“…for twelve years thou shalt leave Tirion where this threat was uttered. In that time take counsel with thyself, and remember who and what thou art. But after that time this matter shall be set in peace and held redressed, if others will release thee.” - Doomsman of Arda, Silmarillion p. 92
The words passed through Fëanáro’s mind and high emotions quickly followed. Anger came first, directed both at the Valar and his half-brother, followed quickly by a frisson of joy because banishment would allow him to work uninterrupted in his workshop without the social requirements of a high prince in Tirion society.
Yet, he had been banished and must set up a new household. He remembered a place where he had explored as a younger elf. Northerly it was, yet not so far north as to be unable to sustain crops and livestock. And so he returned to his people in Tirion, and gathering them together declared that he would build a new life in this northern valley, and that all those willing to come with him freely would be welcomed.
His seven sons, faithful and loving as they were towards him, pledged to accompany him, along with their various households. And, more surprisingly, his father, the King of the Noldor, also pledged to accompany his eldest and most beloved son, declaring his second child, Nolofinwë to be the Steward of the throne; King in all but name and crown.
And so they left, a mighty force of people, wains, and livestock, and they began a great building in that northern vale. When the walls arose, the outward aspect of the buildings became clear – this was no mere home or family compound. This was a fortress. Thus it acquired its name – Formenos – a name that would echo throughout the tales in ages yet to come.
One of the first buildings on the grounds of Formenos was his forge and workshop. As soon as he could, he returned to his own explorations, experiments, and endeavors. He answered to no man save his father and, sometimes, his sons, and his time was his own. He was allowed to work undisturbed. But along with this time to work uninterrupted, also came time to think, not just to create.
Isolation can indeed lead to introspection, and introspection can lead to illumination of motives, goals, and methods. He realized this full well, but deigned to walk that road. Instead of allowing himself to examine the actions that had led to his banishment, bringing the lies of Melkor to the surface and exposing them for the falsehoods that they were, his preference was to bury his emotions deep within him, working himself into exhaustion each day and then falling into fitful sleep. Over time, instead of clarity of vision, the imagined insults given to him by the Valar and his half-brother became magnified in his mind. Therefore the banishment only succeeded in tempering his steel, not softening it.