It can only be found in chests in secluded locations, where you can only get by using Adress Matrix at the Upgrate Station. Put the Matrix into the Adress Connector and flip Upgrate Control Lever. Use the teleport to get to a new location.
Adress Matrix locations:
- Forest Adress Matrix - Central Station, second shelf.
- Boiling Adress Matrix - Medeliu's House.
- Stonehole Adress Matrix - Brass Forest, West Catacombs, near key chest.
- Void Adress Matrix - In the begining of Bottom of the world quest in the box right next to Llavados.
If you don't put any Matrix in Adress Connector, flip the lever and Teleport. You'll get access to one more sidequest in Deep underwater station.
You will need this ingot to temper any equipment from The Wheels of Lull.