Why does aluminum foil have 2 different sides?

Answer

The difference in appearance between the shiny side and the dull (matte) side of aluminum foil is a result of the manufacturing process known as 'milling.' Aluminum is rolled through heavy steel rollers to achieve the desired sheet thickness. In the final rolling stage, two layers of foil are passed through the rollers at the same time because the foil is so thin it would likely break if rolled individually. This process is called 'doubling.' The side that comes into contact with the highly polished steel rollers becomes 'shiny,' while the side that faces the other layer of aluminum foil becomes 'dull' because it is in contact with metal rather than the smooth rollers. Despite the difference in appearance, both sides perform exactly the same for cooking and storage purposes.