Oops, as I mentioned on the other thread, I just skimmed over the technobabble regarding the bridge so I totally forgot that they used aluminium, I only remembered that they needed a large number of chulls over on Narak to operate the bridge and that Navani is using the same principles from the archerstand in OB and spanreed, I just got the basic gist
In Ch 11, we see that they know iron pulls (attractor fabrials), they know there should be a repeller fabrial, but presumably don't know how to go about it. If they don't know steel, they might indeed not know copper. So they haven't quite figured out the elemental metal-alloy connection, in the sense of Allomantic pairings, I'm not saying that the Rosharans are unaware of the element or compound's existence. Navani makes it clear in the Rhythm of War part 1 epigraphs that fabrial science advances are treated as state secrets so some fabrial science methods may even be lost, like the half-Shard shields and bows from Jah Keved, after the development of which King Hanavanar and a no. of his Highprinces were assassinated by Szeth and then a civil war started.