Howdy folks!
Diodorus Siculus states this of the Celtiberian weapons:

And a peculiar practice is followed by them in the fashioning of their defensive40 weapons; for they bury plates of iron in the ground and leave them there until in the course of time the rust has eaten out what is weak in the iron and what is left is only the most unyielding, and of this they then fashion excellent swords and such other objects as pertain to war.41 The weapon which has been fashioned in the manner described cuts through anything which gets in its way, for no shield or helmet or bone can withstand a blow from it, because of the exceptional quality of the iron.

I have never heard of this as a method for ensuring quality of steel, though I am aware of varying qualities of steel corroding differently. Presumably the goal is to be left with higher, or at least homogenized, carbon content but realistically, I'm unsure of how effective it would be. I wouldn't have thought that the better quality steel would be sufficiently rust-resistant to fair much better after 3 years in the ground than the poor-quality iron.

Does anyone have any insight on this practice, if it was practiced elsewhere or if Diodorus Siculus may, in fact, be mistaken?