc causes a significant, long-term, and clinically unacceptable expansion of the amalgam if it is contaminated with saliva or other moisture during placement.

Particles of the silver alloy powder can be either irregularly shaped, spherical, or a mixture of the two (Figure 5-4). The shape of these particles will significantly influence the setting reaction and manipulation of the amalgam. If a mixture of particles is used, the resulting amalgam is referred to as an admixed amalgam. Amalgams containing irregular particles with or without spherical particles added require greater packing or condensation forces during placement than amalgams with spherical particles alone. Most practitioners feel that amalgams containing irregular particles produce better proximal contacts and are easier to carve. Spherical amalgams, containing only spherical alloy particles, require less mercury and set somewhat faster. Practitioners generally select either a spherical or admixed amalgam, depending on the clinical situation. The importance of proper manipulation of each type cannot be overemphasized. For example, if a condensation force that is appropriate for a spherical amalgam is applied to an admixed amalgam, the restoration will likely contain voids and lack adequate proximal contacts.

Today, nearly all practitioners use silver alloy powder and mercury sealed into disposable hard plastic capsules (Figure 5-5). Both the mercury and alloy powder are apportioned by the manufacturer. The mercury is sequestered from the alloy powder by a plastic membrane inside the capsule that is easily ruptured during mixing. Sometimes, a small inert cylinder called a pestle is added to facilitate mixing. This precapsulated method provides the proper ratio of alloy powder to mercury, limits handling of pure mercury by the dental team, produces amalgams with consistent physical properties, and keeps the mercury clean.

The reaction of the silver alloy with mercury is called amalgamation. When high-copper alloy particles contact the mercury, the copper, silver, tin and other elements in the alloy dissolve into the mercury much as sugar would dissolve in water (Figure 5-6). Dissolution rates vary for each element and occur from the surface of the alloy particles. Almost immediately, new solid products begin to crystallize as the dissolved elements react with the mercury. The crystallization of new products continues until all the liquid mercury is consumed, and the amalgam becomes stiffer and eventually hardens completely. The ratio of alloy to mercury is formulated so that the hardening of the amalgam occurs before all of the original alloy particles can dissolve. Thus, the set amalgam contains many of the original alloy particles surrounded by several new products of mercury with tin, copper, or silver (see Figure 5-6). The reaction products are collectively called the matrix of the amalgam. In the set mass of most contemporary amalgams, the original particles comprise more than 50% of the volume and are important to the strength and corrosion resistance of the clinical restoration.

Once the amalgamation reaction is complete, little or no mercury remains unreacted. In its reacted state, mercury is not easily released from the amalgam. This fact is important because free mercury can pose a health risk if it occurs in sufficient concentrations. In practice, minute amounts of mercury vapor (approximately 1 to 2 µg per day) are released from dental amalgams as a result of chewing. Higher release may occur during the setting reaction, during removal of old amalgams, or if the amalgam is heated above 80 °C (Figure 5-7). To further limit mercury release during the setting reaction, newer mercury-indium liquid alloys have been substituted for pure mercury in a few products.

Early amalgams were much lower in copper (2 to 4 wt %) than contemporary amalgams. In low copper amalgams, the chemical reaction produced a tin-mercury product in the matrix called gamma-2 (γ²). Gamma-2 had poor corrosion properties that significantly shortened the clinical lifetime of the amalgams. The current high-copper amalgams have eliminated the gamma-2 product; high-copper amalgams have greatly increased resistance to corrosion and marginal breakdown in clinical use.

Clinically relevant properties of amalgam include strength, dimensional change, creep, tarnish, and corrosion. Some of these properties are listed for spherical and admixed high-copper amalgams in Table 5-2. Several examples of commercial products are listed in Table 5-3.