Healthy urine is 95% water, 2.5% urea, and 2.5% salts, minerals, enzymes, etc. That doesn’t necessarily make it the ideal laundry detergent, but there’s plenty of historical evidence that it was used as such going all the way back to ancient Rome.
The key here is urea, which decays into ammonia. You’ll find ammonia in many household cleaners because it can cut through dirt and grease.
Even after the invention of laundry soap, some people preferred to use urine for tough stains. (Our question remains: what do you use to get out the urine stains and smell?!)
Hey, it’s free and never in short supply, so we can see why you might want to soak a particularly terrible grease stain every now and then when you had no other choice. And one would need to use stale urine to get the correct chemical reaction, so perhaps that’s easier to rinse out? We’re not sure; we’ve never tried.
Urine also had other uses back in the day – disinfecting wounds and softening leather, for example. But there’s really no reason to try this at home to see how it works.
Now, for those who continue to claim that Romans used it for whitening their teeth, we’d like to see some more evidence. The only citation for this “fact” is a poem by Catullus, famous for writing scandalous and filthy poems (and who hardly makes a factual claim).
Source: “From Gunpowder to Teeth Whitener: The Science Behind Historic Uses of Urine” — Smithsonian Magazine