The application of pipe dope prior to assembly will fill the minute voids between the threads, thus making the joint pressure tight.
Pipe dope also acts as a lubricant and helps prevent seizing of the mating parts, which can later cause difficulty during disassembly.
A material safety data sheet reports that the "Permatex" 51D pipe joint compound contains kaolin clay, vegetable oil, rosin, ethanol, etc.
As recently as the 1950s, toxic lead oxide mixed with spar varnish was used as a dope for drinking water pipes.
[6] Pipe dopes used in oil drilling frequently include powdered graphite, lead and zinc, and sometimes copper suspended in grease.