Pewter is an allomatic metal that, when burnt, gives the user extreme strength, resistance and durability. It's uses include combat, moving objects, surviving attacks, running for obscene distances, and healing oneself. The major problem with pewter is that when it runs out, all the pain and injury that you resisted using the pewter (or at least a large portion of it) hits you at once. Pewter is an alloy of lead and tin, 91 % tin and 9 % lead.
When used in Feruchemy, pewter will store strength. The user lessens the size of his muscles now in order to increase their size later.