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Old 01-03-2006, 05:33 AM posted to rec.food.baking
[email protected][_1_] djs0302@aol.com[_1_] is offline
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Default Baking cakes in foil pans


Bob (this one) wrote:
s foil pans reflect heat.

I did as you suggested. *All* the hits were from non-scientific sites,
with the words sounding like they were written by the same person. You
need to evaluate the sources of your information better than you did.
None were authoritative, and merely parroted each other.

Now look at the technical sites...

"If metal is a conductor of heat, why is it that aluminum foil will
insulate food and reflect heat?
"Aluminum may be a good conductor of heat, but its a terrible
emitter or absorber of thermal radiation. When you wrap food in aluminum
foil, you dramatically reduce that food's ability to lose heat via
radiation if it's hotter than its surroundings or its ability to gain
heat via radiation if it's colder than its surroundings. Aluminum foil
doesn't have much effect on heat transferred to or from the food via
conduction or convection because aluminum itself is a good conductor of
heat." http://rabi.phys.virginia.edu/HTW/clothing_and_insulation.html

"...terrible emitter or absorber of thermal radiation" means that it
doesn't reflect it, it simply doesn't absorb it. It doesn't make it go
back where it came from, nor does it capture it.

See that "good conductor of heat" thing? Not "reflector" of heat.

Here's another one:
"2.132 Reflect radiant heat waves
Heat tissue paper with a magnifying glass, as in 2.131. Note the
distance from the reading glass to the tissue paper. Put a tilted mirror
half way between the lens and the paper. Feel with your hand above the
mirror until you find the point where the heat waves are focussed. Hold
a piece of paper tissue at this point. The paper ignites."

That wouldn't happen in the dark. Light is radiant heat. It reflects
light. It doesn't reflect heat as heat, it just doesn't absorb radiant
energy well, nor does it transmit it. You say it reflects heat, so I'm
sure you'd closely wrap your hand in a single layer of foil and hold a
candle under it such that the flame was 1/2 inch below it. It'll reflect
the radiant heat (we call that light) but the conducted heat and
convected heat will fricassee your hand. That's why aluminum cookware
works well. It conducts heat very well; it doesn't reflect it back
towards the flame or the coil.

"23.00 Heat & temperature, internal energy & heat, heat and the first
law of thermodynamics
Heat is a form of energy measured in Joule. The first law of
thermodynamics states when other forms of energy are converted to heat,
or when heat is converted to other forms of energy, there is no loss of
total energy. The second law of thermodynamics states heat always flows
from hot bodies to cold bodies. "
http://www.uq.edu.au/_School_Science_Lessons/UNPh23.html

Note that aluminum is right behind gold and copper in its capacity to
conduct heat (the last column of the chart). Silver isn't mentioned, but
it's at the top of the list.
http://www.ee.byu.edu/cleanroom/thermal_properties.phtml

"Clearly in selecting a conductor these are very significant differences
- so the best materials are those which lie low towards the bottom of
the metals bubble, such as copper and aluminium. Gold is excellent, but
it is so expensive it is way off the scale of the chart. Even so, it is
used for electrical contacts in microcircuits.

"Finally, thermal conductivity and electrical conductivity are closely
related - as the underlying physics is similar. The electrical
resistivity chart also gives an indication of thermal properties - with
thermal insulators towards the top (polymers and foams, and ceramics)
and the good thermal conductors - metals - at the bottom."
http://www-materials.eng.cam.ac.uk/mpsite/physics/introduction/default.html

This means that heat and electricity are treated similarly by metals.
Aluminum is a good conductor of electricity, so it's also a good
conductor of heat. As nothing reflects electricity, nothing reflects
heat, either.

You might want to consider why there are charts for conduction of heat,
but none for reflection of heat.

This whole issue reminds me of that silly device with a hemispherical
mirror (like a bowl) into which you were supposed to put ice to reflect
the cold upwards and chill things in a small basket at the mouth of the
bowl. It was a physical impossibility.

Now you go find out what "heat" is and is not. And learn the difference
between radiant heat and conducted or convected heat is. And don't go to
cooking hobbyist sites for lessons in physics.

Pastorio


You can quote as much so called scientific proof as much as you want
but I know from experience that shiny aluminum baking pans reflect
heat. Besides, there's a big difference between the way a pan heats up
on a stove and the way a pan heats up in an oven. A pan on a stove
heats up entirely by conduction. A pan in an oven heats up by means of
radiation and indirect conduction, that is the oven's heat source heats
the air inside the oven and the heat from the air is conducted into the
pan. Since light colored materials reflect radiant energy, part of the
radiant heat inside the oven is reflected away from the pan.