Fuelling Feruchemy with unkeyed Investiture

[Xanpheon]

A thought occurred when I was reading through some WoBs;
 

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Questioner

As far as the Lord Ruler goes, how did he use the Twinborn thing? Feruchemy and Allomancy?

Brandon Sanderson

What he had to figure out how to do is: Allomancy is powered by Spiritual power directly from the Shard of Adonalsium. Whereas Feruchemy is powered by your own Investiture and effort being transferred into the thing. What he needed to do was figure out a way to power Feruchemy with Allomantic power, right? You could have done the same thing by fueling it with the Dor, or with Stormlight, or another external. But he only had access to three magics. So what he had to do was figure out that.

So what he's doing is, he's basically taking metals, (since he's a Feruchemist and an allomancers), and he is burning metals that he has Invested himself, but then using... basically, switching it so he gets a burst of Allomantic power that is charged with a Feruchemical attribute. So it's powering Feruchemy with Allomancy by burning the metal that he himself has Invested.

Questioner

So he was essentially putting stuff into the metal?

Brandon Sanderson

Basically, priming the pump. He puts it in with Feruchemy. Then he burns it with Allomancy. But that fuels Feruchemy with Allomancy, which allows him to draw on the powers of the Shards, rather than himself. So it's not really a perpetual motion machine, because he's drawing the power from someone else. But it's external, which allows him to break the rules of Feruchemy.

The big question I have is: that works in the book, because you can dig into the technicalities of the book. But that's not gonna work in the movie, right? That explanation right there, that's so many levels over the heads of the audience. So I have to figure out a way to not break the cosmere magic, but make it simpler to understand in the movie. Which is the big headache in writing the screenplay. That's probably the biggest challenge in the screenplay is to figure out how to make that all work.

LTUE 2020 (Feb. 15, 2020)

The key part being that in his first response he comments that Feruchemy could be fuelled by external sources, such as the unkeyed Dor jar that the Ghostbloods used in TLM. This is logical, considering how we know that unkeyed investiture and manifestations of Investiture work in general.

But how does this manifest, exactly? My two major questions (as a connoisseur of Feruchemy, these questions are important to me for... reasons) are:

• How does a Feruchemist use unkeyed Investiture?
  • We know (also from TLM) that Allomancer Mistings using raw investiture from an external source, not burning metals, have that investiture channelled through their Allomantic power  that they manifest, and in the case of Vin/Elend that if you are a full Allomancer drawing on external investiture you can channel it as you choose (Push/Pull, Pewter etc.).
  • It's likely this would work for some things, and wouldn't for others. F-Copper, for example, likely wouldn't be able to be drawn by a Feruchemist from unkeyed investiture because it seems to work much more like some other examples of "memories stored in investiture" that we've seen so far, in that there needs to be actual memory woven in to the investiture - you'd probably just get a jarbled mess of nothing, or maybe the "tone" of the investiture? But F-Pewter is a much more straightforward explanation - you just convert it directly into the attribute you want through your power.
  • Do you, then, need the investiture to be placed in your metalminds? Or can you "digest" it directly? Allomancy is meant to process investiture directly drawn from Preservation, so it makes sense that it can filter unkeyed investiture through whatever power they use. Feruchemists, though, are only seen to be drawing/replacing investiture from their metalminds rather than processing it internally, necessarily. Can we assume it works like Allomancy, or does it need to be shaped by the metal?
• Could you use unkeyed investiture to make unsealed metalminds?
  • The Excisors were user originally to make unsealed metalminds for the Southern Scadrians so they could share heat, but as we now know Harmony's Perpendicularity is not only located in the Malwish empire but is also "closely guarded" - enough that Autonomy didn't want to send her army through it to preserve it's strength. With relatively easy access to a source of nominally limitless investiture, can they fill their unsealed metalminds with it? Could this explain the (frankly, ridiculous) power they will eventually need come the future of the Cosmere?

 

Without getting into the weeds of how unkeyed investiture could fuel each individual ability too much, a full Feruchemist who was able to fuel themselves with unkeyed investiture is just a fascinating concept - the conversion rate, how fast the investiture would drain, there's so much interesting to learn. 

This may in fact be spawned by my suspicion that that is exactly how the Bands of Mourning were made, why they drained quite as fast as they did, and why they were drained by TLM.