While still in his early youth he [Fëanor] wedded Nerdanel, the daughter of a great smith named Mahtan, among thoseof the Noldor most dear to Aulë and of Mahtan he learned much of the making of things in metal and in stone.Nerdanel also was firm of will, but more patient than Fëanor, desiring to understand minds rather than to masterthem, and at first she restrained him when the fire of his heart grew too hot; but his later deeds grieved her, and they became estranged.

(Silm, Chapter 6, Of Fëanor and the Unchaining of Melkor)

And Fëanor made a secret forge, of which not even Melkor was aware; and there he tempered fell swords for himself and for his sons, and made tall helms with plumes of red. Bitterly did Mahtan rue the day when he taught to the husband of Nerdanel all the lore of metalwork that he had learned of Aulë.

(Silm, Chapter 7, Of the Silmarils and the Unrest of the Noldor)