View Full Version : Why won't cooking oil burn by its lonesome?
11-20-2004, 01:48 PM
My son did an science class expeiment to see whether oil floating on top of water would burn (as opposed to plain old poured into a pyrex dish). The oil did not burn in either instance. You drop a match on it, it goes out. You use one of those long-necked charcoal lighters to light it, nothing happens. Floating on water or by its lonesome, you can't get it to light.
My wife pointed out that some of her oil lamps readily use cooking oil as their fuel. So apparently havingf a wick makes a difference. But we can't figure out what that difference is. After all, if you light other flammable liquids -- alcohol, gasoline, etc., they catch on fire easily -- way too easily, in many instances. So why doesn't demonstrably flammable cooking oil burn by it's lonesome?
11-20-2004, 01:53 PM
When frying things, there's been a couple instances where the oil has splattered onto the cooking element and caught fire. Maybe it's just a matter of the oil reaching combustion temprature?
11-20-2004, 01:57 PM
It is not the oil liquid that burns but the vapor concentration above the liquid. Which is why - DON'T TRY THIS AT HOME - a cigarette can be extinguished in a cup of gasoline.
This also ignores heating the oil to its flash point where - with the right conditions - it can spontaneously combust.
The wick brings fresh oil to the burning end by caplillary action where a localized heat source burns the oil vapors and heats the oil to a gaseous state. The wick concentrates the heat to a small area so only a small bit of oil needs to be heated instead of the hole shebang.
11-20-2004, 01:59 PM
Corn oil absolutely will burn. It has an ignition temperature of 460 F. Although your burning match is much hotter than this, you have to understand that the temperature of the oil must reach 460 in order for it to ignite. Because of the relatively high heat capacity of oil, it would take quite a while for a match to heat any appreciable amount of free-standing oil to this temperature. Soak a bit of tissue or paper towel in it and light it on a flamefroof surface, and it will burn quite easily.
11-20-2004, 02:01 PM
Not only does the oil have to reach combustion temperature (~460°F (http://cms.firehouse.com/content/article/article.jsp?sectionId=18&id=36459)), but the resulting flame has to provide enough heat to get more oil to that temperature. Cooking oil conducts heat rapidly, so a blob floating on water will quickly cool to the temperature of the underlying liquid. That puts out the flame.
There's three factors at work here - flash point, fire point and volume-area effect.
Pretty much all liquids evaporate at ambient temperatures, some faster than others. When a pool of flammable liquid such as gasoline is set on fire, the liquid itself isn't burning, its the gasoline-vapour-and-air mixture above the pool that is burning. Not all mixtures of gasoline vapour and air will burn - if the gasoline vapour is too dilute it cannot ignite.
The flash point of a liquid is the temperature at which the vapour above a pool of liquid reaches a combustible concentration. Typically a low energy spark will ignite such a vapour and result in momentary combustion. However, if the pool is reasonably deep, the combustion consumes the vapour without heating the liquid very much so the flame goes out after a moment, hence the "flash".
Th fire point of a liquid is the temperature at which the vapour above a pool of liquid is combustible and will sustain a flame. A low energy spark will ignite the vapour but the liquid will generate more vapour as fast as the flame consumes it, and so you get a burning pool. The fire point is usually a few tens of degrees higher than the flash point.
What does this mean in real life situations? A match flame is generally above the flash point or fire point of most flammable liquids. Apply a match to small shallow pool of such a liquid in a plastic dish, and the heat of the match may be enough to heat the whole lot above its fire point so the starting temperature isn't very important.
However, apply a match to a larger deeper pool of flammable liquid and it hasn't got enough heat to warm that volume of liquid above its flash or fire point,so you can't ignite it. If the liquid is already above its fire point it's a different story.
Cooking oil has a high fire point, so igniting pools of it with a match at room temperature is a tall order. But if it is wicked into a fibrous material, then you have a small quantity of oil at any one time in the wick, with a high surface area. Apply a match to the wick and you will heat the oil trapped in the outermost few fibres above its fire point, so it can ignite. The heat of that combustion is enough to heat the oil in the neighbouring fibres and the flame propagates to the whole wick and becomes self-sustaining.
11-20-2004, 03:30 PM
So why doesn't demonstrably flammable cooking oil burn by it's lonesome?
What everyone else said.
The other obvious example that's in your kitchen is the humble candle. Plain, boring old wax doesn't normally catch fire, but give it a wick and it's easy.
11-20-2004, 04:50 PM
Thanks, guys, you've covered it nicely. Guess we won't have to set fire to the kitchen after all!
11-20-2004, 05:07 PM
And bear in mind that not all "cooking oil" is the same. As anyone who stir fries knows, peanut or corn oil has to get VERY hot before it will catch fire (I didn't know the temperature required to burn corn oil was 460 degrees, but that sounds right). Other oils will burn a lot faster at much lower temperatures. That's why you're always warned not to stir fry with olive or (pure) sesame oil. Add them for flavor later, sure, but don't use them to stir fty.
11-20-2004, 05:12 PM
The reason to cook with Peanut/Canola and not olive oil is not about the flash point neccesarily but about the smoke point. The smoke point is when the oil starts chemcially breaking down and you want to ideally cook as close to the smoke point as possible without going over. If you go over the smoke point, the oil turns bitter and unusable.
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