How the drowsing rod purity work?

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A full load of ore and lime = something like 180% of a bar, so you light the smelter, get a bar at 100%.. Now there's 80% leftover for the next run, so your next run of iron and lime is 180% +60% from last run... So that's 240%.. You get 2 bars, with 40% leftover... And leftovers retain purity, so if the iron was coming out at 5%, then you have 40% of a 5% bar in there that you can't see that comes out in the next bar


now.. Each piece of iron ore gives you 2% towards a bar, so in a fresh smelter, 2 runs of 25 pieces =100%. So after 2 runs without lime, you hit 100% even, with no leftovers

Its mathy at first look, but not as bad as it sounds either,