As the title says, it appears that I cannot forge things that require more than one metal bar. Urist McMetalsmith will go and get 1 of the metal bars as usual, but as soon as he drops off the first bar, I get a "Urist cancels forge anvil: job item missing or destroyed" announcement, and he goes and puts back the one metal bar that he has already collected. I am probably missing something really basic, but I have no idea what it is.
I haven't rigorously analyzed it, so it might not actually be the 1+ metal bars that is making the item unforgeable, but that is what seems to be the problem.
Things I have forged successfully:
copper crossbows
copper picks
copper bolts
silver crafts
artifact steel barrel (although it was really glitchy and took a lot of fiddling; see below)
Things that don't work:
iron anvil
gold statue
copper statue
copper armour stand
iron armour stand
Artifact steel barrel (maybe related?)
So I got a metalsmith mood, and he went and gathered his stuff, but, wanting to increase the value of the artifact, I did a bit of a forbidding dance with metal. It was all going good, until he got to the last item: leather. I had three tanned hides, but whenever he grabbed one, he would just go stand on the workshop holding the leather, rather than putting it into the workshop. I solved it by just fiddling around with forbidding and unforbidding the leather and other stuff until it worked. My other three artifacts so far, all non-metalsmith and 2/3 using multiple reagents, were all successful without a hitch.
If anyone wants me to try any experiments to diagnose the problem, I can probably do so tomorrow.