In the original Mistborn Trilogy, why didn't anyone notice that aluminium was not affected by Allomancy?

Question

In the original trilogy, Kelsier was an expert Pusher/Puller (Coinshot/Lurcher), however nowhere its mentioned that there exists a metal which can't be pushed or pulled. Especially since Kelsier did all the research for the "eleventh metal"

Only after Vin has her metal reserves depleted by Aluminium does she realize that it may be an allomatic metal and plays around with its alloys (hence duralmin).

However I find it strange that its skipped over that Aluminium is immune to Pushing/Pulling and thus the next logical step to Emotion Soothing or Rioting. Is there a reference to this I might've missed?

Answer 1

There's no reference in the books, but Sanderson is using real-world metallurgy by implication.

Aluminum is rare before society reaches industrial levels of tech, because it requires more than basic smelting and shaping to have in sufficient quantities to make cheap shiny hats.

To compare to the real-world (and shamelessly steal from Wikipedia), aluminum as an element is the third most common material in the Earth. The problem is that as a pure metal in the ground, it reacts and bonds with so many other things that you soon have almost no pure metal - it becomes a bunch of other minerals like silicates. The easiest way to get it out of the ground is through a mineral called bauxite (as Valorum notes in comment) and it requires (I'm skimming here) significant temperature, pressure, and a rotary kiln, the first of which a medieval setting might provide, but the rest highly unlikely. Once you have it, it bonds with air just as quickly, but in a way that prevents corrosion to the larger portion of the metal.

So whatever aluminum the Final Empire had - and only the Lord Ruler and his Inquisitors seemed to have any (I presume Vin's metallurgist got it from her, and she got it inside the abandoned Kredik Shaw) - they got from a rare natural deposit, possibly experiments on ores, or a stash left over from the times pre-Ascension whose tech level is poorly understood.

(it is also entirely possible, since the nature of the world was changed at the end of the trilogy, that it wasn't "immune/inert" at first, but there is no way to prove that with the info we have, and Sanderson using metal history as a template is much more likely)

Answer 2

The Lord Ruler kept Aluminum from the public

Any Allomancer who accidentally burned aluminum had their other metal reserves stripped away from them, leaving them powerless. Aluminum had been kept secret by the Steel Ministry; Vin had only found out about it on the night when she’d been captured by the Inquisitors, the same night she’d killed the Lord Ruler.

Vin was one of a hand full of people who knew about aluminum and what it could do when burned. She was actively testing different forms of aluminum leading to the end of the trilogy. Even then, she is told its extremely rare, and she never has more then some powder or shavings. Its not until refining techniques increase in the Wax and Wayne series that it becomes prevalent.

It also appears Vin MAY have known other users for Aluminum as well,

know of aluminum?” Yomen paused, but Vin could see in his eyes that he did. “Duralumin is the Allomantic alloy of aluminum,” Vin said. “Where aluminum dampens the power of other metals, duralumin enhances them. Mix duralumin and zinc or brass, then Pull on the emotions of a koloss, and he will be yours.

Vin's study seems primarily about metals she as a Mistborn can burn, they never appear to try to make new weapons for fighting other Mistborns.