The Silver-Tin System

Figure 15-6 is an equilibrium-phase diagram of the silver-tin alloy system. Conventionally, starting from the left of the phase diagram, each phase is designated with a Greek letter in alphabetical order from 0% Sn to 100% Sn. The ratio of silver to tin in Table 15-1 shows that amalgam alloys have a narrow range of compositions, falling within the β + γ and the γ phases of the diagram shown in Figure 15-6.

Low-Copper Alloys

Silver-tin alloys are quite brittle and difficult to blend uniformly unless a small amount of copper is substituted for silver. Within the limited range of copper solubility, an increased copper content hardens and strengthens the silver-tin alloy. The chief function of zinc in an amalgam alloy is to act as a deoxidizer, which is an oxygen scavenger that minimizes the formation of oxides of other elements in the amalgam alloys during melting. Alloys without zinc are more brittle, and their amalgams tend to be less plastic during condensation and carving. The American Dental Association (ADA) Specification No. 1 for amalgam alloys allows some mercury in the alloy powder.

The ranges of conventional alloy composition by weight in the early 1980s were 66.7% to 71.5% silver, 24.3% to 27.6% tin, 1.2% to 5.5% copper, 0% to 1.5% zinc, and 0% to 4.7% mercury. The structure of these conventional alloys was dominated by Ag₃Sn (γ phase) with some Cu₃Sn (ε phase).

High-Copper Alloys

The first high-copper alloy was formulated by mixing one part of silver-copper, spherical eutectic (Ag-Cu; 71.9% silver and 28.1% copper by weight) particles to two parts of Ag₃Sn, provided as lathe-cut particles. This modification raises the copper content to 11.8% by weight. This is often called “dispersed-phase alloy” or “admixed high-copper alloy.”

A second type of high-copper alloy was made by melting all components of the dispersed phase alloy. This process yields a single composition system. The presence of the higher copper content makes mechanical cutting of ingots into particles difficult. Thus, they are often provided in a spherical form that is produced by an atomization process. The copper content of this group of alloys can be as high as 30% by weight. Various amounts of indium or palladium have been included in some commercial systems.

Gallium-Based Alloys

In an attempt to eliminate mercury from direct metallic restorative materials, gallium, which is also a liquid when alloyed with indium and tin at room temperature, has been considered as a substitute. Like mercury, this metal element can be triturated with alloys for high-copper amalgam.

Lathe-Cut Powder

To produce this powder, an as-cast ingot is first annealed to retain a uniform phase and then it is placed in a milling machine or in a lathe to be fragmented by a cutting tool or bit. The powders obtained from cutting are acid-washed to produce a more reactive surface. Since the stresses induced into the particle during cutting are not uniform and can slowly be self-relieved over time, they will cause the performance of the amalgam to be inconsistent. However, a stress-relief process can be performed by annealing the powder particles at a moderate temperature.

Atomized Powder

The liquid metal is atomized into fine spherical droplets of metal in a chamber of inert gas. If the droplets solidify before hitting a surface, the spherical shape is preserved as spherical powders. Like the lathe-cut powders, spherical powders are given an annealing heat treatment and surface washing.

Particle Size

The average particle sizes of modern powders range between 15 µm and 35 µm. Smaller particles greatly increase the surface area per unit volume of the powder. A powder containing tiny particles requires a greater amount of mercury to form an acceptable amalgam. It is critical to maintain an optimal particle size and size distribution.

The particle size distribution can affect the character of the finished surface. When the amalgam has partially hardened, the tooth anatomy is carved in the amalgam with a sharp instrument. During carving, the larger particles may be pulled out of the matrix, producing a rough surface. Such a surface is probably more susceptible to corrosion than a smooth surface. A smaller average particle size tends to produce a more rapid hardening of the amalgam with greater early strength.

Lathe-Cut Powder versus Spherical Powder

Amalgams made from lathe-cut powders or admixed powders tend to resist condensation better than amalgams made entirely from spherical powders. Since freshly triturated amalgams from spherical powders are very plastic, one cannot rely on the pressure of condensation to establish a proximal contour for a class II restoration. Spherical alloys require less mercury than typical lathe-cut alloys because spherical alloy powder has a smaller surface area per volume ratio than does the lathe-cut powder. Amalgams with low mercury content generally have better properties.

Admixed Alloys

When mercury reacts with an admixed powder, silver in Ag-Cu spheres and silver and tin from Ag-Sn particles dissolve into the mercury. Whereas both γ₁ and γ₂ crystals form, as in lathe-cut alloys, the tin in mercury diffuses to the surfaces of the Ag-Cu alloy particles and reacts with the copper to form a layer of η′ phase crystals on the surface. The η′ layer on the surface of Ag-Cu alloy particles also contains γ₁ crystals, since γ₁ and η′ phases form simultaneously. As in the low-copper amalgams, γ₁ is the matrix phase (i.e., the phase that binds the unconsumed alloy particles together). In this reaction, the γ₂ phase does form along with the η′ phase but later reacts with copper from Ag-Cu particles, yielding additional η′ phase (the secondary solid-state reaction in Figure 15-7). The γ₂ phase can be eliminated with at least 11.8% of copper by weight in the alloy powder.

Figure 15-10 illustrates the microstructure of an admixed amalgam. Included in this structure are the γ phase particles, Ag-Cu particles, ε particles, the γ₁ matrix, and η′ reaction layers. In some admixed amalgams, a small number of the η′ crystals are also found amid the γ₁ matrix.

Single-Composition Alloys

The major components of single-composition particles are usually silver, copper, and tin. The copper content of various single-composition alloys ranges from 13% to 30% by weight. In addition, small amounts of indium or palladium are included in some of the single-composition alloys. A number of phases are found in each single-composition alloy particle, including the β phase (Ag-Sn), γ phase (Ag₃Sn), and ε phase (Cu₃Sn). Some of the alloys may also contain some η′ phase.

When triturated with mercury, silver and tin from the Ag-Sn phases dissolve in mercury. Very little copper dissolves in mercury. The γ₁ crystals grow, forming a matrix that binds together the partially dissolved alloy particles. The η′ crystals are found as meshes of rodlike crystals at the surfaces of alloy particles, dispersed in the matrix. In most single-composition amalgams, little or no γ₂ forms (Figure 15-7). Figure 15-11 shows the microstructure of a typical single-composition amalgam. This structure includes unconsumed alloy particles (P), γ₁ grains (G1), and η′ crystals (H).

Figure 15-12, A, shows a scanning electron micrograph of a high-copper single-composition amalgam fractured a few minutes after condensation when the amalgamation reaction was still taking place. Two kinds of crystals are seen on the surface: polyhedral crystals (γ₁), shown by arrow A, between the unconsumed alloy particles, and meshes of η′ rod crystals, shown by arrow B, for the unconsumed alloy particles.

Figure 15-12, B, shows details of the marked areas in Figure 15-11, A. In addition to a mesh of η′ crystals (arrow B), which formed on an unconsumed particle, η′ rods (arrow C) are seen embedded in a γ₁ crystal (A). Meshed η′ crystals on unconsumed alloy particles may strengthen bonding between the alloy particles and γ₁ grains, and η′ crystals dispersed between γ₁ grains may interlock γ₁ grains. This interlocking is believed to improve the amalgam’s resistance to deformation.

Proportioning of Mercury and Alloy

A wide variety of mercury and alloy dispensers are available. The most common is the dispenser based on volumetric proportioning. Preweighed pellets or tablets are first placed in a capsule. In mechanical mixing, the capsule serves as a mortar. As a liquid, mercury can be measured by volume without appreciable loss of accuracy. The dispenser should be held vertically to ensure consistent dispensing of mercury. The dispenser should be at least half full when it is used. If it is not, the weight of mercury dispensed may be erratic. Probably the most common cause of inaccurate delivery of mercury is the entrapment of contaminants in the reservoir and orifice of the device. This variable could decrease the amount of mercury by 3% or 4% and this may lead to an unusable mix. Regardless of the alloy or triturator used, no more than two pellets of alloy should be mixed in a capsule at one time.

Disposable capsules containing preproportioned aliquots of mercury and alloy are now widely used. To prevent any amalgamation from occurring during storage, the mercury and alloy are physically separated from each other. The older types of preproportioned capsules require activation before trituration to allow the mercury to enter the compartment with the alloy. Some alloys are now available in self-activating capsules, which bring the alloy and mercury together automatically during the first few oscillations of the triturator. Although the preproportioned material is more expensive, it is more convenient, it eliminates the chance of mercury spills during proportioning, and it should result in a reliable mercury/alloy ratio. However, these capsules do not provide an opportunity to make minor adjustments in the mercury/alloy ratio to accommodate personal preferences.

Regardless of the method used, the proper amount of mercury and alloy always must be proportioned before the start of trituration. The addition of mercury after trituration is contraindicated.

Triturators

The main mixing mechanism of a mechanical triturator is a reciprocating arm that holds the capsule under a protective hood. The purpose of the hood is to confine mercury that might escape into the room or to prevent a capsule from being accidentally ejected from the triturator during trituration. A commonly used older model is a single-speed device with an automatic timer for controlling the length of the mixing time. Later models have multiple speed settings. A modern triturator is often microprocessor controlled (Figure 15-5) and contains preset trituration programs for a number of materials. It can also be programmed by the operator to include other materials. A cylindrical metal or plastic piston of smaller diameter than the capsule is inserted into the capsule, and this serves as the pestle (Figure 15-4, A & B). Spherical alloys often do not need a pestle (Figure 15-4, C).

A triturator should be used at the speed recommended by the alloy manufacturer. Older triturators do not operate at a sufficient rate of speed to amalgamate high-copper alloys properly with minimal mercury. Self-activating capsules are usually very sensitive to trituration speed.

Manufacturers often provide a list of recommended time schedules and speed settings for their alloys and various types of triturators. Because of the speed variations in triturators, the schedule should serve only as a rough guide. Dentists and assistants can adjust the amalgamation time required to attain a mix of correct consistency. For a given alloy and mercury/alloy ratio, increased trituration time and/or speed shorten the working and setting times. In addition, alloys differ in their sensitivity to trituration time (Figure 15-13).

A reusable capsule should be clean and free of previously mixed, hardened alloy. At the end of each trituration procedure, one should quickly remove the pestle from the capsule, replace the lid, reinsert the capsule in the triturator, turn it on for a second or two, and then remove the amalgam. This mulling process generally causes the mix to cohere so that it can be readily removed from the capsule with minimal residue in the capsule. It minimizes the need of scraping out partially hardened alloy, which usually produces scratches in the capsule.

Consistency of the Mix

The proper time of mixing can be determined by observing the consistency of the mix. For example, the very grainy mix (Figure 15-14, A) indicates undertrituration. Not only will the amalgam restoration made from this mix be weak, but also the rough surface left after carving of the granular amalgam will increase its susceptibility to tarnish. If the trituration has produced an amalgam of the general appearance shown in Figure 15-14, B, the strength will be optimal and the smooth carved surface will retain its luster long after polishing. Because of the friction between particles during trituration, such an amalgam mix should be warm (not hot) when it is removed from the capsule. This will have no effect on the physical properti/>