Alchemist's fire

Alchemist's fire was a fluid that ignited when exposed to air. It was non-magical and could be created by an alchemist.

Description
Alchemist's fire was a viscous liquid that stuck to most everything it touched. Upon contact with air, it burst into flames and burned until it was consumed or put out. Because of this, it was frequently stored in breakable flasks and used as a thrown weapon and in traps. A creature with alchemist's fire adhered to its body could avoid additional injury if they managed to extinguish the flames before they dissipated naturally. Rolling on the ground improved the chances, while immersion in water put the flames out immediately. In the 1370s DR, the price of a flask of this substance was about 20 gp. A century later, the price was about 50 gp.