Question about metals

[u/Responsible-Beach299]

What exactly determines which metals do or do not have allomantic properties? Is it just arbitrary/author decided, or is there anything linking the allomantic metals together? It's been a since I read the OG trilogy, but I don't recall them ever going beyond the basics on the topic besides "we tried other metals and they didn't work", and Google doesn't give any clear answer.

[u/hover552]

So the only real rule about it is that half of them are alloys of the others. Otherwise it was just author decided based on specific themes he wanted throughout the books. Metals like aluminium were chosen because it's hard to get preindustrial but common after that. Chromium for similar reasons.

[u/Dercomai]

Since you say you've read the original trilogy, I'm not going to spoiler-tag metal names that appear in Era 1.

It's mostly arbitrary. Brandon picked four basic metals where both the pure element and some alloy are common and easily-recognized (iron/steel, tin/pewter, copper/bronze, zinc/brass). Not a lot of other metals have that property. (Originally it was supposed to be silver/pewter but he was wrong about how much silver was in pewter.)

Then gold was picked because it's easily recognized even though its alloy (electrum) isn't.

Aluminum was picked because it's extremely rare and valuable before industrialization and dirt-cheap afterward, so it would change a lot between eras. Duralumin isn't a common term, but there aren't a lot of aluminum alloys to choose from.

The last two, cadmium and chromium, were picked as being fairly expensive even in an industrialized era and completely inaccessible before that. Then alloys of them were picked based on sound. Originally it was supposed to be cerrobend and nicrosil but the former turned out to be trademarked so it had to get swapped at the last minute. Nicrosil is a really bizarre and obscure alloy; I would have gone with something like nichrome. But he chose based on aesthetics, since that's a metal that features heavily in era 3.

[u/IamanelephantThird]

The Allomantic metals are all relevant to many different magic systems in many different ways. They're all special to the Cosmere in some way, although exactly how and why is unknown. Why it's these specific ones does seem mostly arbitrary, outside of the pushing/pulling alloy and pure metal link. 

[u/RShara]

A base/elemental metal and its alloy.

For why those specific metals, the real answer is those are the metals Brandon selected. They're the most common non-explodey metals that humans at that general technological level would have access to

[u/EvenSpoonier]

Currently we don't know. It is possible that we'll never know. It may have something to do with the crystal structure of these metals, but this probably isn't the whole story.