Feb 28, 2011
Medieval Mondays: Siege Warfare and the Myth of Boiling Oil
This hardly ever happened. Oil made from animal flesh, olives, or other materials was labor intensive to produce and expensive, far too expensive to literally throw away. In general, defenders of castles poured boiling water down on attacking troops. Every castle had a plentiful supply of water. It was one of the critical elements when choosing where to build a fortification. There are numerous instances of castles surrendering when their water supply dried up or got contaminated by the besieging forces.
I'm not sure where the myth of boiling oil came from. There's a Roman account of using boiling oil, but in the vast majority of cases the much cheaper and more available water was used. In one case in France, however, some men-at-arms poured molten lead on a knight. That must have seriously hurt!
Boiling liquids of whatever kind were poured out of murder holes like the one seen here courtesy of Wikimedia Commons. These holes opened out onto the outsides of walls of gates, or inside barbicans. Rocks and other nasties could also be thrown down. Medieval warfare wasn't pretty!