I learnt to lead-weld (with oxy acetylene) in the past and found it very satisfying...
The lead refers to the (lead) plumb bob used to determine the direction of vertical and therefore what's level.
https://etymology.en-academic.com/27820/plumb
It appears you are wrong, and the person you are replying to was correct. Perhaps when baldly contradicting someone, you could make some effort to back up your opinion.
https://etymology.en-academic.com/27820/plumb
plumb plumb {{11}}plumb (adj.) "perpendicular, vertical," mid-15c., from PLUMB (Cf. plumb) (n.). The notion of "exact measurement" led to extended sense of "completely, downright" (1748), sometimes spelled plump or plunk.
{{12}}plumb (n.) c.1300, "lead hung on a string to show the vertical line," from O.Fr. plombe, plomme "sounding lead," from L.L. plumba, originally plural of L. plumbum "lead," the metal, of unknown origin, related to Gk. molybdos "lead"
Neither have anything to do with pouring molten lead. Only measuring the vertical.
The purpose of finding the vertical is to determine the horizontal in installing sloped pipe.
