Physics of Cookware: Conduction and Radiation

[scott123]

Are more conductive materials, by nature, better radiators?

I'm in the process of looking at ceiling materials for converting a home oven for pizza baking, that, when preheated sufficiently, will give me broiler-ish radiative transfer in a broiler free gas oven environment (where the broiler is in a separate compartment and the pizza is baked in the main oven chamber).

For instance, for the sake of argument, if I take 1.25" thick firebrick and the identical heat capacity quantity of steel slab and heat the two to the same temp, will food the same distance beneath them brown in the same amount of time? Or will steel's superior conductivity cause it to radiate energy faster?

Also, since color effects radiative transfer, let's assume the firebrick and steel are the same color.

[ray goud]

You cannot assume that the steel and brick are the same color, because they are NOT. If you paint either one to match the other, the paint will burn off at the temp of a pizza oven (I have one).

Steel has much more conductivity than brick, so the surface temp will be more constant ( for as long as it is hot), and the steel will lose heat faster as the heat at the surface is renewed from its (steel) interior reservoir. Long after the steel has cooled off, the brick will still contain much more heat than the steel. For a somewhat silly parallel, consider that the "heat tiles" on the bottom of the Space Shuttle hold their heat for hours, if not days, after protecting the underlying aluminum from the heat of reentry. Those are mineral-based tiles.

For your application, any metal ceiling is a non-starter unless you can get nothing else. You would be far better off with brick. And the shape (dome versus arch) is completely unimportant, again from personal experience.

You can also check this site: http://web.mit.edu/lienhard/www/ahttv131.pdf

It will give you a "heat textbook" you can examine to gain the values for various materials and how heat moves through and to/from them. I have recommended it before.

Ray

[scott123]

The steel is rusted. There you have it. Same color I'm trying to remove emissivity from the equation.

So, if I'm hearing you correctly, the surface temp IS responsible for emitted radiation. The higher the surface temp, the more energy is radiated. Since surface temp is directly related to conductivity, a material's ability to conduct heat is directly proportional to it's ability to radiate heat.

So, although my steel slab ceiling will lose heat faster, for a brief time, it will radiate heat more intensely than my brick ceiling. I'm looking for a sprint, not a marathon. A quick payload release, not a slow one. I'm looking for 4 minutes of intense radiative heat from my ceiling. As long as the steel is thick enough (1/2" range) it should give me better browning during those 4 minutes than the fire brick will. In addition, the oven will be on during those 4 minutes. When I've tested the bottom burner in a single stone formation it gave me almost no browning on top of the pie, even with the hearth on the top shelf, so I don't expect it to feed the steel slab much during those 4 minutes, but I do expect it to help a little. Once the pie is out, the steel will need to recover before the next pie goes in, but, with it's conductivity, that wait shouldn't be very long.

The problem I currently have with a two ceramic stone setup is that, not only does the bottom stone act as an insulator, delaying the top from reaching target temp (550 f), but the conductive heat I get from the hearth will burn the pie long before the radiative heat from the ceiling gives me the color I'm looking for. Reducing the vertical space between the two stones certainly helps, but I still have to have enough space to be able to launch the pie comfortably. That 'comfortable' distance (about 4 inches) from the ceiling to the hearth is giving me almost no color. Having a steel slab ceiling might even the tables. The steel slab will, for a short amount of time, radiate heat more intensely than the brick will, it will recover between pies relatively quickly, and it's superior conductivity will most likely allow it to preheat far more quickly than the hearth, regardless of the hearth's insulating properties.

[ray goud]

How are you going to keep the steel at a high temp? Are you heating it from its "backside" (side not facing the food)? If you have a gas burner on that "backside", then why have any steel at all? A flame radiates far better than any other material, assuming you are heating it from "behind". Give us some more info.

Ray

[scott123]

As I said before, it's a broiler-less main gas oven chamber- the broiler is in a separate compartment below.

From top to bottom.

Oven ceiling

Steel slab on top shelf

Firebrick on shelf 5" below that (with 1/2" to 1" clearance on all sides)

Oven floor with vents

Heating element

Below that, of course, is the broiling chamber, but I'm not using that for pizza.

I'm attempting to recreate the intense heat of a broiler without a broiler in the main chamber.

[slkinsey]

Unless you get that slab red-hot, there is no way you will approach the radiant heat of a broiler. Since the top piece will effectively be heated by the ambient temperature of the oven, I don't think it will make any difference what it is composed of. Heat capacity doesn't really make much difference where radiant heat is concerned. There are extremely powerful broiler elements with a tiny heat capacity.

[MSRadell]

As said by the previous poster putting a piece of any material in the top will not make any difference at all in your scenario. Without additional heat from the top you're not going to accomplish what you want to. The only thing you might try is putting the pizza in and a cold stone so the pizza sees more heat from the top while the bottom stone heats up.

[scott123]

Unless you get that slab red-hot, there is no way you will approach the radiant heat of a broiler. Since the top piece will effectively be heated by the ambient temperature of the oven, I don't think it will make any difference what it is composed of. Heat capacity doesn't really make much difference where radiant heat is concerned. There are extremely powerful broiler elements with a tiny heat capacity.

Okay, so if heat capacity doesn't make much difference, then you're telling me that I can take two 2" thick slabs, one of steel and one of brick, put them next to each other, preheat them to 550 deg., open the door quickly put a hand under each about 2" away, and both hands will get about the same heat? And, that, because neither item is 'red-hot' neither hand will burn?

Is that what you're saying?

[ray goud]

As I said before, it's a broiler-less main gas oven chamber- the broiler is in a separate compartment below.

From top to bottom.

Oven ceiling

Steel slab on top shelf

Firebrick on shelf 5" below that (with 1/2" to 1" clearance on all sides)

Oven floor with vents

Heating element

Below that, of course, is the broiling chamber, but I'm not using that for pizza.

I'm attempting to recreate the intense heat of a broiler without a broiler in the main chamber.

In my opinion, you may get part of what you want, a pizza which cooks more than usual on the top. I don't think it's worth the trouble you will face, though. The steel slab needs to be quite massive (thick) and at least as large as the pizza's diameter. Then you need to support it, ideally from the bottom, on some sort of stilts or side walls, which should be welded to the steel. It shouldn't be situated a large distance from the top of the pizza, giving one less room to slide things in and out. Then, it will take quite a long time to heat up. It will be a user-dangerous contraption. But if you have the money, discount the safety aspects, access to welding or an equivalent support system, and won't be discouraged by less-than-success, then try it.

Ray

[scott123]

Ray, the average oven shelf will support a 1/2" steel slab cut to allow for about 1/2" wall clearance on all sides. I know this because I put 1 1/4" soapstone slabs in ovens all the time and soapstone is a little less than half the density of steel.

A peel will be used. Working with a peel with 5" of vertical space takes some getting used to, and is not without the threat of an unintended calzone or two, but there's no physical danger.

[ray goud]

Ray, the average oven shelf will support a 1/2" steel slab cut to allow for about 1/2" wall clearance on all sides. I know this because I put 1 1/4" soapstone slabs in ovens all the time and soapstone is a little less than half the density of steel.

A peel will be used. Working with a peel with 5" of vertical space takes some getting used to, and is not without the threat of an unintended calzone or two, but there's no physical danger.

When you're all done and tried it out, please post your results and some pics.

Ray

[budrichard]

"For instance, for the sake of argument, if I take 1.25" thick firebrick and the identical heat capacity quantity of steel slab and heat the two to the same temp, will food the same distance beneath them brown in the same amount of time?

Simple answer is yes. Thickness does not enter into the calculation.

Or will steel's superior conductivity cause it to radiate energy faster?"

The simple answer is no.

Radiation depends on the absolute temperaure of the material and the surface area and is described by the Stefan–Boltzmann law http://en.wikipedia.org/wiki/Thermal_radiation. Obviously there are materials that will not survive the temperature that a ceramic or a steel will and they of course will not work, period.-Dick

BTW a broiler does not depend on heat to do its broiling but temperature, that's why for electrical ovens, a broiler consists of electrical elements near the food and for a gas oven the broiler consists of a gas flame near the food, obviously the gas flame will be much hotter than any electrical element giving rise to the superior performance of gas broilers over electrical boilers.

Edited June 21, 2010 by budrichard (log)

[scott123]

BTW a broiler does not depend on heat to do its broiling but temperature

But doesn't heat impact temperature? A body's ability to emit radiation depends entirely on it's surface temperature. It's surface temperature, in turn, is 'fed' by the energy stored in it's core traveling to it's cooler exterior- it's heat capacity. If, for instance, you take a little piece of steel and a larger piece of steel, ten times it's weight (with about the same surface area), and heat both until glowing red, when the heat is turned off, the larger piece will glow longer. That longer glow relates to it's greater heat capacity, does it not?

Even though, at 550 deg. f., the glow of the steel and brick isn't visible to the naked eye, it's still there. The steel should glow brighter for a shorter period of time than the firebrick, because it's surface temp will be bolstered by energy flowing faster from the core to the exterior.

I know that as materials glow red, their radiative output increases dramatically, but I don't think it's that far fetched to theorize the possibility that sub glowing materials of sufficient heat and conductivity might put off enough radiation to brown bread.

Let's say, for the sake of argument, I had a cube of aluminum weighing a ton that I was able to heat to 550. Could I toast a piece of bread 2" away? Being aluminum, it's going to lose heat very quickly, but, assuming I move incredibly fast to get the bread in place, would it toast?

If one can store heat in thick materials for conductive purposes, can't one also store heat for radiative purposes as well?

Edited June 21, 2010 by scott123 (log)

[slkinsey]

Unless you get that slab red-hot, there is no way you will approach the radiant heat of a broiler. Since the top piece will effectively be heated by the ambient temperature of the oven, I don't think it will make any difference what it is composed of. Heat capacity doesn't really make much difference where radiant heat is concerned. There are extremely powerful broiler elements with a tiny heat capacity.

Okay, so if heat capacity doesn't make much difference, then you're telling me that I can take two 2" thick slabs, one of steel and one of brick, put them next to each other, preheat them to 550 deg., open the door quickly put a hand under each about 2" away, and both hands will get about the same heat? And, that, because neither item is 'red-hot' neither hand will burn?

Is that what you're saying?

Yes, both hands will get right around the same amount of radiant heat. Sure, your hands will burn if you keep them in there long enough. But no, I wouldn't expect either hand to burn more than the other.

Let's say, for the sake of argument, I had a cube of aluminum weighing a ton that I was able to heat to 550. Could I toast a piece of bread 2" away? Being aluminum, it's going to lose heat very quickly, but, assuming I move incredibly fast to get the bread in place, would it toast?

No, the bread likely would not brown in this scenario. More to the point, you could brown the bread much better using 10 grams worth of thin metal filament heated until red hot.

In a traditional wood-burning pizza oven, the intense radiant heat that cooks the top of the pizza is not energy radiating out of the hot stone at the top of the oven. Rather it is reflected radiant heat originating from the fire burning on the oven floor. This is the reason wood burning pizza ovens, even after they have been fired and brought up to temperature, still have to have a small fire burning inside to work properly. Take away that fire and you lose that intense top-down radiant heat.

Edited June 22, 2010 by slkinsey (log)

[ray goud]

"Rather it is reflected radiant heat originating from the fire burning on the oven floor. This is the reason wood burning pizza ovens, even after they have been fired and brought up to temperature, still have to have a small fire burning inside to work properly. Take away that fire and you lose that intense top-down radiant heat."

Although I think scott123 should try whatever he wants to, slkinsey is absolutely correct in the above quote, something which I confirmed (after being a little unsure) in my own outdoor pizza/bake oven about six years ago. Because of that phenomenom I can roast veggies while the oven is heating up and long before it is near pizza or even focaccia temp. Did it again on Sunday past.

Ray

[budrichard]

" but I don't think it's that far fetched to theorize the possibility that sub glowing materials of sufficient heat and conductivity might put off enough radiation to brown bread"

What is your training in physics and heat transfer? I'm not trying to be condescending but I would suggest you acquire texts in heat transfer and basic physics and try to understand the fundamentals involved. Unfortunately without a knowledge of Calculus, you will have to take the fundamentals as truth without any derivations.

I encounter this type of problem all the time. Individuals not founded in the fundamentals of science trying to understand how the world works is like the ancient Greeks attempt to divine the world by just thinking.

I've given you the facts as Science understands them today, if you chose to argue them or theorize without understanding them, go ahead but I can't change the facts to suit your theories.-Dick

[scott123]

Iron frying pan (1/8" thick) heated to 600 deg.

Distance from english muffin: 1"

Duration: 4 minutes

There you have it. Browning.

And this was with just the heat of the 1/8" pan. No heat coming from the bottom oven burner OR heat coming from a pre-heated hearth. I was putting my hand above the frying pan every minute or so and noticed that it lost a lot of it's punch pretty quickly. That lost of heat won't happen as quickly with a 1/2" steel slab. With a 1/2" steel slab, a bottom burner at full blast and a stone hearth heated to the same temp, I expect more than enough browning/charring for my needs.

[slkinsey]

So how did you do this, exactly? Preheated the pan to 600 degrees, then took away the heat source somehow and somehow held the english muffin 1 inch away from the surface of the pan for 4 minutes? Was this on the stove? In the oven? I'm just trying to get a picture of how you did this.

You can, of course, brown an english muffin in a 600 degree oven without holding it 1 inch away from anything at all if you leave it in there long enough. And those of us who own toasters know that you can brown an english muffin in around 60 seconds by holding it 1 inch away from a piece of metal that weighs around 1/1000th of what your cast iron pan weighs, so long as that metal is red hot.

Again, no one is saying that sub-red hot substances don't radiate thermal energy. They just don't radiate nearly as much. More to the point, considering that your metal slab will be continuously replenished by the ambient heat of the oven, a large thermal mass isn't required to sustain the radiation. You might be able to get a better effect with a thin sheet of very shiny material. Surely you're not putting the pizza into the oven, turning off the heat and leaving the door open? A ten pound slab of metal in a 600 degree oven won't radiate any more thermal energy than a one pound slab of the same stuff. The only difference a large thermal capacity will make is that it will take longer to come up to temperature (or if you turn off the heat source).

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'm with Ray. I say try it and see what happens. My experience in baking multiple "levels" of pizza in my oven at the same time is that having something on the shelf above makes some difference, but by no means a dramatic difference. You will get nowhere near the effect achieved by reflected radiant heat in a real wood- or coal-burning pizza oven. I also don't see any reason why it would work any better with a steel plate above than it would with a baking stone above (which, it must be said, will be a lot easier on your oven). It might also be interesting to experiment with putting something highly reflective above, so that rather than counting on radiation out of the top piece you would rather be refecting thermal radiation down onto the pizza.

[boilsover]

Host's note: this post refers to a comment in the All-Clad D7 Cookware (7 layers) topic.

 

I don't think it's that silly. Cast iron is great for some things precisely because it is heavy. The sheer mass of cast iron is what makes it so good at what it does.

 

Really?  All these years I was thinking cast iron's chief putative advantage (along with taking on seasoning) was storing heat.

 

In fact, cast iron isn't even terribly good at that.  For equal mass, aluminum is vastly superior at storing heat, and copper is basically a tie.

 

What cast iron has going for it is low cost and the thickness required for it to be cast without cracking or warping, usually 3-5mm.  The mass-to-cost ratio is spectacular--as are the makers' profit margins--but that's about it.  I have a medium-sized straight-gauge aluminum omelet pan that is just shy of 7mm thick, and weighs 3.3 lbs; no cast iron pan of similar mass can compete at holding and bestowing heat, unless you're focused on keeping food warm at table.  Personally, I'd rather have a cozy or rechaud for that.

 

Past the incidental benefit of mass:cost and the aforementioned quasi-nonstick property, cast iron pans are basically intended to be an affordable means of separating food and stove.  IMO, of course.

 

That's why I think it's silly for All-Clad to try to emulate it.  

Edited July 24, 2015 by Smithy (log)

[MSRadell]

Really?  All these years I was thinking cast iron's chief putative advantage (along with taking on seasoning) was storing heat.

 

In fact, cast iron isn't even terribly good at that.  For equal mass, aluminum is vastly superior at storing heat, and copper is basically a tie.

Aluminum isn't even close to cast-iron as far as storing heat goes. It transfers heat very effectively as does copper but cast-iron definitely holds heat much better. There's a big difference between heat transfer and heat storage, just portable textbook and look at the properties of the various materials. The same principle applies in that aluminum and copper are good conductors of electricity but cast-iron isn't.

[JoNorvelleWalker]

Aluminum isn't even close to cast-iron as far as storing heat goes. It transfers heat very effectively as does copper but cast-iron definitely holds heat much better. There's a big difference between heat transfer and heat storage, just portable textbook and look at the properties of the various materials. The same principle applies in that aluminum and copper are good conductors of electricity but cast-iron isn't.

 

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

 

Compare aluminum and cast iron.

Edited July 23, 2015 by JoNorvelleWalker (log)

[Shel_B]

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

 

Compare aluminum and cast iron.

 

What do all those numbers mean?  I don't understand the explanations and descriptions.  Can you just tell us in a couple of simple sentences how iron and aluminum compare in the context of this discussion?  Is the aluminum used in cookware, or, more specifically, the aluminum used in All-Clad, the same as the aluminum tested and shown in the linked chart?  How might any differences effect the heating properties of the cookware?  Likewise, the iron used in cast iron cookware.

[AlaMoi]

you're both right.  that's the problem with science - it can mis-direct a body.

 

there is a "coefficient" aka "number" which represents how much heat energy a material "holds" -

 

according to that number, aluminum is much better at holding heat than cast iron (stainless, copper, etc) - see the table.

but, that number is "per pound/mass" of material.

a body will notice upon first hand inspection that aluminum pans are not as heavy as cast iron - there is less aluminum mass to the pan.

so, aluminum holds heat better but there's less of it to hold some "amount" of heat - the "more mass" factor of cast iron is why it is noted for "holding heat better."

Edited July 23, 2015 by AlaMoi (log)

[boilsover]

 

[sic] a body will notice upon first hand inspection that aluminum pans are not as heavy as cast iron...

 

Well, that's an unfortunate fact of marketing.  Aluminum pans are available which store more heat than same-sized cast iron pans.  See, e.g., the Alegacy Eagleware Point Two Five line, widely available at restaurant supply purveyors.  Likewise, venerable French makers like Gaillard and Jacquotot once offered aluminum extra fort lines that were nearly as good as their fort copper ones, and head and shoulders above iron.

 

A body will also benefit from understanding that, since aluminum stores almost exactly TWICE the heat of cast iron (pound for pound), an aluminum pan weighing HALF as much as a same-size cast iron one will hold EQUAL heat.  Anything more tthan half as heavy, then, necessarily holds MORE.

 

Now, specific heat is not everything, either.  But when you run the numbers for thermal diffusivity and emissivity, copper is the clear winner, and aluminum still beats cast iron all hollow.

 

It's physics.

Edited July 23, 2015 by boilsover (log)

[Shel_B]

[...] since aluminum stores almost exactly TWICE the heat of cast iron (pound for pound), an aluminum pan weighing HALF as much as a same-size cast iron one will hold EQUAL heat.  Anything more tthan half as heavy, then, necessarily holds MORE.

 

 

 

When figuring the weight of a pot for the purpose of estimating heat retention, etc., should the lid be considered?  Might this be more of a consideration when using the pot in the oven vs on the stovetop?