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Feruchemy is the second magic system in the series. Feruchemy works differently from the other magics. While an Allomancer burns metal to gain power, a Feruchemist can use these sames metals as holding units, or minds, to store up their own power. Basically, depending on the metal, a Feruchemist can become worse at something in exchange to be equally better in the future. No power is gained or lost.
The major strength of Feruchemy versus Allomancy is the speed at which one can access its power. While a Feruchemist must provide the power for the magic from himself, storing it over time, an Allomancer can simply consume metals to power the magic. However, an Allomancer has limits to how much power they can draw at once; a Feruchemist can draw out the power of the metalmind with virtually no upper limit, exhausting it in one massive burst if they so choose. For instance, a Feruchemist tapping a pewtermind can be far, far stronger than even a very powerful Mistborn burning pewter, if only for a short time.
Iron: Stores Weight. Less weight slows descent . A Skimmer Ferring using this will decrease the pull of gravity on them in exchange for increasing it later.
Steel: Stores Physical Speed. A Steelrunner Ferring using this will be physically slower now in exchange for being faster later.
Tin: Stores Senses. A Windwhisperer Ferring using this will become less sensitive in one of the five senses of his choice (sight, hearing, touch, smell, taste) in exchange for heightening that sense later.
Pewter: Stores Strength. A Brute Ferring using this will lessen the size of his muscles to increase them later
Zinc: Stores Mental Speed. A Sparker Ferring using this will think very slowly in exchange for thinking faster later.
Brass: Stores Warmth. Firesoul Ferrings using this will cool themselves in exchange for being able to warm themselves later by tapping the metalmind.
Copper: Stores Memories. An Archivist Ferring using this will be able to store memories inside copper, forget it, then will be able to recall it with perfect clarity later while withdrawing it from the metal.
Bronze: Stores Wakefulness. A Sentry Ferring using this will sleep or be drowsier now in exchange for staying awake longer later.
Cadmium: Stores Breath. A Gasper Ferring may hyperventilate while storing breath in exchange for eliminating or reducing the need to breathe later on.
Bendalloy: Stores Energy. A Subsumer Ferring using this can consume large quantities of food and store the calories in the metalmind, in exchange for the ability to forgo eating later.
Gold: Stores Health. A Bloodmaker Ferring using this will feel sick now in exchange for increased regeneration and healing later.
Electrum: Stores Determination. A Pinnacle Ferring using this will become depressed in exchange for a manic state when tapping the metalmind.
Chromium: Stores Fortune. A Spinner Ferring will become unlucky during active storage in exchange for increased fortune later.
Nicrosil: Stores Investiture. Little is known about Soulbearer Ferrings.
Aluminum: Stores Identity. Trueself Ferrings can store their spiritual sense of self within an aluminum metalmind.
Duralumin: Stores Connection. A Connector Ferring can store spiritual connection inside a metalmind, reducing friendship and outside awareness during active storage, in exchange for the ability to quickly form friendships and relationships while tapping.
Atium: Stores Age. A Feruchemist using this will become older now in exchange for become younger later, the same amount of years for the same time.
If an Allomancer Mistborn also can use Feruchemy, then certain synergy combinations can result. This is most evident in the Lord Ruler, who possessed every Feruchemical and Allomantic power. A person who possesses one Feruchemical ability and one Allomantic ability is called Twinborn.
By burning a metal containing a stored attribute, such as burning atium which contains youth, the user effectively makes a profit on the attribute stored in the metal due to allomancy's property of drawing power from the metal. Thus the user gains more of the attribute than invested. A Twinborn who can do this is called a Compounder. An example of this process is the Lord Ruler's immortality which he achieves through periods spent aged and sickly, storing youth in atium and health in gold, which he later burns and stores in the main metalminds which sustain him. With effectively limitless amounts of youth and healing, he convincingly posed as a god. Note that the attribute stored must have originated from the user, hence Vin's inability to use the power in Sazed's pewtermind.
If a Feruchemist taps an ironmind and steelpushes or ironpulls, then they can increase their weight to become the anchor for the push or pull, even if an enemy also manipulating the piece of metal weighs more, or the metal weighs more than the user. One could also decrease their weight to gain more movement of their own body from each push or pull.
If a Feruchemist taps a Pewtermind, Steelmind, Bronzemind, and Goldmind and Allomantically burns Pewter, they will gain increase in Strength, Speed, Wakefulness and Health beyond what one would normally be able to achieve. This will work if you tap smaller combinations of those minds, however, only for the attributes you tap.
Likewise, if a Feruchemist taps a Tinmind and Allomantically burns Tin, then their senses will increase beyond what one would normally be able to achieve.
If one Allomantically burns a metal then fills the Metalmind of the same attribute, they can fill the metalmind without lessening that attribute. This is why one might burn pewter to fill a Goldmind, then fill an Atiummind and tap the Goldmind, possibly creating the Lord Ruler's ability to live forever. This is most prevalent with burning pewter, which can fill Pewtermind, Steelmind, Zincmind, Brassmind, Bronzemind, and Goldmind.
If a Pewtermind is filled using Allomancy burning pewter, then the Feruchemist won't experience an increase in muscle size when they tap their collected strength from the Metalmind. This allowed the Lord Ruler to tap enormous strength without revealing his use of Feruchemy to onlookers.