Physics of Cookware: Conduction and Radiation

[JoNorvelleWalker]

What I didn't realize until just now:  on a molar basis, at culinary temperatures, the specific heats of common metals are all about the same.

 

https://en.wikipedia.org/wiki/Dulong%E2%80%93Petit_law

[MSRadell]

http://www.engineeringtoolbox.com/specific-heat-metals-d_152.html

 

Compare aluminum and cast iron.

You are certainly correct, aluminum has a much higher Specific Heat than cast-iron does. It excepts heat much quicker than cast-iron, no question about that. However on the same basis it gives up heat much quicker than cast-iron to for the same reason. Thus cast-iron holds heat better which many times for cooking you want.

 

Both materials make excellent cooking surfaces but for very different reasons. It all depends on what you need for what you are cooking as well what type of heat source you have available.

[paulraphael]

Cast iron retains heat a long time because of its thermal mass. This is the specific heat multiplied by the mass. Aluminum has a much higher specific heat than iron, but also has a much lower density, and in clad pans like these, is usually much thinner. The result is that iron will usually be the thermal mass champion.

 

Which is either good or bad. If you want to sear a big piece of meat it's good; if you want temperature control it's bad.

 

Iron has poor conductivity, so even fairly thin aluminum will generally give you more even browning on the stove.

 

Disk-bottom pans are a different story. The aluminum disk can be thick enough to have a very high thermal mass, and be good for searing; potentially even better than iron. Heating will be exceptionally even. Responsiveness will be slow, but generally better than iron.

 

I like iron for browning big chunks of things where I don't need any control and don't need to worry about the color of the pan drippings. It also excels in the oven for maintaining even temperatures of a braise.

 

Thin clad aluminum like AC is great for high control and general purpose stovetop use. Their 10" fry pan is great for quick sautées. I don't have any of their saucepans, but can tell they'd perform very well.

[boilsover]

Thus cast-iron holds heat better which many times for cooking you want.

 

 

 

I've yet to hear a cogent theory as to why and for what this might be true.  If we're agreed that cast iron stores half the heat of aluminum for a given weight, and accepts heat 4x slower in reheating, then please explain why, on an active hob or in an active oven, anyone would want a lesser, slower heat delivery.  The answer better not come back that the appliance can't return the pan to set heat quickly, because both aluminum and copper soak it up faster, too.   Every prep I've heard touted as superior in cast iron pans (distinguish seasoned linings) hasn't been when I've compared.  Things like pizza cooked on thick sheets, and no-knead bread a la Fahey are but two examples.  Low, slow and long can be done at least as well in other materials.

 

I totally get the desirability of, say, holding your chili or fondue at an acceptable temperature for a little longer when the power goes out or you're driving your food across town.  I suppose the self-cook-on-a-rock-at-table thing could be OK if you could find a 1" slab of cast iron.  For me, these are not really cooking functions.    

Edited July 24, 2015 by boilsover (log)

[MSRadell]

  If we're agreed that cast iron stores half the heat of aluminum for a given weight, and accepts heat 4x slower in reheating, then please explain why, on an active hob or in an active oven, anyone would want a lesser, slower heat delivery. 

I don't think there is an agreement on the first part of your statement. I think everyone will agree that cast iron is slower to heat up but I don't think there's any consensus about cast-iron be able to store half the heat per given weight. I think it can be proven that it stores more heat based on how long it takes it to cool down. Even a thin section of cast-iron takes a considerably longer time to cool down and the same thickness of aluminum does! The ability to store the heat is why cast-iron works better for cooking certain items.

[dcarch]

"------ I think it can be proven that it stores more heat based on how long it takes it to cool down.---"

 

Not correct.

 

If you withdraw money from you bank account slower, that will not mean you have more money in the bank.

 

dcarch

[boilsover]

I don't think there is an agreement on the first part of your statement. I think everyone will agree that cast iron is slower to heat up but I don't think there's any consensus about cast-iron be able to store half the heat per given weight. I think it can be proven that it stores more heat based on how long it takes it to cool down. Even a thin section of cast-iron takes a considerably longer time to cool down and the same thickness of aluminum does! The ability to store the heat is why cast-iron works better for cooking certain items.

 

LOL, you must have a different Periodic Table than everyone else.

 

Let's be clear.  For equal weights, aluminum *stores* twice as much heat.  And unless your cast iron pan weighs morethan twice what your aluminum one weighs, it's not storing more heat.  Howver, it's true that, once heated, aluminum *gives up* its stored heat a little faster than does cast iron.  These are separate things.

 

How far into thermodynamics do you want to go?  We know from Sam Kinsey's work right here on eGullet that Thermal Diffusivity (conductivity divided by volumetric heat capacity) is the measure of a pan material's ability to conduct thermal energy relative to its heat capacity.  When those calculations are run, aluminum winds up a little more than 4x better than cast iron.  We also know that, if we take the next step and look at Thermal Emissivity (the pan material's ability to exchange thermal energy with its surroundings), computed as the square root of the product of conductivity and volumetric heat capacity, we see that cast iron is only 70% as effective as aluminum--and only 45% as effective as copper.

 

That's the physics of it.  If you look at practical cooking considerations, if the concern is food cooling too quickly after it's cooked, it depends on the mass of the food.  Air is a terrible heat conductor, and water has a very high heat capacity.  Therefore, a full aluminum stockpot isn't going to cool much more rapidly than will a cast iron one (The story changes radically if you compare in an icebath, however) .  Will one sauteed mushroom cool more slowly in cast iron?  Sure.  But that is not much different than saying it's cooking longer.

 

In naval architecture, cast iron is classified as a thermal insulator.   And it makes sense to think of cast iron pans as excelling at this, and not as a great way of delivering heat to food.    I suppose, if someone only cooked by bringing a pan to heat, putting in the food, and then immediately shutting off the heat, this kind of slow dissipation might work OK for a few things--if you wanted the cooking to take longer.    Resting a steak in its CI pan off the heat sort of qualifies, but you could do the same thing faster in a more conductive pan or platter.

Edited July 24, 2015 by boilsover (log)

[JoNorvelleWalker]

There is also the issue of distance. The intensity of radiation decreases as the square of the distance from the source. It took you four minutes to brown that english muffin holding it 1 inch away from the surface of the radiating body. Now imagine that you have a 1/8 inch thick slab of iron on a shelf in your oven at 600 degrees. It will still take about four minutes to brown that english muffin at one inch away from the slab. How close together are your oven shelves? Ten inches at least? That's ten times the distance, which means a sigificant reduction in the intensity of thermal radiation. How long do you think it would take to brown that english muffin at ten inches?

 

I question this for the reason that neither scott123's frying pan nor the oven shelves are point sources of radiation.  The inverse square law does not apply.

[Barrytm]

Scott, heading back to your original design question, though veering off in a different direction,  you should be able to comfortably launch and retrieve a pie with a 3 inch clearance.  IIRC,  I have done it with less.  Either buy, or make, a superpeel, you should have no trouble launching in a very tight space.