What is foil made up of?

Online Answer
Aluminum foil is made by rolling sheets of 98.5 percent pure aluminum metal between pairs of polished, lubricated steel rollers. Successive passes through the rollers squeeze the foil thinner. Household aluminum foil is so thin (0.0005 of an inch) that the rollers can't handle it without tearing it.
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According to Reynold's Kitchen, the difference in appearance between the two sides of aluminum foil is simply a result of manufacturing and serves no real purpose. Meaning, whether you are cooking your food with the shiny side up or the dull side up, you're doing it right.
Aluminum foil can also be used to cover doorknobs and hardware. Wrap a bit of foil around the knobs to protect them from stray paint drips, and save your painter's tape for baseboards and door frames, where those straight blue edges are far more critical..
Aluminum foil is actually made of two layers: one of aluminum, another that is coating. The surface of the aluminum layer that is in contact with atmosphere oxidizes and turns into aluminum oxide — hence the matte side. In that case, we should use the shiny side, since aluminum oxide is not that healthy.
The difference in appearance between dull and shiny is due to the foil manufacturing process, he said. "In the final rolling step, two layers of foil are passed through the rolling mill at the same time. The side coming in contact with the mill's highly polished steel rollers becomes shiny.
Good news! According the the experts at Reynolds, there really isn't a difference. "Regardless of the side, both sides do the same job cooking, freezing and storing food," Mike Mazza, marketing director for Reynolds Wrap, told TODAY Home via email.