Who thinks up uses for urine? And fermented urine to boot!
That must have been one of those inventions that happened by accident.
I can imagine, some Roman left a bedpan up on a shelf in the summer for a bit too long, then went to take it down and spilled it on themselves. Then, when they went to wash it off, they noticed what a beautiful glow their skin had :)
Indeed...one has to hold onto something for a while in order for it to ferment.
According to one poet -- I think Catullus or Ovid -- the Spanish. They apparently also used it for laundering clothes and teeth whitening.