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Comment by johng | Hacker News Reader

I always heard the shiny side reflected heat better. So that side should face food you are trying to heat up in the oven.

Any truth to that I wonder?

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No. Aluminum foil has the same material properties with respect to convection and conduction of heat no matter which side faces out. The only heat that would be different would be radiated heat, which your food won't have a ton of, and even then, the dull side is still quite reflective. It's maybe one of those "technically" correct statements that the shiny side reflects more heat, but for the application of cooking, the impact is effectively zero. The retention of steam is going to be such a larger factor the side you use will effectively make no difference.

> The only heat that would be different would be radiated heat, which your food won't have a ton of

Your food won't radiate much heat, but your oven certainly does. Aluminum + broiler is like photolithography for food.

I can speak for myself: when I ask if the shiny side reflects the heat better, I don't mean to also ask if the difference is significant. It's really just curiosity, whether my school physics intuition holds up or lies to me, that's all.

So, "technically yes" is good enough answer for me.

Is it technically true, though? The matte side has a difuse reflection, which does not mean it reflects less. It just scatters more.

The shiny and dull sides look like perfect mirrors in IR wavelengths.

The feature size of the matte vs shiny sides are much smaller than the wavelength of the bulk of the radiated light in either a microwave or conventional oven.

Is it? I can see the broiler glowing, so at least a decent chunk is close to visible light.

An alternate intuition pump, at least if you're old enough to remember incandescent bulbs: consider how bright a 1000-watt bulb is, compared to how bright (in the visible spectrum) the 2-3000 watt oven element gets.

Look up the emission spectrum for a body at the temperature of a typical oven element: yes, it starts emitting some light that is visible, but the bulk of the energy is still in the IR band.

> microwave or conventional oven

I worry for you if you're microwaving aluminum foil.

I recall similar advice around mylar heat blankets. Perhaps those got mixed up?

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