Why does
tinfoil hurt your fillings?
Having a bizarre
physiology that has gifted me with incredibly strong dental enamel, I have always wondered what having a
filling would feel like. The fact that I could
shock and
scare my friends by chomping the foil wrapper from some
Wintergreen gum always mystified me. They would stare and cringe, as though
fingernails where being dragged across the blackboards of their soul. To me, it was just a crinkly
hors d'oeuvre having no effect on my dental or mental
health. To find the meaning of their discomfort, I turned to
research.
Seems that people have had trouble with their
teeth for a long time. The filling, as it turns out, is an ingenious
Chinese invention, having been in use since as early as the
7th century. Chinese dentists used a
mercury paste to fill decayed teeth, often to the ill health of their patients. Used as a
suspension medium, the quicksilver paste held other metals, such as
silver,
tin and
copper, in place until they were
adhered to the tooth. The practice spread throughout the world, and by the 1830s, found its way into common
North American usage. Slightly more aware of the hazards of
mercury poisoning, a brief period of concern about the use of dental
amalgam (
hg/
silver paste) was quickly hushed by its endorsement by the
American Dental Association.
Tin was also added to the common mixture, to combat the
expansive properties of silver, which has been known to
burst teeth under
pressure. Ouch!
So, until recent advances in
dental composites in the 1980s, you were very likely to have received a amalgam filling in North America. They are easily identified by their
silvery black color. Thus, dental science has allowed you to become a living
battery!
I'm told that the
pain tinfoil creates feels like an electric shock in the sensitive
root nerves of the filled tooth. This description is very
accurate. It is an
electric shock, of up to 2 volts.
Some dental amalgams contain up to five
dissimilar metals.
Galvanic action between the metals different
electrical potentials creates a electric
cell in the tooth, which can generate currents of up to 10
microamps, a huge spike over normal body
nerve currents of 3
microamps. People with extensive fillings can actually measure a
weak voltage in their mouth from this action. This is
all fine and dandy, until an outside element is introduced.
When the
aluminum is introduced by the foil, it acts as an
anode to the filling's
cathode. The natural
electrolytes present in
saliva create a '
salt bridge' and the magic happens. The contact essentially
short circuits the cell. A weak current flows between the electrodes (
tooth and
tinfoil) and is detected by the sensitive
nerve of the tooth as an unpleasant sensation.
Zap. To accompany the shock, a metallic
taste can also be generated by
tin ions released by the reaction.
Another interesting dental electrical reaction is the reason that it is
rare for people to have both
gold and amalgam fillings at the same time. In this case, a gold filling would act as a
cathode, and the amalgam would be the
anode. Zap again, with the added bonus of having the
anode filling
corrode each time. It would soon need replacement.
Isn't dentistry cool?