The party reaches the top of the tower where they finally expect to free the captured princess only to find she must be hidden in another castle, for in her place stands Dagorath, a menacing warrior in full plate armor and wielding a massive two-handed greataxe. Rather than being terrified, the bard Avonia steps forward, “Don’t worry, I got this! I cast heat metal on the axe.”
Let’s take a look at the text of Heat Metal, a second level Transmutation spell:
Choose a manufactured metal object, such as a metal weapon or a suit of heavy or medium metal armor, that you can see within range. You cause the object to glow red-hot. Any creature in physical contact with the object takes 2d8 fire damage when you cast the spell. Until the spell ends, you can use a bonus action on each of your subsequent turns to cause this damage again.
If a creature is holding or wearing the object and takes the damage from it, the creature must succeed on a Constitution saving throw or drop the object if it can. If it doesn’t drop the object, it has disadvantage on attack rolls and ability checks until the start of your next turn.
A greataxe isn’t a metal weapon. It’s a metal axe head with a wooden shaft. In fact, the vast majority of the 37 weapons listed in the Equipment section of the Player’s Hand Book would have wooden shafts. The exceptions are the dagger and the various swords.
So let’s replace the Dagorath’s greataxe with a badass sword instead, something comparable to Andúril, Flame of the West, Aragorn’s sword reforged from the shards of Narsil, the broken blade that cut the One Ring from Sauron’s Hand in the Battle of the Last Alliance.
Notice the leather grip? Heat Metal specifies there must be contact with the metal object, so all Heat Metal would accomplish is removing the Versatile property (assuming Dagorath wants to totally avoid the spell’s effect). And unlike Aragorn in the films, anyone swinging a sword around in combat is going to have some sort of gloves on. Dagorath’s got gauntlets, and Heat Metal doesn’t heat adjacent metal objects.
“Did I say his axe sword?” Avonia’s player asks, “I meant his armor. Duh, he’s wearing full plate. I’ll just cook him alive while he rolls with disadvantage every round.”
This is really the ideal situation to use Heat Metal in. Dagorath can’t simply drop his armor, it would take 50 rounds of combat to doff, and so he’ll take damage and debuffs every round, just at the cost of Avonia’s bonus action, until his goose is quite literally cooked. His only option is to try to break her concentration… but then she can just re-cast it.
Too bad for her that a suit of plate armor wouldn’t make contact with the wearer. They’d have padding on underneath, and that’s going to buffer them from the effects of the Heat Metal spell. Inside a metal gauntlet would likely be leather, and there’d be padding under a helmet as well. Medieval armor isn’t going to have any direct contact between the wearer and metal because (a) that’d be terrible uncomfortable, and (b) it’d negate most of the utility of the armor, allowing the force of blows to go right into the wearer.
Rules as Written, Heat Metal isn’t going to be effective against weapons or armor, the very things it’s supposed to be used for. The second paragraph uses more broad “holding or wearing the object” language, but that’s only in reference to creatures that have taken damage from Heat Metal, and the damage is only to things in physical contact.
Richardson vs State Tax Commission
Naturally, in deciding what to do with a spell like Heat Metal, our minds go to the manufacturing and processing exemption to the Idaho state sales tax. In Richardson, the Court held a very important canon of statutory construction:
In interpreting legislation, this Court will not presume that the legislature performed the idle act of enacting a superfluous statute.
Using this rule, we can say that the creators of Dungeons and Dragons 5th Edition did not perform the idle act of writing a superfluous spell — that they did not intend to create a spell that had no actual use cases (or so rare of ones as to render the spell generally useless). They plainly wanted it to be useful against at least some weapons and some armor, though not all weapons and armor.
So, how do we rule on it?
Metal armor (even with padding underneath) — Yes
Studded leather armor — No (This fictional armor seems to be based on a misunderstanding of pictures of brigandine, which would be a heavy, metal-based armor. Also, Heat Metal specifies it would work on medium and heavy armor, while studded leather is light.)
Daggers, swords, and other weapons with a metal handle (and leather grip) — Yes (Even while wearing sensible gloves.)
Axes and maces — Maybe (If the DM just wants to make the spell relevant in more situations, the close proximity to the metal head may be enough to justify it.)
Polearms — Probably Not
Clubs — Definitely Not
Blood — No (Contains iron, but blood is not a manufactured object.)
Bananas — No (Contains potassium, calcium, manganese, and copper, and domesticated fruits produced on commercial farms push the boundary of what counts as being manufactured, but you can’t see the metal in it.)
Nickelback — Sorry, but no. (Like bananas, the metal content is not perceivable.)