Practically the opposite of Ferrum, Mercurius follows the honing of the soul and mind to better focus themselves. Many of those who follow the Refinement of Quicksilver are cunning and wiley, and tend to celebrate those who are likewise. The manipulation of Pyros is a reflection of that nature, as they are curious about its very nature and try to see how far they can push it to do their will.
The Refinement of Quicksilver is one of the most mentally demanding of the Refinements, and a Promethean rarely turns to it without some substantial thought. Most who ultimately become Mercurius wonder about the limits of their Transmutations, wish to refine them further, or wish to learn new ones. It is also transitional; once a Promeathean comes to a deeper understanding of Pyros that satisfies them, they are likely to move on to another Refinement.
The Osirians and Galateids are the most likely to follow this path for long periods; the Nepri are usually curious about the world around them and want to know more about how things may be manipulated, while the Galateids follow Mercurius out of a desire to attain perfection in every way possible. Frankensteins are rare along the Refinement of Quicksilver, simply as the elements of fire and therefore Pyros are part of their very natures and they see no need to study it further.