Silver-based amalgam alloys are classed according to the shape of the particles in the powder as irregular, admixed, or spherical (Figure 9-1). Each of these particle shapes contributes certain handling characteristics to the amalgam, and to some degree the amalgam type is selected by the dentist according to these characteristics. Irregularly shaped particles are formed by shaving fine particles off an ingot of the alloy with a lathe (lathe-cut alloy). The particles are sifted to separate them into fine and ultrafine particles. Spherical particles are produced by spraying molten alloy into an inert gas (spherical alloy). Spherical particles are formed as the atomized droplets cool. Admixed particles consist of a mixture of lathe-cut and spherical particles (admixed alloy).

Modern dental alloys are considered to be high copper in content (13% to 30%) compared with their predecessors, which had 4% to 6% copper by weight (Table 9-1). They generally contain 40% to 70% silver and 12% to 30% tin. They are mixed with mercury 43% to 50% by weight. Spherical alloys require less mercury to wet the particles and generally set faster than lathe-cut particles. Manufacturers may also add indium (1% to 4%), palladium (0.5%), and zinc (0.01% to 2%). Zinc may inhibit corrosion by reducing the oxidation of the other metals in the amalgam. In the low-copper amalgams, zinc was responsible for gradual expansion of the amalgam over time (delayed expansion) when moisture contamination was present during placement. Contact of moisture with the zinc caused the formation of hydrogen gas within the amalgam, which caused it to expand. Delayed expansion could cause the restoration to expand beyond the cavity walls, causing cracking in the adjacent enamel. Because of this undesirable property, most high-copper alloys contain no zinc or very low levels of zinc and are called zinc-free alloys.

Tarnish is an oxidation that attacks the surface of the amalgam and extends slightly below the surface. It results from contact with oxygen, chlorides, and sulfides in the mouth. It causes a dark, dull appearance, but it is not very destructive to the amalgam (Figure 9-2). The rougher the surface, the more it tends to tarnish. Metals such as palladium are sometimes added to help reduce the tarnish. Polishing of the restoration can also reduce the tarnish. Polishing of amalgams is best done after the restoration has set for a period of 24 hours or longer. Some clinicians have advocated polishing fast-set amalgams in as little as 20 minutes after placement. However, amalgams polished this soon after placement usually do not achieve a high shine. High-copper amalgams have a smoother surface after carving than low-copper amalgams and tend to tarnish less. Polishing is not as critical to their longevity as with low-copper amalgams. Because of this fact, controversy exists among dental educators and clinicians as to whether high-copper amalgams need polishing if they are well carved and contoured at the time of placement. Generation of excessive heat during polishing can cause a release of mercury from the silver-mercury phase that results in a mercury-rich surface that will corrode more readily and deteriorate at the margins.

Corrosion can occur from a chemical reaction between the amalgam and substances in saliva or food, resulting in oxidation of the amalgam. It can also occur when two dissimilar metals interact in a solution containing electrolytes (saliva is such a solution). An electrical current is generated between the metals (much like a battery) in a process called galvanism. The result of the galvanic reaction is oxidation of one of the metals. This oxidation is responsible for the corrosion of the amalgam. Corrosion also takes place within the amalgam through interaction of its metal components. It weakens the amalgam over time, can stain surrounding tooth structure as corrosion products enter the dentinal tubules, and can lead to deterioration of the margins (Figure 9-2). The high copper content of newer alloys eliminates the formation of the gamma-2 reaction product that caused weakening of the amalgam. High-copper alloys have virtually replaced low-copper alloys, because high-copper amalgams are more durable with less deterioration at the margins (better marginal integrity), less corrosion, and higher strength.

Clinically, a galvanic reaction may occur when a newly placed amalgam contacts another metal restoration such as a gold crown. The patient feels a mild electrical shock and may experience a metallic taste. This problem may persist until the amalgam completes its setting reactions, until enough oxides build up on one of the metals to stop the electrical current, or until the offending restoration is replaced with a nonconducting restoration such as composite or with a restoration of a metal similar to the one next to it. Corrosion, however, can occur within an amalgam without the patient ever being aware of the process.

Ideally, the dimensions of a newly placed amalgam should not change. If it contracts excessively, it will open gaps at the margins, contributing to leakage of fluids and bacteria and causing sensitivity. If it expands excessively, it can put pressure on the cusps and cause pain with biting pressure or may result in fracture of the cusps. Some expansion and contraction occur during the setting reaction of the amalgam. It is the net effect of these two processes that is important. The composition of the alloy particles, the ratio of the mercury to alloy powder by weight, and salivary/moisture contamination are other factors that contribute to dimensional changes. The low-copper amalgam, especially those containing tin, were prone to expansion over time if they were exposed to moisture during placement. This gradual expansion after placement is called delayed expansion (Figure 9-3).

Amalgam is among the strongest of the directly placed restorative materials. It has the ability to resist the strong forces of the bite repeatedly over many years when it is properly placed. Amalgams are stronger in compression (approximately 400 to 450 MPa) than composites or glass ionomers. However, they are relatively weak in tension (about 12% of compressive strength) and shear and therefore require adequate bulk to resist breaking. If thin excesses of amalgam are left over the cavosurface margins, they will chip away over time, creating an irregular margin that tends to collect plaque and contribute to recurrent caries. Generally, high-copper amalgams have a higher early compressive strength (1 hour) than low-copper amalgams. This is advantageous because it helps resist breakage if the patient inadvertently bites on a newly placed amalgam. Some high-copper amalgams gain approximately 80% of their strength in the first 8 hours. Low- and high-copper amalgams are comparable in compressive strength once they have completely set at about 24 hours. See Table 9-2 for American National Standards Institute/American Dental A/>