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There is no way to do that, other than through the use of national spells, but there are no national magic items.

I'm ok with anybody being able to forge black steel as magic items. What is special about Ulm is that they can make it with their normal blacksmiths, whereas for everyone else it is a magic item requiring earth gems. And even for Ulm it can only be made in middle era, the rest of the time it might as well be magic to them too.