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  #1  
Old 02-21-2006, 10:29 AM
Fridgemagnet Fridgemagnet is offline
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Aluminium foil - shiny side/dull side

In cookery lessons years ago I was taught that there's a subtle difference in cooking characteristics between the shiny side of the foil and the dull side - if you want crispier food, wrap in aluminium (US: aluminum) foil with the shiny side towards the food.

I've also heard it makes no difference at all, and the shiny/dull sides of the foil are merely a product of the manufacturing process.

Should I bother observing foil polarity when wrapping my jacket potatoes? Or can I be random from now on?
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  #2  
Old 02-21-2006, 10:34 AM
Mangetout Mangetout is offline
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It doesn't make any difference; the difference in the amount of radiated heat reflected back is going to be a drop in the ocean, as the matt side is still reflecting quite a bit (or it would look black).

I have heard that the shiny side may carry oil residues from the manufacturing process, but this doesn't seem a massive worry; you'll ingest more hydrocarbons walking through a city centre.
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Old 02-21-2006, 10:57 AM
Patty O'Furniture Patty O'Furniture is offline
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Old 02-21-2006, 11:03 AM
bibliophage bibliophage is offline
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This column of Cecil's may be of interest: Should a baking potato be wrapped in foil shiny side in or shiny side out?
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Old 02-21-2006, 11:05 AM
bibliophage bibliophage is offline
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My dull side is out today, as you can see.
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Old 02-21-2006, 11:13 AM
David Simmons David Simmons is offline
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Quote:
Originally Posted by Fridgemagnet
In cookery lessons years ago I was taught that there's a subtle difference in cooking characteristics between the shiny side of the foil and the dull side - if you want crispier food, wrap in aluminium (US: aluminum) foil with the shiny side towards the food.

I've also heard it makes no difference at all, and the shiny/dull sides of the foil are merely a product of the manufacturing process.

Should I bother observing foil polarity when wrapping my jacket potatoes? Or can I be random from now on?
Bsed on the test described in the cited Cecil's column you can confidently state (with 95% confidence) that between 19% and 81% of the time shiny-side-in will bake faster,or vice versa as the case may be.
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Old 02-21-2006, 11:40 AM
Biffy the Elephant Shrew Biffy the Elephant Shrew is online now
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Conversely, when constructing an aluminum foil hat, the shiny side faces out, the better to repel the mind control rays.
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Old 02-21-2006, 12:16 PM
Mr. Blue Sky Mr. Blue Sky is offline
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Originally Posted by Biffy the Elephant Shrew
Conversely, when constructing an aluminum foil hat, the shiny side faces out, the better to repel the mind control rays.

If you are an evil alien bent on world domination, but don't want to do so just yet, place the shiny side in to keep your evil thoughts in your evil head.
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Old 02-21-2006, 02:21 PM
Fridgemagnet Fridgemagnet is offline
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Thanks guys, that's one less detail in my life I have to bother with. From now on, the foil is wrapped in whatever way I find aesthetically pleasing.
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  #10  
Old 02-21-2006, 03:35 PM
Chronos Chronos is offline
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The basic argument of those who say that one side is better than the other is that heat will pass through the foil more readily in one direction than in the other. But the existance of a material which passed heat more readily in one direction than in the other would be a violation of the Second Law of Thermodynamics. And in this house, we obey the Laws of Thermodynamics!

Since everything in the Universe transmits heat equally well in both directions, aluminum foil will retain heat equally well, whichever side is out. Lacking any other mechanism, then, there's no difference which way you wrap it.
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Old 02-21-2006, 03:43 PM
scr4 scr4 is offline
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Quote:
Originally Posted by Chronos
But the existance of a material which passed heat more readily in one direction than in the other would be a violation of the Second Law of Thermodynamics.
Not if the two sides are at very different temperatures. Consider a mylar film aluminized on one side. Wrap our body in it, and stand outside in a cold, clear night. You'll be a lot warmer with the aluminized side facing out, becuase the aluminized surface emits very little IR. With the bare mylar side facing out, the body heat conducts easily through the aluminum coating, and then get radiated away by the mylar.

That said, ovens work mostly by convection, not radiation. And both sides of the aluminum foil are bare aluminum. So I expect the difference would be negligible.
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Old 02-21-2006, 05:00 PM
Chronos Chronos is offline
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True. I was forgetting that a material might (and in fact probably will) have different thermal couplings via the different methods of heat exchange. Metals, as you point out, conduct heat well but radiate it poorly, so a multi-layer material composed of metal and something else (like your aluminized mylar) could have different results depending on which side were in. And even with bare metal on both sides, you will have different radiative qualities.

I would, however, dispute the assertation that normal ovens work primarily by convection. There are some ovens (called, appropriately enough, convection ovens) which heat largely by convection, but in most of them, radiation is dominant. Think about the different results in baking a dish covered with a metal lid or aluminum foil, vs. one covered with plastic oven wrap.
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Old 02-21-2006, 05:18 PM
scr4 scr4 is offline
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Quote:
Originally Posted by Chronos
I would, however, dispute the assertation that normal ovens work primarily by convection. ... Think about the different results in baking a dish covered with a metal lid or aluminum foil, vs. one covered with plastic oven wrap.
I admit I'm not much of an oven user - is there much difference, and not just because one retains moisture better than the other?

I found some interesting numbers here it says the shiny side has 20% higher emissivity than the dull side. If that's true, and if radiation is dominant, shiny side out should heat up the object up to20% faster, at least initially. Of course it'll reach the same equilibrium temperature.
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  #14  
Old 02-21-2006, 07:22 PM
mangeorge mangeorge is offline
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A potato lesson for you.

Why anyone would wrap a potato(e ) in foil to bake it is beyond me. Might as well boil it, because that's what you're doing. Rub that russet with Crisco and put it right on the oven rack. Temperature isn't really very important in case you're also baking something else. Lower temps just take longer.
By the way, Reynold's Release foil actually works pretty good. You do have to pay attention to which side contacts the food, though, unlike regular foil.
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Old 02-22-2006, 03:39 PM
Chronos Chronos is offline
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A baked dish with exposed top, or covered only by a transparent cover, will brown much more than one with an opaque top.

On the other hand, when I think of wrapping something in foil to cook it, I'm much more likely to think campfire than oven. There, I suspect that conduction is the dominant transfer mechanism, since the foil-wrapped food (not necessarily a potat) is directly in contact with the hot coals.
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  #16  
Old 02-22-2006, 03:51 PM
kunilou kunilou is offline
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Let's get to the real question. I wish to inflict maximum pain on my little brother. When I get him to bite down on a piece of aluminum foil, should his dental fillings make contact with the shiny side, or dull side?
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  #17  
Old 02-22-2006, 06:37 PM
mangeorge mangeorge is offline
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Oh, that!

Quote:
Originally Posted by kunilou
Let's get to the real question. I wish to inflict maximum pain on my little brother. When I get him to bite down on a piece of aluminum foil, should his dental fillings make contact with the shiny side, or dull side?
You need to do sort of a "Z" fold, so that the shiny side contacts his top teeth, and the dull side contacts his lowers.
CAUTION: Be very careful. If you get it wrong (backwards), you'll turn him gay. That's how it happens, you know.
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