Quote Originally Posted by Cass
NaOH (Sodium Hydroxide, Lye, caustic soda) does not congeal. The hardning comes from the exotherm process. The lye heats up and fuses together when you don't add enough water to dry lye by putting to much dry lye in the trap . It fuses together forming a hard lump that will disolve very slowly if fresh water is introduced. This is hard to do in a trap.
A friend of mine (no, honestly!) has put some caustic soda down his shower drain, forgot about it (!!!) and now it has solidified and water isn't shifting it.

Do you know what will dissolve it? I have sugested vinegar and advised him to beware of any fumes...

Thanks for any help you can give.