Can an item an an oven (eg a roast) get hotter than the temperature you've set the oven to?
August 7, 2012 1:52 PM   Subscribe

Can an item an an oven (eg a roast) get hotter than the temperature you've set the oven to?

If I set my oven to 80 degrees Celsius, and I put a roast in there, does the roast ever get hotter than 80 degrees?

I understand that if I put a glass of water in a fridge, it gets cold, but if I leave it there for a few days it doesn't freeze. If I put a pan of water on a stove top, it gets warm, but if I leave it there for a few minutes it starts to boil. It doesn't get over 100 degrees C because it turns to steam.

So what happens with a roast if I leave it in an 80 degree oven for a few hours/days? Will it stay at 80 degrees, or will it retain the heat and eventually catch fire/burn/dry out/etc?

What if I put a block of metal in the oven - will that get to over 80 degrees and eventually turn red hot and melt?

I feel pretty silly for asking this question - I'm sure if I knew anything about thermodynamics this would be obvious, but I don't, so it's not...

(Using the Science & Nature category because this isn't specifically about food, more about the thermodynamics side of things.)
posted by UltraFleece to Science & Nature (20 answers total)
Keep in mind that ovens cycle on and off, and the temp inside can vary a few (or even many if your oven is old/dirty) degrees above and below what you set it at depending on the cycle.
posted by royalsong at 1:59 PM on August 7, 2012 [1 favorite]

No. Once an object in the oven has reached the same temperature as the oven is providing, it won't get any hotter. It's basically the zeroth law of thermodynamics. If you leave the roast in for several days it will probably dry up and get exceedingly unpleasant, but it won't go foom!.

(On a purely practical note, your oven might have a time-out timer. Mine turns off after 12 hours.)
posted by scruss at 2:05 PM on August 7, 2012 [3 favorites]

royalsong is correct about actual ovens cycling on and off to modulate the temperature in the oven up and down a few degrees. Also, most ovens have automatic turn off timers so you don't leave them on all the time.

If you had a perfect oven that heated to the air to precisely 80C and never varied from that temperature, the hypothetical block of metal (or roast) would reach, and stay, at 80C when it reaches thermal equilibrium with the oven environment. The equilibrium can take an exceedingly long amount of time depending on the mass of the metal (or roast), but would eventually happen.
posted by saeculorum at 2:07 PM on August 7, 2012 [1 favorite]

The roast won't get (much) hotter than 80 degrees, and to the extent it does, it's because your oven gets hotter than 80 degrees.

What you're really asking about is the difference between systems with and without feedback. Your oven has a feedback system that works like this: you set the desired temperature, and the oven automatically turns on the gas/electricity if the current temperature is lower than your set temperature. When the oven reaches the set temp, it turns off. (The turn-off may be a little slow, so it may not actually happen until a few degrees above your set temp.) Then the oven cools off for a while, but when it gets a few degrees below the set temp, the feedback system turns it on again.

Your refrigerator works the same way: it cools down to your set temp, then turns off. While off, it warms up. Eventually, the temperature gets above your set temp, and the cooler turns on again.

This is in contrast to a (gas) stove, which you simply turn on. If you put something on the flame, it keeps absorbing heat and getting hotter until something stops it from getting hotter. (For water, the boiling phase change keeps the temperature at 100 degrees until the pan is dry, but after the water boils off, the pan will get MUCH hotter.) There is no feedback system here, so there's no (immediate) limit to how hot things will get.

(Of course, a gas flame of a certain size only puts out a certain amount of heat, which is incompletely transferred to the pan; and the pan will radiate some heat itself, so there's an equilibrium temperature above which a dry pan on a stove will not get any hotter, but it's probably hotter than is good for the pan or the stove.)
posted by spacewrench at 2:11 PM on August 7, 2012 [5 favorites]


*As mentioned ovens cycle on and off so limited areas of the oven may get hotter than the set temperature. However, it is rather unlikely that those would raise the temperature of a large thermal mass like a roast much at all before the cycle kicks off. Depends on your oven.

Think of it in terms of a closed system with energy coming in via the electrical or gas heating element. Energy can escape the system over time through vents and conduction (very small amount due to the materials used to insulate it). Inside the oven it has somewhere a temperature sensor. As that energy escapes the temperature decreases. When it goes below a certain threshold the oven controls tell it to kick on and it overshoots a bit to get back up to the correct temperature (depending on your oven this overshoot can vary quite a bit).
posted by Feantari at 2:12 PM on August 7, 2012

Agreeing with everyone else that the answer is basically no, but unless your oven is perfectly airtight (it isn't), food held at an elevated temperature *will* eventually dry out, even though it never gets hotter than the surrounding air.
posted by jon1270 at 2:22 PM on August 7, 2012

Well, if your oven is hot enough and your roast is dry enough, it could catch fire; combustion is an exothermic process which converts chemical energy into heat energy, thus raising the temperature inside the oven chamber above what it was before the fire started.

So technically it's possible, though hopefully you are not at risk of finding yourself in this situation.
posted by Joey Buttafoucault at 2:31 PM on August 7, 2012 [1 favorite]

There are substances whose autoignition point is fairly low. If you put one of those in the oven set to 80 C, it might catch fire, at which point it would combust and get quite hot. White phosphorus is an example. But a roast isn't going to get hot enough.
posted by brianogilvie at 2:37 PM on August 7, 2012 [2 favorites]

I think I see two possible confusions.

1| The oven is continuously applying heat, it should continue to build up right?

Well your oven is built to insulate heat but it doesn't do it perfectly, or even that predictably, so its got a device in it called a thermostat. It is basically the same concept as the thermostat that controls your house but for your oven and without an air conditioner, when it senses that the oven is colder than what you've set it to it puts more energy into the system. It keeps putting in energy until the oven is either as hot or hotter than where you've set it and then it turns off. The heat then slowly leaks, which the thermostat senses and before applying more energy.

2| But Blasdelb, the meat hunk is in a hot environment, it would only continue to soak up heat ad infinitum right?

Well, whatever temperature you set your oven at isn't actually hot, at least not in an objective sense. You can only make your oven hot in relation to what would be the comparatively cool environment of your kitchen. In an objective sense, when you move a roast beast from a 23°C room into an 80°C oven, all your roast does is slowly go from an equilibrium of 8-15°C (because it did just come out of your fridge right? Food safety and all.) to an equilibrium of 80°C. For us hot and cold only have meaning in relation to the fundamentally arbitrary temperature that is comfortable for us. The only temperature that isn't fundamentally arbitrary is 0°K, which is cold enough that time stops in weird ways.
posted by Blasdelb at 2:50 PM on August 7, 2012 [1 favorite]

Yes, if it is an electric oven.

The thermostat measures air temperature. If your oven is electric, there is a heating element. The element heats the oven with infrared radiation. If an object is close to the element as it heats, it will be heated directly by the element until the thermostat shuts the element off.

To demonstrate this effect, DO NOT put a sheet of paper on the bottom rack of your oven and set the temperature to 350F, because it will catch fire.

This effect is also why you want to use silver baking sheets for cookies, not black ones; your cookies would have burned bottoms as the black sheets would absorb more IR.
posted by seanmpuckett at 2:55 PM on August 7, 2012 [1 favorite]

N.B. a glowing cherry red element has a temperature of about 1500F.
posted by seanmpuckett at 2:58 PM on August 7, 2012

It depend on how the oven measures temperature in the oven, how cycling is controlled and the types of heat transmission permissible in the oven (i.e. radiative, conductive and/or convective heat), but yes it's possible -- it's just unlikely to get very much hotter.
posted by imagineerit at 3:19 PM on August 7, 2012

To demonstrate this effect, DO NOT put a sheet of paper on the bottom rack of your oven and set the temperature to 350F, because it will catch fire.

Fahrenheit 351 is one of my favorite novels.

Blasdelb and most others have got this right. The object in the oven can become no hotter than the energy imparted by the oven will allow. If the oven is set to 80 C, assuming complete accuracy, items in the oven will get no hotter than 80 C.

Another example of this effect would be the inside of a car on a hot day. The driver might remark that the seat belt buckle is too hot to touch, but it is no hotter than anything else inside the car. However, it feels hotter because of the specific heat and thermal conductivity of the metal buckle as compared to the upholstered seat.
posted by Tanizaki at 3:20 PM on August 7, 2012

Oh, I just thought of something else, the manufacturers wouldn't claim to be able to let you set a temperature if that wasn't a thing their oven could actually do, and not oven type things can. For example microwaves, lacking a thermostat, would work just like your hypothetical block of metal that just keeps on heating.

Microwaves work by imputing energy into the spin state of water, which then releases as heat. [More awesome detail on how microwaves work] They are safely prevented from making dangerously hot things by the fact that water will not reach more than 100°C without pressure, it just turns into steam. (This is also why microwaves are great for reheating potatoes as the won't burn and great for steaming corn as they're really good steam generators).
posted by Blasdelb at 4:06 PM on August 7, 2012 [1 favorite]

Note that the OP said the oven was off, then set to 80C with a roast in it. You must allow for the initial heating cycle, which will dump massive amounts of heat into the oven all at once in order to attain the setpoint.

Therefore, paraphrasing myself: the heat source in an in-capsule thermodynamically heated oven* will be hotter than the rest of the oven, otherwise it could not heat the oven. Therefore objects closer to the heat source will become hotter than objects farther away from the heat source until the heat source is turned off, at which point all objects in the oven will converge on a temperature that represents the net energy imputed by the heat source.

This is why you pre-heat an oven before you put food in it. Otherwise the food will get hotter than the thermostat set point, and likely burn. And yes, I said 350F intentionally. The paper will catch fire well before the thermostat will turn off the heater.

* All standard residential ovens are this type. Commercial air/turbo ovens are not (elements in a separate compartment heat air blown into the cooking compartment), and neither are microwave ovens (as explained by the usually reliable Blasdelb) or the really fancy bunker-style wood burners that externally heat a cast iron compartment from the outside. They weigh a ton and are great for certain kinds of baking because they produce a really moist, even heat.
posted by seanmpuckett at 4:39 PM on August 7, 2012

Wouldn't it be possible to put a convex lens in the oven to achieve higher localized temperatures? I dun know science...
posted by Yowser at 5:38 PM on August 7, 2012

no, unless something else is adding heat to the system.

scruss is correct in the zeroth law of thermodynamics.
posted by Under the Sea at 5:44 PM on August 7, 2012 [1 favorite]

it's also fun to consider that each time the flame cycles off, the roast is then re-radiating some of its heat, cooling down slowly, until the thermostat kicks the flame back on, so roughly half the time, the roast is heating the oven!
posted by Abinadab at 3:36 AM on August 8, 2012

Not unless it catches fire.
posted by chairface at 8:32 AM on August 8, 2012

For example microwaves, lacking a thermostat, would work just like your hypothetical block of metal that just keeps on heating.

Most microwaves will eventually shut themselves off if they get too hot. It's possible to melt glass in one if you can prevent it from doing this.
posted by yohko at 9:07 AM on August 9, 2012

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