A dental dam is a diaphragm of rubber that is used to prevent mercury amalgam material and other toxic substances from being swallowed while your teeth are being worked on by a dentist. The dentist punctures a very small hole in the dam and pulls it over the specific tooth that is being worked on, which then sticks through the dam, visible to the dentist, while the dam blocks off the rest of your mouth. Nothing can get past the dam and down your throat. Some dentists (a few) are now removing mercury/silver/amalgam fillings from people's teeth and replacing them with less toxic and more natural appearing material. The mercury amalgam has been banned entirely in some European countries because it is very poisonous, giving off mercury vapour in the mouth. I had all my old fillings removed about 5 years ago, and now have a mouthful of totally natural looking teeth, and a big worry off my mind...but it's an expensive process.