Reminds me of a recent visit from our local plumber. I had attempted to unblock the bath drain with caustic soda, which was a complete failure. As a result I now had a solid lump of caustic soda, old hair and calcium in the U-bend.
The plumber, who I know quite well arrived with a plumbers grade fix for the problem in the form of sulphuric acid.
We poured in a tiny amount and there was an interesting explosion. Rather than do the obvious and desist from continuing this potentially disastrous line of repair, we agreed the best course of action would be to remove the entire blocked U bend and continue the experiment outside in the adjacent barn.
Safely away from more sensible adults, in said barn, more acid was added, and a fountain of interesting highly caustic brown goop ejected itself violently from the poor unfortunate plumbing piece. Naturally we tried again with similar results and much amusement from two individuals who should know better. In the end we just replaced the U bend, as for some strange reason it had cracked. Do not try this at home children.