Mercury alloyed with a number of metals forms amalgam, which is primarily used for dental fillings. The common constituents of amalgam are mercury, tin, silver, copper and other trace metals. The use of amalgam in the dental sector dates back to the 1800s since it is easy to apply, has good durability and strength and is economical.
The popularity of amalgam has diminished in recent years especially due to its Mercury content and since reliable composite materials have become widely available.
There are several different types of Amalgams, that include the following:
- Dental amalgam
- Potassium amalgam
- Sodium amalgam
- Aluminium amalgam
- Ammonium amalgam
Dental amalgam is classified into the following two types:
- Low-copper dental amalgam – These were being used in the past and have almost fully been replaced by high-copper amalgams.
- High-copper dental amalgam - High-copper alloys exhibit properties of high strength, less corrosion and tarnish, less creep and minimal sensitivity to handling variables producing long-term clinical results. When compared to low-copper amalgam, high-copper amalgam restorations exhibit fewer incidences of marginal failure.
The composition of amalgam is given in the table below:
High-copper amalgam includes 40-60% silver, 27-30% tin and 13-30% copper and 1% zinc set with mercury. Indium and palladium are also included. Silver enables setting expansion and causes an increase in strength and resistance to corrosion. Tin may cause setting contraction whereas copper improves strength, minimizes corrosion and tarnish, brings down creep and brings down cases of marginal leakage. Zinc brings down the oxidation of other alloys in the metal. It has been proved that zinc-containing amalgams have a longer life when compared to non-zinc amalgams. Indium reduces creep and increases strength whereas palladium reduces corrosion and tarnish.
Tarnishing is the loss of luster from a metal or alloy surface because of the formation of a surface coating. The alloy remains unchanged and the mechanical properties also remain the same. A sulphide layer formed in the surface causes amalgam to tarnish.
Galvanic corrosion takes place when two dissimilar metals are present in a wet environment. The flow of electric current between the metals results in one of the metals getting corroded. Surface corrosion may cause a change of color of an amalgam restoration and may result in pitting. Surface corrosion fills the amalgam/tooth interface with corrosion products bringing down microleakage. Internal corrosion will result in marginal breakdown and fracture. Galvanic corrosion is promoted by an acidic environment.
An amalgam restoration needs to be strong enough so as to resist the biting forces of occlusion. Dental amalgam has high compressive strength, which is 380MPa for low-copper amalgam and 414MPa for high-copper amalgams. However shear and tensile strengths are quite low. Hence it in essential that tooth structures support the amalgam to ensure long-term clinical success.
Creep is a gradual change in shape due to compression because of dynamic intra-oral stresses. Creep causes the flow of amalgam, hence unsupported amalgam will protrude from the cavity margin. These unsupported edges are quite weak and corrosion may cause further lowering of strength. Creep may also result in an overhang on fillings causing food to be trapped leading to secondary decay. The gamma-2 phase of amalgam is key to the considerably high creep values exhibited by certain materials.
Dimensional change is the net expansion or contraction of an amalgam. In case the amalgam contracts, dimensional change is negative and is positive in case of expansion during setting. According to the ADA/ANSI specification, the dimensional change should be a contraction or expansion of not more than 20 µm/cm.
Dimensional change is impacted by a number of factors such as the alloy/mercury ratio and also condensation and trituration techniques. Contraction and expansion take place simultaneously during the amalgamation reaction.
For the preparation of dental amalgams, liquid mercury is mixed with a powdered alloy mainly comprising tin and silver. Lathe cutting or milling of a cast ingot of the silver-tin alloy is done to produce the powder. The particles formed are shaped irregularly.
Alternatively, the liquid alloy may be atomized and condensed, a process that causes particles with a spherical morphology. Both these alloy types are used in clinical amalgams as a mixture of spherical and lathe-cut particles. For clinical purposes, amalgam is mixed with mercury in a process called trituration. Previously, they were formed manually; however, presently vibratory mixers are present and manufacturers prepare the unmixed amalgam in two chambers of a tiny capsule.
Before mixing, the thin membrane separating the alloy powder from the liquid mercury is destroyed and the capsule is placed in the mechanical mixer arm and vibrated for a specific amount of time to enable thorough mixing of liquid and powder. The extrusion of the freshly mixed amalgam with a plastic consistency is done from the capsule into the cavity. During the trituration process, the surface layer of the silver-tin alloy dissolves in the liquid mercury and the reaction causes new phases to be formed. These new solid phases cause solidification of the plastic amalgam paste.
The applications of amalgams are the following:
- Dental amalgam is an excellent and versatile restorative material and is used in dentistry for several reasons. It is economical and quite easy to use and can be manipulated during placement. Amalgam remains soft for a brief time period so it is possible to fill it into any irregular volume and then a hard compound is formed. Amalgam has a greater longevity when compared to other direct restorative materials, like composites.
- Sodium amalgam has been used in organic chemistry as a powerful reducing agent, which is much safer than sodium. Sodium amalgam is also used while designing the high pressure sodium lamp providing sodium to obtain the right color and mercury to fine-tune the electrical features of the lamp.
- Mercury has been used in gold and silver mining since it amalgamates very easily with them. In gold placer mining, mercury is used for the separation of gold from other minerals.
- Aluminium amalgam is used as a reducing agent.
- Thallium amalgam finds use in low-temperature thermometers.