Turn other metals into iron by way of a crucible. It should be noted that transmutation requires that metal as a catalyst. This means that you have to have iron to make more iron out of other metals. This research is a prerequisite for other transmutation recipes.
"You have discovered a way to multiply iron by steeping iron nuggets in metallum harvested from other metals."
Crucible: 2 metallum, followed by an iron nugget. Note that this recipe will mostly prevent using iron nuggets for metallum in the crucible. (You can use 2 at a time, for example to purify two ore blocks.)