Now, if you are confused as to what manufactured metal means, then for the purposes of the usability of the spell it means all weapons and armor. Once the metal is taken out of its ore form and worked into something useable, then it becomes manufactured. In that case, you can use heat metal to affect the item. Nails, horseshoes, posts, beams, keys, and tools all also go under the umbrella too. You might be able to use this information to use the spell out of combat too!