Ask A Scientist General Science Archive

name         Connie S.
status       other
age          50s

Question -   Hello. I have a question that I am sure you will have the answer to. What side 
of the aluminum foil should be on the outside when I wrap something up in it to keep warm or 
cold? The shiny side or the dull?
I was at a grill out last week and was told that the shiny side should be on the inside and 
the dull side of the aluminum foil should be on the outside.
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Connie,

The shiny side is slightly better reflector of heat. Face that side of the foil in the 
appropriate direction. To keep things cold, put the shiny side on the outside -- that 
will reflect incoming heat. To keep things warm, face the shiny side inward toward the 
hot food -- to reflect the heat that is trying to escape back into the food.

Note that people often bake potatoes with the shiny side out -- that is because it makes 
for a better (prettier) presentation. In reality, baking them that way reflects the 
incoming heat on the outside, slightly slowing the cooking process.

Of course, in reality, which side of the foil faces where makes little difference. It does 
make for lively conversation, though.

Regards,
ProfHoff 686
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There is no difference in which side you use.  The difference in the surface of aluminum 
foil is a result of the manufacturing process and has no bearing on heat retention or 
reflection.

Chris Murphy
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As a practical matter, it probably does not matter because convection is overwhelming 
any difference in reflectance of shiny vs. dull side of the foil. The science is that 
you are balancing the E = K* T^4 law, where T is the absolute temperature in kelvins, 
K is a constant, and E is the energy transferred. The value of 'K' depends upon the 
details of the reflectance of the surface. But the target "hamburger-in-foil-on-the-grill" 
is, in practice, very complicated. The charcoal is radiating heat to the burger from below 
(which is highly variable depending upon the surface temperature of the coals. On top the 
burger-foil is losing heat energy to the surroundings, but it is virtually impossible to 
assign an "average" temperature to the surroundings. To make matters worse, the aluminum 
foil is "crinkled" so that the reflecting surface is not uniform, and on the "dull" side 
the reflectance is not even specular, i.e. there is a range of angles of reflection for a 
specified angle of incidence, and that "range" depends upon the angle of reflectance.


Now what DOES make a difference with respect to aluminum foil in general, either on the 
grill or in the oven is this. DO NOT let the aluminum foil contact an acidic ingredient, 
like tomato sauce, catsup.. At oven (or grill) temperature the acidic ingredient will 
dissolve the aluminum foil. Although Al(+3) is not especially toxic, contact with acidic 
foods at high temperature should be avoided.

Vince Calder
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Connie,

Short answer:  I believe that the dull side should be out.

Long answer:

That would be my answer too.  The only real scientific basis that I can think of for this 
answer is:  Radiative heating / cooling.  I have done no experiments to prove one way or 
the other though.  The goal is to maximize heat transfer.  Heat transfer can happen by 
CONDUCTIVE, CONVECTIVE, and RADIATIVE means.

Conductive heating will happen by the hot grill in direct contact with the foil.  This 
mode is limited by the thermal conductivity of the foil.

Convective heating will happen by the hot air movement around the foil and transferring 
its heat to the surface.  Then it will also be limited by the a heat transfer coefficient 
based on how fast the air is moving and of course the temperature difference between the 
foil and the hot air.

Radiative heating will happen by the absorption of infrared radiation (IR radiation) or 
heat emanating from all around the food (within the BBQ).  Now please consider infrared 
radiation to be in the wavelength range of 880 nanometers or so.  Realize that this 
wavelength of "light" is very near the color red in the visible portion of the spectrum. 
Further, this means that the radiation will act like light and the foil shall act as a 
mirror.  Knowing now that you want to transmit (foil to absorb all radiation) as much IR 
through the foil as possible.  It should be clear to you that the dull side should be 
OUTWARD facing so as not to minimize the reflection of IR radiation.

Hope this helps,
Darin Wagner
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Shiny side IN if you want to keep the contents warm (eg. human body); shiny side OUT if 
you want to keep heat out.

Howard Barnes.
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