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?

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