What Is Harpic?

I may be quite wrong, but I thought that Harpic powder was mostly
caustic soda.  Sodium hydroxide in other words, or lye in the U.S.
Readily available as a drain cleaner everywhere.  Needless to say, very
dangerous stuff if not used correctly.  Powder into cold water, never
the other way around.

See http://ptcl.chem.ox.ac.uk/MSDS/SO/sodium_hydroxide.html
Also http://www.wisegeek.com/what-is-lye.htm

Take care!
Nigel Perry