Alloy

The alloy used in the traditional dental amalgams consists of a mixture of silver, tin, copper and sometimes zinc and/or mercury. A typical composition may be as shown in Table 2.1.1.

Silver is the main constituent, present in combination with tin as the intermetallic compound Ag₃Sn, known commonly as the γ phase. The phase diagram for the Ag–Sn system is shown in Figure 2.1.1, and shows that the Ag₃Sn phase is the third pure phase in the system, hence the Greek symbol γ.

This γ phase reacts readily with mercury to form the dental amalgam. Copper is present to increase the strength and hardness of the amalgam, and a more pronounced effect is produced when the copper content is increased beyond 6%, but this will be dealt with later. Zinc may be present as a result of the initial production of the alloy, and is not considered to serve any useful purpose in the amalgamation process. Mercury is sometimes added to provide a more rapid reaction, in what is referred to as pre-amalgamation.

The alloy is used in the form of a powder, and the size and shape of the particles in this powder are critical to the handling characteristics and the final properties of the restoration. The alloy powder is available as either lathe-cut particles or spherical particles, as shown in Figure 2.1.2.

Setting reaction

The setting reaction between the Ag–Sn alloy and the mercury is initiated by a vigorous mixing of the two ingredients. This mixing causes the outer layer of the alloy particles to dissolve into the mercury, forming two new phases, which are solid at room temperature. The reaction is as follows:

As can be seen from the reaction, not all of the alloy particles dissolve in the mercury. On the contrary, a considerable amount remains, so that the final structure is one of a core of γ held together by a matrix of predominantly γ₁, which is interspersed with γ₂. The structure of the set material is shown in Figure 2.1.3.

The copper in the lathe-cut alloy is present in the form of discrete areas of Cu₃Sn, and remains mainly within the original alloy in its unreacted form.

In the case of the spherical particles, the copper is uniformly distributed, and the alloy could be more accurately regarded as a ternary alloy of silver, tin and copper. Hence, in the final structure of the spherical alloy amalgam, the copper is not present as a discrete phase but is widely distributed throughout the material. Although some voids will inevitably be present, in a well-condensed amalgam there will be very little porosity.

Strength

The strength of an amalgam is extremely important, since the restoration has to be able to withstand the considerable loads generated during mastication, and any lack of strength is likely to lead to marginal ditching of the restoration or even gross fracture.

Although most attention has been paid to the final compressive strength of the set material, it is perhaps more important to consider the tensile strength and the rate at which the final strength is acquired.

As might be imagined, the final strength of the amalgam will be a function of the properties of the individual phases. It is not easy to determine the properties of the three main phases of an amalgam, but micro-hardness measurements suggest that the γ phase and the γ₁ phase have a similar hardness, while the γ₂ phase is considerably softer. The tensile strength of the γ₂ phase has also been measured to be only a fraction of that of the original γ phase, with the γ₁ phase falling in between (Table 2.1.2).

This means that the weak link within the amalgam structure is the γ₂ phase, and if its proportion in the final composition could be minimized, a stronger amalgam would result. The amount of γ₁ and γ₂ formed is strongly dependent on the amount of mercury in the final composition. The higher the mercury content, the weaker the material will be, because larger amounts of the weaker phases will be produced.

The final mercury content of the amalgam is dependent on the quality of the condensation technique more than anything else, with a properly condensed amalgam having a mercury content of just less than 50%. Besides the condensation technique, the size and shape of the alloy particles will also affect the final mercury content. The initial ratio of alloy to mercury is lower in amalgam made with spherical alloy particles than with lathe-cut alloy particles because the material is more easily condensed. With spherical alloy particles, a final mercury content of about 45% is readily achievable.

The particle size is also important. For a given amount of alloy that is to be amalgamated with mercury, choosing smaller alloy particles results in more of the alloy surface being exposed to the mercury. This means that more of the alloy will dissolve in the mercury, producing more of the mercury-containing phases. Consequently, too small a particle size is contraindicated.

Whatever the form of the alloy used, the conscientious removal of excess mercury during the placement of a restoration is vitally important.

Flow and creep

It has been postulated that the excessive flow of an amalgam, resulting from repeated occlusal loading, can cause flattening of contact points, overhanging margins, and protrusion of the restoration from the tooth surface at the margin. The latter has been implicated as a major source of marginal breakdown. Although flow is measured for amalgam in laboratory tests, the measurement is usually carried out over a short period very soon after mixing and is therefore of limited clinical relevance. A more appropriate measurement would be that of creep. This is the flow caused by loads acting over long periods. Creep is dependent on both the yield strength of the material and the temperature of the environment, and only becomes a serious problem when the environmental temperature is greater than half the melting temperature of the material.

Since the amalgam phases have very low melting temperatures (about 80°C) and the restorations are subjected to repeated loadings, there is the possibility of creep occurring. The phases most prone to creep will be the mercury-based γ₁ and γ₂ phases. Consequently, the lower the proportion of these phases present (as may be achieved by proper condensation), the less susceptible the amalgam will be to creep.

Corrosion

It is well recognized that amalgams corrode in the oral environment. Indeed, corrosion is often cited as an advantage, in that the corrosion products help to produce a good marginal seal. However, crevice corrosion, caused by the formation of an oxidation cell in the marginal gap, can cause a rapid deterioration in the properties of the amalgam. The corrosion process is especially associated with the γ₂ phase.

The γ₂ phase is considerably more electronegative than the γ and γ₁ phases. This means that, in the presence of an electrolytic solution, the γ₂ phase will act as the anode of the oxidation cell and will gradually dissolve. The reaction is as follows:

tin-mercury phase + oral fluids → tin salts + free mercury

Sn₇Hg + oxygen → oxides & chlorides + Hg

Normally, the formation of oxides would help to slow down the corrosion process by forming a protective surface coating. However, in the gap between the amalgam and the tooth tissues, a surface oxide is not formed, as the reaction products from the corrosion process precipitate out. The process is also very insidious, since the production of free mercury allows further reaction with γ, and the formation of more γ₁ and γ₂. This process will severely weaken the amalgam structure, and is often cited as a cause of marginal breakdown.

Dispersed phase amalgams

In the early 1960s, attempts were made to increase the strength of dental amalgams by increasing the copper content of the alloy.

The idea was that the copper would act as a dispersion-strengthening agent. A spherical alloy (basically silver and copper) with a high copper content was added to the conventional lathe-cut alloy. The choice of the spherical particles was made essentially because it was easier to alter the composition of spherical particles than lathe-cut particles on an experimental basis. The potential advantages, in terms of easier condensation, were also recognized at that time.

As it turned out, the increase in the copper content of the alloy resulted in a modification of the setting reaction, which proved to be highly beneficial.

The first reaction is the same as for the traditional alloys, but this is followed by a second reaction:

γ2+Ag −− Cu→Cu6Sn5+γ1

Thus, the final amalgam contains little or no γ₂. The structure of this amalgam is shown in Figure 2.1.4.

Initially, it was thought that all of the γ₂ was eliminated by this reaction, but it has since been recognized that some γ₂ will remain, although it is only a small and probably insignificant amount.

The modification in the setting reaction has resulted in a number of interesting and important changes in the properties of the amalgam, namely:

Other high-copper-content formulations

With these sorts of improvements, it was not long before the idea of the all-spherical high-copper amalgams came into being. These combine all of the advantages of easier condensation with those mentioned above, and a number of such products are now available.

The powder in these cases is a ternary spherical alloy of silver, tin and copper and has a setting reaction as follows:

Ag −− Sn −− Cu+Hg→Ag −− Sn −− Cu+γ1+Cu6Sn5

The reluctance of many dentists to convert from a lathe-cut alloy to a spherical alloy has led to the introduction of high-copper-content all-lathe-cut alloy formulations. The composition of these alloys is essentially the same as for all of the spherical alloys, except that there are wide differences in the total copper content, which can vary from 12 to 30%. As yet, it is not known what the optimum percentage of copper is.

For those dentists who prefer the dispersed-phase type of alloy, there are now a number available that have a mixture of spherical and lathe-cut particles, where both particle types have the same composition of ternary Ag–Sn–Cu alloy.

Manufacturer’s variables

The variables under the control of the manufacturer are the composition and the particle size and shape of the alloy.