Why Does Aluminium Foil Have a Shiny Side and a Dull Side?

One side of household aluminium foil looks almost mirror-bright while the other is matt. It is tempting to assume that the manufacturer designed the two sides for different jobs. In ordinary foil, however, the contrast is mainly a trace of how the material is made.

Aluminium is rolled repeatedly into an increasingly thin strip. At the final stage, a single layer is so thin that it can tear easily, so two layers are rolled together. The outer surfaces touch the smooth rollers and are pressed to a smoother finish, reflecting light more directly and appearing shiny. The two inner surfaces touch each other rather than the rollers. Their microscopic texture scatters light more broadly, so they look dull. The layers are then separated, leaving every sheet with one finish of each kind.

The two faces are not different grades of aluminium, nor are they deliberately assigned separate heating and insulating functions. Manufacturers state that either side of standard and heavy-duty household foil may face the food without changing normal cooking performance. Special non-stick foil is an exception because one surface has an added coating; its packaging should identify the side intended for contact with food. The important distinction is the coating, not shine by itself.

https://www.alufoil.org/facts-and-properties
https://www.reynoldsbrands.com/products/aluminum-foil/heavy-duty-foil