Ah, I'm sorry, I forgot to mention this, but it's impossible to be born with/engineer an affinity for something that is entirely man-made. You could have an affinity for metal, for example, but not swords.

Some magi have tried to create affinities for man-made objects, some with more reasonable methods and others with... dubious ones, but it's never quite worked out.

Um, sorry, I completely forgot to mention this anywhere. ^^;