Caustic Soda
From London's Ghost Acres
Sodium hydroxide, also known as lye or caustic soda, was used to remove hair and any fat or flesh from hides and skins (Watt 1885, 127). Caustic Soda was also used in soap-making. It was produced locally and imported from Europe.
https://books.google.ca/books?id=2fMHAAAAQAAJ&dq=leather%20manufacture&pg=PA127#v=onepage&q&f=false
http://en.wikipedia.org/wiki/Sodium_hydroxide
https://archive.org/stream/artsoapmakingap00wattgoog#page/n45/mode/2up
Imported from