Quote Originally Posted by smaughazard
Good point, Caradoc, but the urine used for fullering was well-aged and quite, uh… aromatic!
From the link I posted:

For example, Roman aristocrats paid dearly for urine imported from Portugal. Portuguese urine was reputedly both the strongest smelling and the best whitener.

Importing urine to Rome from Portugal would not have been an overnight process. This might be why it was the "strongest smelling," and "the best whitener."