From The Notebook: A History of Thinking on Paper:
One shows a parchment-maker scraping dead flesh off a stretched hive laborious, meticulous piece-work. Behind him, another hide waits in a bath of urine, where it softened, losing hair and tissue, for about a week.
…
The only downside was that, like the parchment-maker's hides, the papermaker's linen rags also needed long soaking in vats of human urine. Like many premodern manufacturers, both depended on buckets of piss.
Wow, writing materials used to be super gross.