2. How are dental amalgams made?
Mercury is a metal that can exist as the pure element or combined with other elements to form inorganic and organic compounds. The pure, ‘un-combined’ form, elemental mercury, is liquid at room temperature and slowly forms a vapour in the air. Chemical reactions and biological processes can transform mercury from one form to another. In general, organic compounds are the most toxic form of mercury, followed by elemental mercury and by inorganic compounds.
Most dental alloys contain a mixture of silver and tin in a 3 to 1 weight ratio, as well as a lesser portion of copper and zinc. A conventional dental amalgam alloy will contain between 67% and 74% silver, with 25-28% tin, and up to 6% copper, 2% zinc and 3% mercury. Other types of alloys exist, such as the so-called dispersion type amalgam alloys which have around 70% silver, 16% tin and 13% copper. A further, quite different, group of amalgam alloys may contain up to 30% copper, and are known as high-copper content amalgam alloys.
In the past, dentists mixed amalgam by hand which could expose them to mercury vapours. Today, manufacturers usually supply the materials to make the amalgam in a sealed capsule with two compartments. One side contains liquid mercury and the other side holds a mixture of alloy as a powder or tablet. The capsule can be placed in a machine that breaks the membrane between the two compartments and mixes the contents. The dentist collects the mixed amalgam, which at this stage has the consistency of a thick paste, and presses it firmly into the prepared tooth cavity. During this process, some mercury rises to the surface and to make the restoration strong the dentist needs to remove as much of this excess mercury as possible. The amalgam begins to harden almost immediately but there is some retained mercury that continues to react very slowly within the filling.
The mercury contained in dental amalgam once it is placed is in a very different form to that in the liquid mercury. The release of mercury vapour from a set amalgam is much lower than that from the liquid metal. More...