Seatpost stuck - where would I get Caustic soda from?

After several weeks of using a super-saturated solution (i.e. so strong that it wasn't disolving any more) I gave up and sent my frame to Mercian, who nuked the seatpost out.

Yeah, I made this mistake with one batch. If you make it too strong it won't work. Why did you give up? Just use a weaker solution no?

You say nuked. How did they actually get it out?