Why did the blacksmiths urniate on their welds?It is a question for a metallurgy class
Apparently, some still do! Variants include using cow urine, pregnant cow urine, urine from a red-headed girl, urine from a prepubescent boy, and even human blood!
Blacksmiths would quench their hot metal work in a slack tub that contained water. At some point, someone discovered that urine in the water effected the rate of the metal's cooling, which in turn, determined its hardness. How this discovery could have come about is humorously discussed at the link.
Urine (like human blood) contains trace elements of salt, which seems to be the critical component, acting like a catalyst, to cool the metal.