MynameisGONE

I’ve thought about this some more and I think it could also work in reverse. It could be that aluminium has no identity, this could prevent it from being soulcast as you may need an identity to change when soulcasting. This would also explain why feruchemically you can store identity, as not having its own would leave space for somebody else’s. Also I don’t think my original idea would work if you can’t compound aluminium as just doubling the strength of your identity doesn’t seem like it should be that overpowered.

Aluminium feruchemy’s attribute is to store identity and i think this is related to why it acts strangely to investiture. So when soulcasting you communicate with an objects identity in the cognitive realm and convince it to change its identity, when its convinced it changes both in the cognitive ream and the physical realm. I think the reason aluminium can’t be soulcast is because it has such a strong sense of self that it’s impossible to change. This rigid sense of identity would conflict with any type of investiture that would also inadvertently alter its identity making it highly resistant to invested arts. I think the reason Hoid can’t be easily killed with a shardblade is because he’s somehow manipulating his identity so the blade loses it supernatural sharpness. It would be like trying to kill a knights radiant with a normal sword. This would also mean that a fullborn could compound and tap aluminium to make themselves immune to the supernatural sharpness of a shardblade.