Wednesday, April 20, 2011

Lamp Oil Doesn't Burn

Pure fantasy: blazing walls of lit oil, bombs fashioned from bottles of oil with a lit wick, tossing oil on an enemy and setting them on fire.

None of this is even close to realistic. Lamp oil is made from vegetable oil or maybe fish oil. These oils have a flash point in the range of a two to three hundred degrees. What does this mean to the non-engineer? OIL DOESN'T BURN.

Flash point is a property of liquids; it is the temperature at which the vapors from a liquid will burn when in contact with an ignition source. When oil is underneath this temperature, it cannot be ignited. So all of these oil bombs and puddles of oil suddenly turning into a raging conflagration are all wrong.

Quick science experiment for people who do not believe me: Grab some vegetable oil from your pantry and pour it into a dish. Light a match and hold it above the oil. Does the oil burn?

No, seriously, go and do this then come back if you don't believe me. I'll wait.

Oil does burn, that is why lamps were fueled by oil. However, the oil has to be heated first before it will ignite. How do we accomplish this? With a lit wick. The wick heats the nearby oil above the flash point causing the oil to give off enough vapors to sustain a flame. That's why you start an oil lamp (or candle, same principle) by lighting the wick for a few seconds with an external fire source before the flame will catch.

[On the other hand, modern petroleum products ignite easily. The flash point of gasoline is around -44 degrees. Kids: don't play with gasoline.]

If you just had a bottle of oil and poured it on the ground, you are never going to get it to ignite with a torch. If you stick a wick into a bottle of oil and light it, the wick will burn. But if you throw that bottle and it shatters, the oil will not ignite and the flame will be snuffed out. Remember, only a tiny portion of the oil very close to the wick is hot enough to burn.

Ok, so that is a mild pet peeve off my chest. If you are bothered by basic physics, don't let your players and monsters toss around molotov cocktails made from vegetable oil. I don't.


  1. Everyone knows that ancient civilizations used NAPALM in their lamps, not vegetable oil. That's why lamp warfare was so effective during the Zombie Hordes War of 1345 AD.

    Vegetable oil? That would have never killed a zombie. Although it would have lubricated the undead sufficiently to modify a Slip-and-Slide into projectile weapon. I believe Archimedes wrote a treatise on that exact application.


    - Ark

  2. That's why discerning adventurers use Froghemoth Oil. The low flashpoint guarantees ignition.

  3. So maybe we can name it Firebomb Oil and sell it separately from lamp oil. Or just assume the PCs are buying the type of oil they really want, be it lamp or firebomb, for the same price.

  4. The new edition of Hackmaster put the kabash on that nonsense as well.

    Good article.

  5. Firebomb oil doesn't exist in any appreciable quantity until the 19th century with basic distillation and the discovery of kerosene from coal oil.

    Or you can cop out and just make it poof into existence, creating something as volatile as petroleum but as stable as olive oil. Substances like that didn't exist until the 20th century.

  6. There were a lot of options.

    In a setting which has dragons and myriad other fantastical flora and fauna, I don't think it would be a cop out to suggest that there may be some sort of plant or animal oil with the desired properties. Froghemoth, dragon, war olive, whatever.

  7. And not one of those early thermal weapons listed were practical to use on any man-to-man scale. Unless you want to carry around cauldrons and heat your oil, pitch, or sand before battle. In fact, that is a common theme with 'early thermal weapons.' None of them really worked unless you spent time processing and heating them and then hoped your enemy happened to be right next to/underneath you.

    Look, there are ways to set shit on fire. Lamp oil is not one of them. Those early thermal weapons are huge contrivances. They would only work under a rigid set of circumstances instead of being the catch-all death weapon of D&D oil bomb fame.

    Oh, Greek fire is as much myth as anything else. And this is a fantasy setting yes, but it still operates under basic physical assumptions.

  8. fantasy (plural fantasies)
    2. (literature) The literary genre generally dealing with themes of magic and fictive medieval technology.

    Fantasy settings consistently operate under violations of basic physical assumptions.

  9. This is literature, bound by style and genre conventions? If you want to continue being combative without actually putting any thought into a discussion or position, you can kindly leave.

  10. This is a discussion of fantasy settings, as you said, which derive from and exist in the fantasy genre. But if you would prefer to employ ad hominem rhetoric to discourage dissenting opinion, don't worry, I'm leaving.

  11. No it is not. Read the link in the top bar about the purpose of this blog.

    Or you know, you could try to offer some kind of dialogue on why the early thermal weapons link you posted was meaningful instead of reading from the dictionary?

  12. Funny that you equate "vegetable oil" with "lamp oil." No, you aren't going to light vegetable oil with a 'match.' But then, if this is D&D, what are you doing with a match?

    A candle, on the other hand, burns at 1000 degrees celsius. So sustained fire, as opposed a 'match' burns at a much higher temperature than the flash point of say, whale oil. True, you can't hold a torch to whale oil and have it light, but I believe the principle has been to light a large cloth wick, create a sustained fire and then throw the package together.

    I have no doubt that the arguments will go against this, so let's say I don't believe any of this. But how is it that in a world of magic it isn't obviously possible for a few bright-thinking mages to invent the equivalence of kerosene or sterno?

    Oh, and incidentally, no credited historian anywhere thinks Greek Fire is a myth. There's too many references and too much evidence. It was quite probably naphtha, which has a flash point of 30 to 90 degrees.

  13. I'm sticking to my knowledge of thermodynamics here. To create flaming oil, you need hot oil and this simply isn't going to happen for an adventurer on his merry way through the world. Even a sustained fire hanging out of a bottle of oil is not going to achieve this. I wager my degree and experience in an oil refinery against gamer logic, sorry.

    Intelligence aside, magic users aren't going to be inventing sterno. In fact, they can create all the flame they need with magic rather than mucking around with petroleum products. If bright minds didn't get around to it in our history, they won't in this faux history either. I am not going to employ a sliding scale of technology irrespective of year.

    Pardon me if I am wrong here, but didn't you suggest that the existence of magic and mages would suppress technologies in gunpowder and artillery? I feel you could turn that towards petroleum products as well.

    And yes, you could get your hands on naptha in any time period. It wouldn't be too hard to do a rough distillation on surface-occurring crude. But we are a few hundred years shy of any major push in distillation methodology for crude cuts.

    What is boils down to is that no where in this post did I say adventurers are never going to be able to use some kind of flaming weapon. It just isn't going to happen with common oils and will require quite a bit of hazard and hassle to make it happen. And it will certainly not be available in Ye Olde Adventuring shop as has been proposed.

  14. Fair enough. But one might think that in the process of inventing the spell firewater, a mage might have stumbled across ...

  15. I agree, that's not to say any individual person hasn't stumbled upon such a thing. I'm sure there are tons of undocumented examples of people inventing something ahead of their time, but the invention never really goes anywhere or they successfully employ it in their home but then it dies with them and so on and so forth. Leonardo comes to mind as a cliched example of wild inventions that couldn't go anywhere at the time.

    The end result, however, is that these items are not going to be in any equipment list. If a player sits down at my table and wants to create something like this, more power to them. We'll work it into the system and if it just so happens to fall in my area of expertise, I can confidently blow them up when they make a misstep in their formulation ;)

  16. For my current campaign I assume that Greek Fire works as advertised and that the traders from the Byzantine Empire are the medieval equivalent of sleazy black market arms dealers.

  17. If I'm running an action movie game, cars are going to run off cliffs and blow up instantly with giant fireballs. If I'm running kung fu, dudes are going to be jumping up on top of roofs. And while your scholarship is illuminating and well appreciated, flaming oil with whatever handwave necessary is as much a part of dungeon crawl as eternally stuck doors and wandering monsters ...

  18. something as volatile as petroleum but as stable as olive oil. Substances like that didn't exist until the 20th century.

    I am not personally familiar with these substances, and I probably wouldn't allow them in my game. Which might be a problem.

    OTOH, I'm perfectly happy to have kerosene-bearing whales providing highly volatile spermaceti oil for adventurers' lamps. I look skeptically at my players and say "are you sure you want to be carrying an amphora of that around with you?"

    How about bellows and nozzles and dry wheat flour? Could the players conceivably dream up a powder explosion device? Or is the available grind not fine enough?

  19. Off the top of my head, in order to get powders to ignite, you need to have a sufficient concentration of dust particles to achieve combustion. I don't know what the explosive limits are on flour (which are most definitely a function of particle size) off of the top of my head either.

    However, this contraption would definitely not work as a flamethrower. Ignited powders do not produce sustained flames but rather a nearly instantaneous explosion/mass combustion. And I can't see getting a high concentration of powder into a stream of low pressure air that bellows would produce.

    As I understand it, dust explosions typically occur in contained spaces where a high concentration of particles can accumulate.

  20. If you are curious:

    Lower explosive limit on different types of flour are around 50 - 60 g/m3 (assuming a particle size of roughly 420 um).

    If you want me to interpret that, I can bill you standard engineering rates :D

  21. 50g/m3 isn't all that much: I wonder what would be the best way to stir it up, if not bellows. A fan? Rapid vibration of the floor?

    It could make a great pit trap, except then you're in the situation of having to explain to your players that yes, in fact, flour can explode. I'm already toying with the idea of a coal mine dungeon and wondering if the constant threat of incineration would actually be fun or just miserable.

  22. Bellows/fans would work for creating a dust cloud which could be ignited, but not as a projectile weapon. The violence of the explosion, however, comes from a confined space. Sure, the resulting fireball from a dust explosion will create a nice crisp ball of warmth, but the rapid expansion of gases that have no where to go is what really wreaks havoc. In an open space, the explosion's energy is diffused and I would surmise most damage would result from heat.

    I think it would work as a pit trap. Person falls into a flour laden pit and the closest torch or lantern could provide ignition. Unfortunately, the flour would not last long in a natural environment before it would be eaten.

    If you are moving from flour to coal dust, it only becomes that much more fun.

  23. Just got here from a commentator's link after I wrote a similar thing on my blog today. Great writeup, and I totally agree, and I will pile yet more gameplay-based grievances on top:

    (1) The fact that burning oil is "hidden" in the DM's book is unfair to novice players.
    (2) The astonishing damage level of oil in the AD&D DMG (say) renders all other weapons obsolete.
    (3) If we want to think about the "fantasy magical world" of D&D in particular, Gygax set the rule that gunpowder/dynamite/similar explosives are "inert junk" in the fantasy world [DMG p. 113]. So if anything, the precedent is that chemical-reactive stuff like this is even LESS functional in our presumed fantasy world.

  24. In 2E AD&D the players' guide separates out Lamp Oil from Greek Fire oil. Lamp oil is used in lamps, and costs 6 copper coins per flask. Greek Fire costs 10 gold coins per flask, and is used as a lit projectile or else thrown and then the oil slick is lit.

    Regardless of whether the Greek Fire should exist, it's an appropriate cost for what it does (3 to 18 damage total spread over two rounds) compared to other weapons which have no recurring costs (1 to 6 or 1 to 8 damage per round).

    I consider Greek Fire to be available in smaller quantities and of course it's more expensive than lamp oil. If you suffer significant crushing damage (as from falling into a pit or being struck by a giant's club) your flasks might crack. Certainly being subjected to magical fire can cause damage to equipment and if the container of your Greek Fire is breached then you're just screwed. It also takes a round to light and a round to throw, burn damage on the throw round and the round after, so the damage is actually spread over 3 rounds. This all puts it right back into the realm of regular swords and spears in terms of effectiveness.

    I love flaming oil, though. In my little fantasy heartbreaker game the oil is just oil, all the same, and cheap, but the damage is much more reasonable. It's more for automatically dealing a little damage each round, for example to distract a spellcaster, or to deter pursuit and deny access, than to cause a ton of damage. Same with acid. Also what if the monster you're fighting is weapon-resistant but is vulnerable to fire or acid? These are worth having even if they aren't going to be used all the time.

  25. The simple way to turn something like vegetable oil(a thick, heavy oil which is not predisposed to readily igniting at room temperature) into a firebomb fuel, is to preheat it. It certainly couldn't be carried like a modern grenade, but if a force of soldiers knew they were going to enter battle, preparations could easily be made. This is especially true in the case of a siege, where both sides typically would prepare for the event for several days, if not weeks in advance. Catapults could be set ready directly next to a camp fire. Over the fire would be constructed a stand, and on the stand would be set the firebombs. When the battle order was given, the bombs would be loaded on the catapult, ready and hot(presumably with the oil already heated beyond it's flash point). Final impact after the projectile's short flight would be extremely likely to shatter the ceramic, exposing the readily ignitable fuel to oxygen, immediately rendering it into something not unlike napalm.