If you examine a typical roll of aluminum foil, you'll notice that one side is shiny, but the other is not. Why is that? Melissa of Today I Found Out explains that the way that it's manufactured makes this necessary:
In what is called the Bayer Process, after pure molten aluminum is obtained from aluminum oxide, it is placed in furnaces with a small amount of other elements (typically the final product will be between 99.8% and 99.9% aluminum). This liquid is then poured into “chill casting devices where it cools into large slabs called ‘ingots.’” Next the ingot is treated with heat (annealed), and then rolled between heavy rollers.
This initial foil is sent through still more rollers, several times, until it reaches the desired thinness. For the type of foil that is bright on one side and matte on the other, it is so thin that during some of the last rollings, two sheets of the thin foil must be placed together lest they tear or crimp during the final rolling of the sheets.
One consequence of this is that while the sides that touch the highly polished rollers are burnished to a bright finish, the inner sides that touch the other aluminum foil remain matte.
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