At the end of the 19th century, a famous dentist, J.V. Black, formulated the familiar silver filling material that has become so common today.  It was an amalgamation, or blending of, silver, mercury and other metals.  Black discovered that when mercury was mixed vigorously with certain metals, it combined to form a mass that could be molded and shaped before it finally hardened.   This made the mixture an ideal dental filling material.  As the formulation gained popularity, it was simply referred to as Amalgam.

Modern Amalgam formulations include: silver, mercury, copper, tin and sometimes other trace metals in varying quantities.  The solid metals are ground into fine shards or made into tiny spheres.  Depending on the shape of the solid metals, the character of the soft mass is altered which helps with handling and packing into the cavity.

Amalgam is placed into a cavity after all the decay is removed.  A liner, or varnish, is usually used to coat the walls of the cavity.  This prevents the silver and mercury from leaching out staining the tooth.  The cavity must be shaped to retain the amalgam.  To do this the bottom of the cavity is made slightly larger than the top.  This creates undercuts that the material locks into.  It also requires the removal of more tooth structure to achieve this.

When the silver filling sets up and becomes hard, it begins a long, slow corrosion process.  This corrosion is the magic that helps the filling seal to the tooth.  This corrosion seal can take six months or more to occur.  During this time before the corrosion seal is complete, the filling can experience leakage about its edges and cause irritation and sensitivity in the tooth.

Amalgam, since it is metal, conducts thermal changes very well.  Hot and cold in the mouth that come in contact with the filling can transmit this stimulus to the inner portion of the tooth which can cause pulpal pain.  This becomes a significant problem when the filling is deep.  Pain can be significant and continued painful thermal changes can even damage the pulp of the tooth, causing it to die.  As such, deep fillings that will be restored with Amalgam, often need an insulating base beneath them.  These non-metallic, insulating bases are usually cements which harden quickly once mixed.   In addition to thermal conduction problems, amalgam also conducts electricity.  This can create another problem when a dissimilar metal comes in contact with it--like gold.  The saliva and the two different metals acts very much like a battery.  This scenario can create something referred to as electro-galvanic shock--which is uncomfortable, even painful.

Amalgam is a good material because it's strong, it's easy to manipulate, it's inexpensive, and it has good resistance to abrasive wear.  For these reasons it became the "king" of restorations in dentistry for almost a century.

While amalgam has many good properties, it has one significant drawback--it's silver-gray in color and is unsightly when placed in front teeth.  As such, dentistry had to find a white filling material that would be more friendly to smiles.  A few different compounds were developed, but they had many drawbacks and problems.

All matter expands and contracts when it is heated or cooled.  In the mouth, thermo-cycling refers to the repeated heating and cooling the mouth experiences with hot and cold food and beverages.  In this hostile environment, filling materials must not be too different in their expansion and contraction rates than that of the teeth, otherwise the seal of the filling to the tooth will break open and cause leakage.

As fillings get larger, their mass increases.  The larger the mass, the more expansion and contraction that occurs with the same temperature change.  This is dramatically illustrated when we consider a roadway.  A road bridge's expansion joints are interwoven teeth-like devices that allow for the expansion and contraction of the road bed.  These allow for many inches, even feet, of size change.  This significant size change occurs on such a large span, in the heat of summer and in the cold of winter.  While the dimension changes associated with heating and cooling in the mouth are dramatically smaller, they still exist and can affect the seal of a composite restoration to the tooth.

Amalgam has the unique ability to form very large, multi-surfaced fillings with some success, because if a margin opens up due to excessive expansion or contraction, it can corrode again and then finally produce some sort of seal.  Composite resin, however, has an instant, solid, bonded seal.  When a composite restoration gets too large, the differential between the tooth and restoration size-change, causes the margins to break open.  In a nutshell, a dentist must place only smaller, conservative fillings when using composite if longevity is to be achieved.

Large amalgam restorations were never considered good dentistry, in that leakage could occur.   In addition tooth strength is compromised and the deep silver fillings can cause pulp problems.  With composite, similar sized fillings simply fail because of the restorative's bonded seal breaking open.  A bonded restorative seal is only superior as long as it remains in tact. 

As such, teeth that need crowns--due to heavy decay--cannot be patched successfully with white restorative material, as it was once was (with limited success) with amalgam.  In cosmetic practices, amalgam has been eliminated, and as such, an appropriate esthetic restoration must be selected for teeth with heavy damage.con

Another driving force in the change from Amalgam to Composite is the environmental concerns about heavy metals.  Mercury, a liquid, heavy metal at room temperature, is an hazardous element.  While it is probably safe in the mouth bound up in the restorative, the mercury is liberated upon being ground out of a tooth.  This waste is then evacuated out of the mouth and it goes into the community sewage system where it must be dealt with.  In addition to this concern, the unmixed mercury in the dental office poses a risk to dental workers if a spill occurs or the unset material is constantly touched with bare skin.  The Environmental Protection Agency, and other federal regulatory entities,  have enacted many regulations with regard to the safe handling and disposal of this heavy metal.  Some of these regulations are onerous and costly to implement.  Hence, the sooner dentistry can eliminate this restorative, the sooner these regulations have less bearing on the rendering of dental care.