Why are most Aluminum Foil Raw Materials shiny on one side and gray on the other?

The production process of aluminum foil is a delicate electrolytic process, which mainly involves the electrolytic refining and rolling of aluminum. In this process, aluminum ore (such as bauxite) is first smelted into aluminum compounds, and then undergoes an electrolysis reaction to obtain pure aluminum metal. During the electrolysis reaction, aluminum ore is placed in an electrolytic cell and mixed with an electrolyte, such as a molten salt of aluminum oxide. When electric current passes through the electrolyte, aluminum ions are attracted to the cathode (usually a block of carbon), where they are reduced to metallic aluminum. This process produces large amounts of heat and gases, including oxygen.
During this process, the generated metallic aluminum gradually forms a thin metal film on the cathode. This metal film is continuously pulled out of the electrolytic tank and rolled through a series of rolling machines to form our common aluminum foil raw material. Rolling is an important step in the aluminum foil production process, which makes the aluminum metal layer thinner and flatter. However, due to the design and operation of the rolling mill, aluminum foil experiences varying pressures and temperatures during the rolling process. These factors lead to different physical and chemical properties on both sides of the aluminum foil.
Specifically, the side of the aluminum foil that is in direct contact with the electrolyte will form an oxide layer due to the action of oxygen and other gases generated during the electrolysis process. This oxide layer is mainly composed of aluminum oxide, which makes this side of the aluminum foil rough and appears gray or dark.
The other side of the aluminum foil has no direct contact with the electrolyte, so no oxide layer will form during the electrolysis process. This side maintains the luster and smoothness of the aluminum metal itself, so it shows a bright luster.
To sum up, the reason why most aluminum foil raw materials are shiny on one side and gray on the other side is mainly due to the different physical and chemical conditions experienced by both sides of the aluminum foil during the electrolysis and rolling processes. This property allows aluminum foils to have different surface properties in applications, for example the glossy side may be more suitable for food packaging as it is easier to clean and reflects heat, while the gray side may be more suitable for applications requiring a certain amount of friction, such as Industrial packaging or construction materials.