Compounding

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Compounding refers to using Allomancy to enhance Feruchemy. When a Metalborn has an Allomantic and Feruchemical ability in the same metal, they may store an attribute inside the metalmind, then Allomantically burn the power. This gives a supercharged burst of the Feruchemical attribute.

The Lord Ruler used this ability extensively, as he was both Mistborn and a full Feruchemist, so he could Compound in every metal. He stored and burned age in atium to effectively live forever, and also used gold to heal very rapidly, leading to rumors that he survived flaying and decapitation. In the Alloy of Law, Miles Dagouter also uses gold Compounding to heal almost infinitely, hence his nickname, Miles Hundredlives. Marsh can perform the same age trick as the Lord Ruler, so he also has some ability to Compound.

During the Final Empire, Steel Inquisitors did not Compound, though it was sometimes possible for them to figure it out. Compounding was the reason the Lord Ruler hunted down Feruchemists so extensively--he did not want Feruchemy to mix with Allomancers and create another Compounder.

While typically, Compounding refers to Allomancy enhancing Feruchemy, there is a way for Feruchemy to enhance Allomantic abilities as well.