There were times when he was very secretive with his craft. More often Fëanor simply didn't have the patience to explain what he does... Until he had to teach it to one of his sons who was actually interested. Only then he realized sharing one's knowledge is not that bad and he's actually capable of saying the same thing twice and not combusting on the spot.

Not that Mio needed any repeating. He clearly is a master in his own craft. Being able to repay him for his kindness feels good. It feels right.

Then a thought strikes him. Being able to work in a forge again gave him ideas. So many he could easily stay in the place for another century. Not an option right now. But there's one thing that's fast enough to create and he suspects could bring joy to another man who showed him kindness.