Why can Vin fuel Elend’s atium-burning, even though Atium is Ruin’s Body and Vin is using Preservation? Or did I misread that and he was just burning atium and had run out of everything else?
Yes, as has been pointed out:
A powerful peace swelled in Elend. His Allomancy flared bright, though he knew the metals inside of him should have burned away. Only atium remained, and the strange power did not—could not—give him this metal. But it didn’t matter. For a moment, he was embraced by something greater. He looked up, toward the sun. (From the text.)
As a note here, the powers granted by all of the metals—even the two divine ones—are not themselves of either Shard. They are simply tools. And so, it’s possible that one COULD have found a way to reproduce an ability like atium’s while using Preservation’s power, but it wouldn’t be as natural or as easy as using Preservation to fuel Allomancy.
The means of getting powers—Ruin stealing, Preservation gifting—are related to the Shards, but not the powers themselves.
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