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In the Mistborn universe Compounding takes place when a Twinborn burns a Feruchemical storage, greatly increasing the Feruchemical power within. Now this works simply for most stores like health, which just gives you more health. But Copper stores specific memories, so what would happen if it was compounded?

As far as I can tell, Brandon hasn't stated what the effect would be. There is a brief Word of Brandon about it from a September 2012 interview

FireArcadia

Is there any use to being a Copper compounder, from a feruchemical point of view? I think the same point would also apply to an Aluminium compounder.

Brandon Sanderson

Some combinations, like some abilities themselves, aren't really that useful. That said, being able to compound copper...that could do some things. Aluminum, not so much.

You can find this WOB here. It's the 5th entry down

There's another WOB from the same interview about copper compounding where he states he's deliberately not detailing all the forms of compounding until he's ready to address them in the series. You can find that one here

Douglas

And now some magic mechanics questions:

1) What benefit does compounding copper get? Exceptionally clear and detailed memories? Memories that can be split into a new coppermind while still remaining in the feruchemists mind? Something else?

2) How does Feruchemical luck work? If a chromium compounder tried his hand at day trading on the stock market, what would happen? Would it make him choose stocks that were coincidentally going to go up anyway? Would it change stock prices by altering the world around him? Would it fail because the required scale of action is too large? Something else?

3) This might have been specified in the books, I don't remember, but does Duralumin expend itself as well as the metal it's used with? If it does, I've got this theory that its effect is actually just to cause a regular flare, not a superflare, but it affects itself in a feedback loop that keeps forcing the flare higher until it runs out.

Brandon Sanderson

So, I've said before that I want to hold off on talking about different forms of compounding and types of twinborn until I can address them in the series. So I'll have to RAFO the first two. However, in answer to the third one, yes you DO expend Duralumin in the process.

So, unless he's changed his mind, there would definitely be some effect, but he's being intentionally vague about it.

I've found forum posts where people have speculated what the effect might be. Some suggest it would sharpen the details of whatever memories are stored in the copppermind. Others suggest that recalling a memory that way would allow you to permanently remember it. The forums are here and here