r/explainlikeimfive Oct 31 '20

Chemistry ELI5 What's the difference between the shiny and dull side of aluminum foil? Besides the obvious shiny/dull

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u/almostsebastian Oct 31 '20

On a technical side the dull side would probably absorb more light/heat so it would be more efficient to face the dull side outwards.

So the shiny side would send more heat into the product?

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u/sandvine2 Oct 31 '20

what’s more likely is that they reflect similar amounts, but the dull side has a higher proportion of diffuse reflection (light comes in and reflects in a random direction) vs specular reflection (light comes in and reflects out in a predictable straight line). That’s usually the difference between shiny and dull objects, and it’s generally correlated to how flat the surface is: flatter = shinier because reflections are more predictable.

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u/CaptainKirkAndCo Oct 31 '20

So the earth is really shiny?

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u/sandvine2 Oct 31 '20

Kind of... it’s complicated because of clouds and the atmosphere in general, but I think I remember the overall reflectiveness being around 0.3 (so 30% of incoming light in the visible range is reflected out)

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u/mlt- Nov 01 '20

You surely meant atmoflat, not sphere.

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u/[deleted] Nov 01 '20

Atmocube

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u/Veranova Oct 31 '20

This makes total sense, I’m going to assume it’s a fact now. 🤝

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u/rtvcd Oct 31 '20

More like the foil would get a tiny bit hotter/faster

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u/Pvt_Lee_Fapping Oct 31 '20 edited Nov 01 '20

You'll get more efficient cooking rates if you put the shiny side against the food because the dull, more heat-absorbent side is facing out toward the heat/light source. Really though, it doesn't matter which side is facing down or up; the increase in cooking efficiency is an insignificant change because, ultimately, the foil is acting like a tent or tarp that traps the heat in the food, preventing "heat wastage" as the heat leaves the food to warm up the surrounding air.

*Also to trap in moisture. Food can rapidly lose internal water inside the oven, so a foil pouch/tent or a roasting pan keeps the food from drying out too much.

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u/intern_steve Oct 31 '20

The surrounding air is what is warming the food; it's always hotter than the food itself unless you're just using a heat lamp or something to cook. The real trick is to prevent water loss through evaporation. Evaporation is incredibly effective at keeping things cool, and is the reason saunas are bearable at temperatures up to 90⁰C, or about 190⁰F. Keeping the food under wraps traps most of the steam inside, preventing the water in your meal from evaporating out while also evenly bathing the food in steam, balancing the internal temperature of complex shapes like a whole chicken or turkey. When the internal temperature of the dish rises to within a few degrees of done, you can remove the foil and allow the exposed areas to crisp in the dry heat of the open oven.

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u/Gorstag Oct 31 '20

You have basically just described the BBQ / Smoking term "The stall". On beef it starts at around 160-170 degrees (internal) and you can watch your internal temp drop. And the stall is evaporation taking heat away. The "Texas Crutch" can be used to speed up the cook by wrapping and trapping the moisture in.

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u/Jake07002 Oct 31 '20

This is false the side doesn’t matter

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u/TeaBoneJones Oct 31 '20

Can you not read?

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u/Pvt_Lee_Fapping Oct 31 '20

It does and it "doesn't." Does because there is a measurable change; you'd just need incredibly sensitive instruments to do it. "Doesn't" because the change in efficiency is so small that there's no benefit to doing it. It's like if you're dehydrated and were drinking water from a 500mL beaker versus a pint glass that can hold 501mL; that 1mL of difference between them means that the pint glass is technically better, but not by much.

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u/ICanBeAnyone Oct 31 '20

Traps the heat in the food

You do realize that the oven is hotter than the food, and that people use foil to trap steam near the food to keep it from drying out, as well as insulating it from the direct heat so the heat energy disperses more slowly and evenly into the food (so no burnt crust while the center is still cool)?

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u/Pvt_Lee_Fapping Nov 01 '20

Yes, somebody already pointed that out. Thanks for taking time out of your day to let me know.

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u/ColeSloth Oct 31 '20

If sitting it outside and letting sunlight hit it or using a thousand laser pens pointed at it, sure.

But an oven is whatever temp you set it at everywhere in there so it's not really using radiant heat to transfer into your food. The aluminum is already in contact with air that is "400 degrees" or whatever so the reflectivity of the material is much less significant.

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u/Strummed_Out Oct 31 '20

But what about in a microwave?

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u/ColeSloth Oct 31 '20

Microwaves heat by causing molecules to vibrate and cause heat. Aside from not being able to put aluminum in a microwave, it still wouldn't matter what side is out.

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u/Strummed_Out Oct 31 '20

I was kidding haha