Aluminium foil is aluminium prepared in thin metal leaves with a thickness less than 0.2 mm; thinner gauges down to 6 micrometres are also commonly used. Standard household foil is typically 0.016 mm thick, and heavy duty household foil is typically 0.024 mm. The foil is pliable, and can be readily bent or wrapped around objects. Thin foils are fragile and are sometimes laminated to other materials such as plastics or paper to make them more useful.

Aluminium foil is produced by rolling sheet ingots cast from molten billet aluminium, then re-rolling on sheet and foil rolling mills to the desired thickness, or by continuously casting and cold rolling. To maintain a constant thickness in aluminium foil production, beta radiation is passed through the foil to a sensor on the other side. If the intensity becomes too high, then the rollers adjust, increasing the thickness. If the intensities become too low and the foil has become too thick, the rollers apply more pressure, causing the foil to be made thinner.

The continuous casting method is much less energy intensive and has become the preferred process. For thicknesses below 0.025 mm, two layers are usually put together for the final pass and afterwards separated which produces foil with one bright side and one matte side. The two sides in contact with each other are matte and the exterior sides become bright; this is done to reduce tearing, increase production rates, control thickness, and get around the need for a smaller diameter roller.

Some lubrication is needed during the rolling stages; otherwise, the foil surface can become marked with a herringbone pattern. These lubricants are sprayed on the foil surface before passing through the mill rolls. Kerosene based lubricants are commonly used, although oils approved for food contact must be used for foil intended for food packaging.

Aluminium becomes work hardened during the cold rolling process and is annealed for most purposes. The rolls of foil are heated until the degree of softness is reached, which may be up to 340 °C (644 °F) for 12 hours. During this heating, the lubricating oils are burned off, leaving a dry surface. Lubricant oils may not be completely burnt off for hard temper rolls, which can make subsequent coating or printing more difficult.

The rolls of aluminium foil are then slit on slitter rewinding machines into smaller rolls. Roll slitting and rewinding is an essential part of the finishing process.

Aluminium foils thicker than 25 μm are impermeable to oxygen and water. Foils thinner than this become slightly permeable due to minute pinholes caused by the production process.

Aluminium foil has a shiny side and a matte side. The shiny side is produced when the aluminium is rolled during the final pass. It is difficult to produce rollers with a gap fine enough to cope with the foil gauge, therefore, for the final pass, two sheets are rolled at the same time, doubling the thickness of the gauge at entry to the rollers. When the sheets are later separated, the inside surface is dull, and the outside surface is shiny.

USES: 

Packaging: Aluminium is used for packaging as it is highly malleable: it can be easily converted to thin sheets and folded, rolled or packed. Aluminium foil acts as a total barrier to light and oxygen, odours and flavours, moistness, and germs, and so it is used broadly in food and pharmaceutical packaging, including long-life packs for drinks and dairy goods, which allows storing without refrigeration. Aluminium foil containers and trays are used to bake pies and to pack takeaway meals, ready snacks and long life pet foods.

Insulation: Aluminium foil is widely used for radiation shield (barrier and reflectivity), heat exchangers (heat conduction) and cable liners (barrier and electrical conductivity). Aluminium foil’s heat conductive qualities make it a common accessory in hookah smoking: a sheet of perforated aluminium foil is frequently placed between the coal and the tobacco, allowing the tobacco to be heated without coming into direct contact with the burning coal.

Electromagnetic shielding: The shielding effectiveness of aluminium foil depends upon the type of incident field (electric, magnetic, or plane wave), the thickness of the foil, and the frequency (which determines the skin depth). Shielding effectiveness is usually broken down into a reflection loss (the energy bounces off the shield rather than penetrates it) and an absorption loss (the energy is dissipated within the shield).

Cooking: Aluminium foil is also used for barbecuing delicate foods, such as mushrooms and vegetables. Using this method, sometimes called a hobo pack, food is wrapped in foil, then placed on the grill, preventing loss of moisture that may result in a less appealing texture.

Art and decoration: Heavier foils made of aluminium are used for art, decoration, and crafts, especially in bright metallic colours. Metallic aluminium, normally silvery in colour, can be made to take on other colours through anodisation. Anodising creates an oxide layer on the aluminium surface that can accept coloured dyes or metallic salts, depending on the process used. In this way, aluminium is used to create an inexpensive gold foil that actually contains no gold, and many other bright metallic colours. These foils are sometimes used in distinctive packaging.