I am trying to find out what is coating the outside of the 3/4 copper domestic cold water supply pipe in my basement. The house was built around 1950. The length of copper pipe that runs from an ell after the meter up to the floor joists above has been coated with some kind of very thick gunk, about 1/8" thick, possibly some kind of cementitious paint or just plain paint. A lot of it appears to have flaked off, and there are also some short sections of the pipe that do not have this gunk anywhere on them. Was it common practice to coat that supply pipe from the mains, and if so what was the purpose?