Can aluminum block a Rosharan Shardblade?

Question

We know from Oathbringer that

aluminum can block Nightblood.

Does the same hold true for Shardblades from Roshar?

Answer 1

Right now, the jury seems to be out on this one. Not necessarily in the usual RAFO way that Sanderson treats a lot of these questions though, which is somewhat surprising.

The answer is "contentious at team Sanderson", as of 2 years ago at least. You can see the question and conversation on Twitter:

Yata:@BrandSanderson Hi, the Comunity has a doubt, We have two WoB: Shardblades cut Alluminium Shardblades can't cut it Witch one is true one?

Brandon Sanderson(7:57 AM): Hm. Yes, I wondered last night if I'd ever answered this before. Truth is, the answer is contentious at Team Sanderson.

I've been pushing for one answer, but Peter (whom I trust) is pushing back. We will see what ends up in the books as canon.

Problem with magic like I do is sometimes you have to wait for the scientific consensus... :) Err on "no" for now.

source (emphasis mine)

So as you can see... it's not very clear at all, because it's not clear what he meant by "no".

It seems Brandon is in the "shardblades can cut aluminum" camp:

Q: "Can Shardblades cut aluminum?"

A: "Well-um-yes, yes they can."

But then, in the other camp, there's Peter Ahlstrom's take on it, which is about as close to cannon as something can get without coming from Brandon himself, since Peter is highly involved in keeping continuity in the stories. His opinion:

Peter: Oh, I think Aluminum would stop Shardblades from magical cutting. But if it's too thin like foil, a sword... would cut it anyway. What I'm arguing is that something else that Shardblades don't cut doesn't need... to necessarily be made of aluminum, for various reasons.

Me: For example Invested objects (metalmind,spike,ecc) or polestones (from some SA's Quote) ?

Peter: RAFO

source (emphasis mine)

So there are a few ways to interpret what is happening here. If one leans towards Peter's side, Brandon's statement about "Shardblades cutting aluminum" could be him suggesting that yes, a shardblade could cut through aluminum because it is a blade, but Peter does not believe it would cut though it the same way it cuts through normal objects without resistance. The reason this theory has quite a bit of community support, is because aluminum seems to have the property of resisting investiture in all the Cosmere (as is hinted at in Mistborn).

Brandon's words may suggest that aluminum cannot block shardblades, but looking at exactly what he says, this is not actually ruled out.

My take: Looking at all the evidence, it seems most likely to me that aluminum can block shardblades, in the sense that they will not magically cut through the aluminum as they do other metals and non-invested things in general. In the same sense though, aluminum wouldn't resist the regular blade nature of a shardblade. Aluminum isn't really the strongest metal, so an extremely sharp and likely unbreakable blade can still probably cut through something like aluminum armor. Thick aluminum walls would probably be beneficial under these assumptions though.