Modern dental practice has become very dependent on its materials, such that the dentist's greatest challenge is choosing the right combinations of them for the benefit of their patients.
Metallic Fillings
Silver Amalgams

For over 150 years, silver amalgam has been used to fill the cavities made by dentists during the removal of dental decay from teeth. When pure silver (melting point 961°C) is mixed with mercury (mp -39°C) it produces a paste of slowly forming intermetallic compounds. When this is packed into the cavity at body temperature (37°C), the intermetallic compounds interlock and the amalgam hardens. However, setting is accompanied by a considerable expansion, and 100 years ago it was discovered that this can be controlled by adding tin to the silver. Unfortunately, this produces corrodible tin-mercury intermetallic phases, and their loss can cause breakdown of the filling.

By adding copper, the tin-mercury phase is eliminated and modern dental amalgams are made by mixing silver-tin-copper alloy powder with mercury. This results in fillings that resist both the mechanical and chemical onslaughts within the mouth for many years.

Although the amount of mercury lost from such fillings is like the contamination of a drink by a drowning midge, those determined to deny the benefits of having usefully restored teeth have over emphasised the risk, and this has generated a search for an alternative, metallic, mercury-free, filling material.
Alternatives to Silver Amalgams

Gallium (mp 30°C) has been combined with indium and tin to produce an alloy that is molten at normal room temperature, and when this is mixed with powdered silver-tin-copper it produces a paste that sets. However, packing this paste has proved to be a challenge, and the best results have been obtained when packing under ethanol. The fillings produced from these alloys are also very prone to dramatic corrosion. The jury is out over the long-term toxicological effects of gallium, which has a clean record so far.

Experimental silver-coated intermetallic particles have been cold welded under pressure to fill well-supported cavities. Unfortunately, these are not the ones in which amalgam is the most useful.
Resin-Based Composite Fillings

These tooth-coloured filling materials have reached a high degree of sophistication since their appearance on the dental scene in the early 1960s. A modern dental composite consists of a paste created by combining a mixture of dimethacrylate monomers and cross-linking agents (known in dentistry as resins) with up to 80% by weight of silane-coated, ceramic particles (the filler), whose sizes range from 0.04-4 microns. This composite paste is packed into a dental cavity and the dentist exposes it for about 30 seconds to intense visible blue light. The light activates a chemical initiator within the composite and the resins undergo free radical addition polymerisation via their vinyl groups, turning the paste into a durable, solid filling.
Disadvantages of Resin-Based Composite Fillings

Composite fillings have similar strengths to amalgam but they tend to wear away more rapidly. They also shrink as they polymerise, and efforts have to be made to prevent gaps forming between the composite and the tooth. Incremental packing and curing helps, but the dentist uses other techniques and other materials to help form a seal.