What is ink sheen?

Sheen is a metallic glint of a second color where ink pools and dries slowly — red-gold fire on a deep blue is the classic. It comes from dye crystallizing at the drying edge, so it needs volume (wet nib), time (slow-absorbing paper), and the right formula.

If a sheening ink looks flat for you, the ink is fine — the paper is absorbing too fast. Move to coated stock and the fire comes back.