Don't worry about it, you built what you needed for now. Once you hit the Swamp biome and come across surtlings (the little dudes on fire) you can harvest them for cores. If you dig out the ground around their spawner (the flame shooting out of the ground) they'll instantly die when they hit the water. Easy-peasy Surtling core and charcoal farm ftw. ;)

Edit to add: the forge does not take cores, just 6 copper, 4 stones, 4 charcoal, and regular wood. You only need 8-10 cores to build a smelter and a charcoal furnace. After that, you're just building additional furnaces, smelters, and portals with 'em.