In addition to the others above, I believe Hugin also says in one of the messages that you need a kiln & smelter to refine the ores. Any time you're stuck, check through the log of bird messages to see if there's a tip there.

On that note, you will need a total of 10 cores to make both the forge and kiln, but one thing that might not be obvious is you don't actually need both at the same time... So if luck is against you and you aren't finding enough, as long as you have at least 5 you can build the kiln for some charcoal and then demolish it to make the smelter.

(Edit - I was saying forge when I meant to say smelter. Fixed)