Why does aluminum foil have a shiny side and a dull side?
The short answer
When makers roll very thin foil, they may pass two layers through the final rolling step together. The outer faces touch the smooth metal rollers and become shiny. The inner faces touch each other and come out with a matte finish.
For ordinary household foil, there is no special “front” or “back.” You can use either side when wrapping food or cooking.
How does rolling make foil so thin?
To make aluminum into a thin sheet, it is squeezed between large rollers. The sheet can pass through the rollers again and again until it reaches the required thickness. In the final stages, two webs of foil can be rolled together. This helps very thin foil withstand the rolling process without breaking.
The two outer faces touch the polished working rollers. The two inner faces touch one another. After rolling, the layers are separated: each sheet has one smooth, shiny face and one matte face. The difference comes from how the surfaces were made; it does not mean that the sheet has two different materials.
Which side should face the food?
With ordinary household foil, either side can face the food. The difference in appearance is mainly a result of manufacturing, and it does not tell you which side must go in or out.
There is an exception: some foils have a special non-stick coating on one side. Check the package or instructions to find the coated side. Some products use the matte-looking side for this purpose, but appearance alone is not a reliable guide.
A common misconception
The shiny face is not a required “heat-reflecting side” for ordinary cooking. Its appearance comes from the rolling process. With special products, follow the label because their treated surface may have a specific use.