|Teflon thread seal tape.
When you screw together cast iron water or gas line pipe, you have to put thread seal compound or tape on the threads first, or else it will leak. The most popular choice is PTFE (polytetrafluoroethylene, otherwise known as Teflon) tape, which is a soft, stretchable material that is very slippery and very resistant to chemical attack. This particular roll is pink, which is not characteristic of Teflon, it's a dye added to indicate that this is thicker-than-usual tape.
Amazingly, on a weight-for-weight basis this tape is over 75% fluorine (the other 25% is carbon). Since pure fluorine is violently, explosively dangerous and extremely difficult to store, this was my best fluorine sample for a long time (see below for some real elemental fluorine).
PTFE tape is also a curious demonstration of the power of convention. It always comes on a particular kind of plastic spool, with a particular kind of snap-on cover to keep it clean. Every brand, every manufacturer, every store you look in, it's always exactly the same type of container. It would be fascinating to know the story of how this came about, but at this point the convention is so strong that if any manufacturer tried to sell Teflon tape in a different package, they probably wouldn't sell more than a roll or two. Those would be returned as purchasing errors, because anyone who is looking for Teflon tape will be looking for a particular shape of package, and won't even see any that's the wrong shape.
Source: Hardware Store
Contributor: Theodore Gray
Acquired: 15 April, 2002
Text Updated: 20 November, 2008