Aluminium foil is aluminium prepared in thin metal leaves, with a thickness less than 0.2 millimetres (8 mils), thinner gauges down to 6 µm (0.2 mils) are also commonly used.[1] In the USA, foils are commonly gauged in mils. Standard household foil is typically 0.022 millimetres (0.9 mils) thick and heavy duty household foil is typically 0.033 millimetres (1 mil).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[1] and 600,000 tonnes (660,000 tons) in the USA in 2003.[2] 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).[2]
In North America, aluminium foil is known as aluminum foil. It is also sometimes called Reynolds wrap after 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. Metallised films are sometimes mistaken for aluminium foil, but are actually polymer films coated with a thin layer of aluminium.
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Foil made from a thin leaf of tin was commercially available before its aluminium counterpart. It was marketed commercially from the late nineteenth into the early twentieth century. The term "tin foil" survives in the English language as a term for the newer aluminium foil. Tin foil is less maleable than aluminium foil and tends to give a slight tin taste to food wrapped in it. Tin foil has been supplanted by aluminium and other materials for wrapping food.[3]
The first audio recordings on phonograph cylinders were made on tin foil.[4]
Tin was first replaced by aluminium in 1910, when the first aluminium foil rolling plant, "Dr. Lauber, Neher & Cie."[5] was opened in Emmishofen, Switzerland. The plant, owned by J.G. Neher & Sons, the aluminium manufacturers, started in 1886 in Schaffhausen, Switzerland , at the foot of the Rhine Falls - capturing the falls' energy to produce aluminium. Neher's sons together with Dr. Lauber discovered the endless rolling process and the use of aluminium foil as a protective barrier in December 1907.
The first use of foil in the United States was in 1913 for wrapping Life Savers, candy bars, and gum.[6] Processes evolved over time to include the use of print, colour, lacquer, laminate and the embossing of the aluminium.
Aluminium foil is produced by rolling sheet ingots cast from molten 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.[7] For thicknesses below 0.025 mm (1.0 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.[8] 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.[8]
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.
Aluminium foils thicker than 25 µm (1.0 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 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 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.[9] The reflectivity of bright aluminium foil is 88% while dull embossed foil is about 80%.[6]
As aluminium foil acts as a complete barrier to light and oxygen (which cause fats to oxidise or become rancid), odours and flavours, moisture, and bacteria, it is used extensively in food and pharmaceutical packaging. Aluminium foil is used to make long life packs (aseptic packaging) for drinks and dairy products which enables storage without refrigeration. Aluminium foil laminates are also used to package many other oxygen or moisture sensitive foods, and tobacco, in the form of pouches, sachets and tubes, and as tamper evident closures. Aluminium foil containers and trays are used to bake pies and to pack takeaway meals, ready snacks and long life pet foods.
Aluminium foil is widely sold into the consumer market, often in rolls of 500 mm (20 in) width and several metres in length.[10] It is used for wrapping food in order to preserve it, for example when storing leftover food in a refrigerator (where it serves the additional purpose of preventing odour exchange), when taking sandwiches on a journey, or when selling some kinds of take-away or fast food. Tex-Mex restaurants in the United States, for example, typically provide take-away burritos wrapped in aluminium foil.
Aluminium foil is widely used for thermal insulation (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.
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).
Aluminum is a good conductor, so even a thin sheet reflects almost all of an incident electric or wave. At frequencies less than 100 MHz, the electric field is attenuated by more than 80 decibels (dB) (only 10-8 = 0.00000001 of the energy gets through).[11]
Thin sheets of aluminum are not very effective at attenuating low-frequency magnetic fields. A refrigerator magnet will hold a sheet of aluminum foil to the refrigerator. The shielding effectiveness is dependent upon the skin depth. A field traveling through one skin depth will lose about 63 percent of its energy (it is attenuated to (1/e) = (1/2.718...) of its original energy). Thin shields also have internal reflections that reduce the shielding effectiveness.[12] For effective shielding from a magnetic field, the shield should be several skin depths thick. Aluminum foil is about 1 mil (25 µm); a thickness of 10 mils (250 µm) (ten times thicker) offers less than 1dB of shielding at 1 kHz, about 8 dB at 10 kHz, and about 25 dB at 100 kHz.
Aluminium foil is also used for barbecuing more delicate foods such as mushrooms and vegetables; food is wrapped in foil then placed on the grill, preventing loss of moisture that may result in a less appealing texture.
As is the case with all metallic items, aluminium foil reacts to being microwaved. This is due to the effect of electric fields of the microwaves causing a build up of charge to form between the sharp points in the aluminium; if enough charge accumulates it will discharge to a different place on the foil, creating a spark (i.e., arcing). The design of modern microwave ovens has been corrected so microwave energy cannot be reflected back into the magnetron, and aluminium packages designed for microwave heating are available.[13]
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 anodization. Anodizing 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.
Foil is used by organic/petroleum geochemists for protecting rock samples taken from the field and in the lab, where the sample is subject to biomarker analysis. While plastic or cloth bags are normally used for a geological sampling exercise, cloth bags are permeable and may allow organic solvents or oils (such as oils imparted from the skin) to taint the sample, and traces of the plastics from plastic bags may also taint the sample. Foil provides a seal to the ingress of organic solvents and does not taint the sample. Foil is also used extensively in geochemical laboratories to provide a barrier for the geochemist, and for sample storage.
Aluminum foil is used to remove rust from steel and polishes steel surfaces by hand by rubbing it with aluminium foil dipped in water. The aluminium foil is softer than steel, and does not scratch the surface. Heat is generated by rubbing friction, the aluminium oxidizes to produce aluminium oxide. Aluminium has a higher reduction potential than iron, and therefore leaches oxygen atoms away from any rust on the steel surface. Aluminium oxide is harder than steel, and the microscopic grains of aluminium oxide produced create a fine metal polishing compound that smoothes the steel surface to a bright shine.
The material used in many ribbon microphones is aluminium leaf or "imitation silver leaf" as it is sometimes called. This is pure aluminium and is around 0.6-2.0 micrometres thick. It is in fact virtually the same material that the BBC used on Coles ribbons, with the exception that they also hand beat the leaf even more. They did this by sandwiching the ribbon between toilet paper and beating with a ball peen hammer. This "cold forges" the leaf. Corrugations must also be imparted into the ribbon, Coles used 25 per inch (1 mm cycle), RCA 44BX has 19 per inch (0.7 mm cycle) and is around 50 mm (2.0 in) long. RCA 77 has 13 corrugations per inch (0.5 mm cycle). RCA ribbon material is around 1 - 1.5 micrometers (0.00005 inch). The new Nady ribbon plus AEA both clearly advertise the fact they use 2 micrometre aluminium ribbon in their mics.
The extensive use of aluminium foil has been criticized by environmentalists because of the high resource cost of extracting aluminium, primarily as a result of the large amount of electricity used to process bauxite ore. This cost is reduced via recycling, reduced energy requirements during transport due to lighter weight packages, and the fact that many foods that would otherwise perish can be protected over long periods without refrigeration. Some aluminium foil products can be recycled at around 5% of the original energy cost,[14] although many aluminium laminates are not recycled due to difficulties in separating the components and low yield of aluminium metal.
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