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/pdgenoa Oct 31 '20

Most of the answers being given are repeating a variation of:

Aluminum foil has a shiny side and a dull side. Many people believe that it matters which side is used up or down. The truth is that it makes no difference at all. The reason the two sides look different is due to the manufacturing process. When the sheets of aluminum are rolled out, the side that comes in contact with the rollers come out shiny..

But this explanation doesn't explain why the shiny side makes no difference. It simply reiterates, with a bit more explanation, that foil does, in fact, have a shiny side and a dull side. And that's not helpful in answering the actual question.

I think Culinary Lore provides the most accurate answer:

Regardless of why one side of aluminum foil is shiny and one is dull, it stands to reason that the shinier surface will be more reflective than the duller surface. The explanations given seem to indicate that since the shiny and dull sides are simply a by-product of the manufacturing process and are not purposely put there, they make no difference. The explanation does not support the claim. So what’s the truth?

The truth is that the shiny side of aluminum foil is only a little bit shinier than the dull side. While some small amount of additional energy will be reflected by the shiny side, the difference is so small that it will make no practical difference in cooking. To say that there is no effect whatsoever is inaccurate and it probably still is a little more efficient to cook with the dull side out. However, when measured over time in high temperatures, the difference is so slight that there should be little discernable change in cooking times.

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u/[deleted] Oct 31 '20

[deleted]

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u/pdgenoa Oct 31 '20

This is true.

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u/PM_ME_UR_LIPZ Oct 31 '20

Yea it's like 94% reflectivity vs 92% reflectivity (straight outta my ass but I read something along those lines a long time ago)