What effect do nonmagical burning weapons have?

Question

If a character soaks their weapon in oil and ignites it, what kind of damage does it do?

Looking at the description of how oil & torches work my guess is that it does normal damage, +1 fire damage per round the weapon is aflame (2 rounds?). If the target is soaked in oil (say from a hurled flask) they take an additional 5 fire damage (for 2 rounds?)

Related to How much damage do torches used as improvised weapons do? but not a duplicate. That question specifically focuses on torches and whether they can be used as a blunt object in addition to doing fire damage. That question's answer is kind of a punt because torches ultimately aren't made to be used as clubs, and anyway it doesn't answer this question.

Answer 1

It depends on how realistic a game you're running.

In real life, when you pour oil on a sword, the oil basically just runs down the sword, and only a very thin coating will remain on the blade. This coating will either evaporate before you can do anything useful with it, or burn for a moment or two before burning away. In the best case, you get a bunch of burning oil on your hands.

Now, you might be running a more cinematic game, where you want people to be able to pour oil onto their swords and have it be an effective tactic. If that's the case, then you would probably use the same rules that a torch does, and have the sword do 1 point of fire damage in addition to its normal damage. How long this buff will last depends entirely on how cinematic the game is, but it probably shouldn't last more than a few rounds in anything but the most ridiculous of game worlds.

Burning weapons in real life are almost exclusively based on projected fire, or blunt impact of a flaming object. It's not really possible to light a sharp blade on fire without either providing a continual source of flame (like a flamethrower), or using a material that is inherently flammable, which would screw up the weapon's efficacy as a blade.

It's a little bit easier to use a non-bladed weapon as a burning weapon, but it's more likely to damage the weapon that way. The same point about the oil running off applies: even materials that would normally absorb the oil, like wood, will generally be made non-porous when crafted into a weapon. A wooden club that gets moldy when you use it in the rain isn't so useful in the long term. However, if you soak a rag in oil, tie it around the business end of a blunt weapon, and light it on fire, then that weapon can almost certainly be used to cause fire damage in the same way a torch can.

If you soak someone in oil through a thrown flask and then light them on fire, then they will probably take some damage from that fire until they can put it out. If they're wearing flammable clothing, then the fire will likely last until it is actively put out. If they're not, then it will only last a few rounds. The DMG doesn't give a specific number for how much damage fire does, but your instinct of 5 damage per round sounds reasonable.

Answer 2

Probably a DM call, but there are some precedents

This is going to be a DM call either way, but the DM probably wants to use the torch and the improvised weapon rules to inform that ruling, so it can go one of several ways:

• +1 fire damage: The burning weapon is similar to a torch, so it should act as a torch on a hit as well as performing its own "job."
• 0 additional damage: Despite being on fire, the amount of flame that a burning weapon produces is likely far less than a torch, a weapon not designed to be lit aflame isn't as good at holding a fuel supply as a torch...as well as probably a number of other reasons mean that the heat from the flame isn't enough to do a significant amount of damage; it's more akin to jumping through a campfire than falling into a campfire. A flaming weapon like this should still be able to set highly flammable things on fire (like flaming arrows over the castle wall meant to set thatch aflame).
• Choose 1 or +0: Hitting someone with a flaming weapon might not leave the heat in place long enough to cause damage, but a weapon could be held up to a target, foregoing its normal effect in favor of acting like a torch in combat. You'd only really use this tactic if you were facing an enemy that could be damaged by fire but not by the weapon.
• Convert one damage to fire: The weapon is no better than before at hurting someone, but now at least part of its effect is the result of the flame, so out of a character's damage roll, the weapon deals one fire and the rest according to the weapon type.

...there are probably other ways to rule this; as with most DM rulings, the key is something that fits the theme of the game, that everyone can accept and that's fairly consistently applied throughout.