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It is revealed that the Maesters have acess to Valyrian Steel when Maester Luwin shows Bran his Valyrian steel link. We are told that when Maesters learn a new skill, they forge a new link, I assume this is literal.
Marwyn the Mage, also has a Rod, ring and mask of Valyrian Steel. I assume the mask and ring would be reforged every time a new arch maester is appointed.
These pieces of evidence show that the Citadel does have a seemingly large stockpile of Valyrian steel and that perhaps, they have someone who can rework it. Tywin says that there are only three living smiths capable of doing so. Is there any direct reference to a stockpile of Valyrian Steel in the Citadel or a Maester who knows who to rework it?