Instability of Amalgams

When a dental amalgam or "silver filling" is prepared a metallic alloy powder is usually mixed with a certain amount of liquid mercury. The composition of the amalgam as given in scientific literature refers to the alloy powder before being mixed with approximately 50% of mercury. When placed in the cavity the mass is "condensed" by the dentist. This term has nothing to do with the physical phenomenon of condensation. It simply means that the dentist applies a mechanical pressure to the mass to pack it and press some excess mercury to the surface of it. This top layer is then scraped away. One of the reasons for not specifying the composition of the final filling as used in the mouth of patients is the great variation in mercury content depending on a number of parameters:

Varying degree of condensation between dentists (1).
When not using capsulated amalgam: Alloy/mercury ratio chosen to achieve a consistency to the liking of the individual dentist.
The use of minimal mercury technique (Eames technique) (2).
The use of the "wet technique" and the "hanging drop technique" meaning the use of considerably more mercury than specified by the manufacturer (3).
Different possibilities to apply condensation pressure in different types of amalgams. Lathe-cut alloy powder accepting much more condensation pressure than spherical alloys (4).

There is one exception to the rule given above and that is the old type of copper amalgam described later. In this case the amalgam is manufactured as a ready to use product after heating (3). The composition given for this amalgam is roughly the same as in the final filling.

This web-document is dedicated to technical aspects of dental amalgam. It is however appropriate to mention that in later years it has been revealed that feces is the main excretion route for mercury from dental amalgam.

Measurements of mercury in faeces of subjects with dental amalgams have been lacking. To fully understand the decomposition of amalgam it is of course of great importance to take into account all routes of excretion - especially the main one. Uptake of mercury in routes with high amounts of metal but low uptake may very well be equal to or exceed those of low amounts of mercury but with a high uptake. Another reason for obtaining total mercury excretion-rates is that the WHO has a provisional limit for total intake of mercury by food - 300 micrograms of Hg/week or 43 micrograms of Hg/24h (5).

Skare and Enquist investigated emission-rates of mercury into the oral cavity and both faecal and urinary excretions in subjects with and without dental amalgams (6). They found an excretion rate of 190 micrograms of Hg/24h in feces and 19 micrograms of Hg/24h in urine in the worst affected individual. The control without amalgam had by far the lowest value or approximately 1/30 of the WHO-limit for daily intake. In this investigation the median excretion in subjects with amalgam by far exceeds the WHO-limit. The faecal excretion was on average 20 times that of urine.