Aluminium foil (or aluminum foil), often referred to with the misnomer tin foil, is aluminium prepared in thin metal leaves with a thickness less than 0.2 mm (7.9 mils); thinner gauges down to 6 micrometres (0.24 mils) are also commonly used.
In the United States, foils are commonly gauged in thousandths of an inch or mils. Standard household foil is typically 0.016 mm (0.63 mils) thick, and heavy duty household foil is typically 0.024 mm (0.94 mils). 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 supplanted tin foil in the mid 20th century.
Annual production of aluminium foil was approximately 800,000 tonnes (880,000 tons) in Europe and 600,000 tonnes (660,000 tons) in the U.S. in 2003.
Approximately 75% of aluminium foil is used for packaging of foods, cosmetics, and chemical products, and 25% used for industrial applications (e.g. thermal insulation, cables and electronics). It can be recycled.
In North America, aluminium foil is known as aluminum foil. It was popularised by Reynolds Metals, the leading manufacturer in North America. In the United Kingdom and United States it is, informally, widely called tin foil, for historical reasons (similar to how aluminium cans are often still called “tin cans”). Metallised films are sometimes mistaken for aluminium foil, but are actually polymer films coated with a thin layer of aluminium. In Australia, aluminium foil is widely called alfoil.
A sheet of aluminium foil
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 (1 mil), 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.
Microscopic close-up of aluminium foil on the back of an intumescent rubber strip.
Aluminium foils thicker than 25 μm (1 mil) 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 aluminium sheets are later separated, the inside surface is dull, and the outside surface is shiny. This difference in the finish has led to the perception that favouring a side has an effect when cooking. While many believe (wrongly) that the different properties keep heat out when wrapped with the shiny finish facing out, and keep heat in with the shiny finish facing inwards, the actual difference is imperceptible without instrumentation. Increased reflectivity decreases both absorption and emission of radiation. Foil may have a non-stick coating on only one side.
The reflectivity of bright aluminium foil is 88% while dull embossed foil is about 80%.