Dipping your Weapon
As an action or bonus action, a character can dip their wea-pon into a hazardous source: a roaring fire, a vat of acid, a barrel of oil, or a freshly dead creature. The source of the object the character is dipping into must be within melee reach of the weapon they are holding.
Damage Examples
| Source | Added Weapon Damage |
|---|---|
| Fire (A roaring campfire, molten slag, or covered in burning oil) | 1 / 1d4 fire |
| Acid (Alchemical acid or acid fresh from a monster's stomach) | 1 / 1d6 acid |
| Blood (From any dead body) | Nothing |
Don't forget under these rules, most Poisons can be dipped or applied to with a weapon.
When you dip your weapon, it follows the logic behind Weapon Sundering, with this variant: Any time you dip the weapon into a damaging source to deal additional weapon damage, it immediately takes the amount of damage listed for added weapon damage. If it still has 1 or more hit points afterwards, it then deals the Added Weapon Damage.
Entirely Subjective Rules
Dipping a wooden weapon might cause it to become destroyed more easily, as if it had vulnerability to fire damage.
Magical weapons that aren't artifacts or grant the users damage resistances or grant extra elemental damage most likely just have resistance to damage, though it must be terrible to lose your super special +1 sword because you kept sticking it in fire or acid.
Though Longbows might be considered to be having 2d8 hit die, that's more for the Bow than the arrow. An arrow under these rules instead has just 1 hit point, unless it's been buffed with other sources that allow it to be aerodynamic and unable to burn super easily. Under the wise words of Internet Forum Users, a flammable arrow is used more for destroying flammable items than dealing extra damage.