For those with no idea... metal sheet is rolled between big shiny rolls, like being squeezed through a mangle.
This is standard practice for rolling of all sorts of metal sheets, but when it gets ultra thin, it creates problems, because the gap between the rolls is so small, and the loads get high, which flexes the big rolls. In Aluminium foil, this problem is overcome by doubling it up.... having 2 separates sheets going through the roll gap at the same time, so the rolls feel like they are rolling aluminium that is twice as thick. Both are squeezed thinner. They put a liquid between them to make sure they don't fuse together.... so that when they come out of the other side of the mangle, they easily separate, and you have 2 thin sheets of foil. The side which were squashed against the other aluminium foil come out a bit matt... and the sides that were directly in contact with the shiny roll come out shiny.
