It’s from the manufacturing process.
Basically, during the last stage of making it, they have to do two sheets stacked on top of each other otherwise it would break. The dull side is where those two sheets are touching and the shiny side is where they aren’t touching.
There is no *meaningful* difference between each side when it comes to the way people normally use it. I’m sure if you actually got super precise and technical each side is a little different but not in any way that would affect anything a normal person would be doing. Like might hear people say you should have the shiny side up for such and such, that’s all bullocks.
From the Reynolds Brands page on aluminum foil:
**What side of the foil should I use, shiny or dull?**
With standard and heavy duty foil, it’s perfectly fine to place your food on either side so you can decide if you prefer to have the shiny or dull side facing out. For Reynolds Wrap® Non-Stick Aluminum Foil, you’ll want to have the dull side facing up. A tip is that if you can read the writing on the foil then you have the right side up to get all of the benefits of non-stick foil.
Essentially both sides are the same. The shiny/dull difference is a side effect of the manufacturing process for regular aluminum foil, there’s no technical difference.
Shiny side up looks nicer. The superstitious will place shiny side in, to bounce the heat around. Complete hogwash. As are Columbia’s foil ‘dots’ lined long underwear, which makes them colder than regular long underwear… because it’s METAL, against your SKIN. People get funny about the properties of aluminum foil.
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