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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.[citation needed]
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.[citation needed]
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.[citation needed]
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.[citation needed]
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.[citation needed]
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 (which cause fats to oxidise or become rancid), odours and flavours, moistness, and germs, and so it is used broadly in food and pharmaceutical packaging, including long-life packs (aseptic packaging) 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 food.[citation needed]
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, when baking, 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.[citation needed]
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.[citation needed]
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).[citation needed]
—however actual energy absorption is minimal: the remaining high-frequency rf energy is almost perfectly reflected from uniform flat aluminium surface, and thus, reflected signal may continue to propagate internally, and if holes or passages of suitable geometry exist in the shield, signal propagation may continue out through those, the aluminium being good material for implementation of a microwave-frequency waveguide.[citation needed]
For effective shielding from a magnetic field, the shield should be several skin depths thick. Aluminium foil is about 1 mil (25 μm); a thickness of 10 mils (250 μm) (ten times thicker) offers less than 1 dB of shielding at 1 kHz, about 8 dB at 10 kHz, and about 25 dB at 100 kHz. At these frequencies a ferromagnetic material such as mild steel is much more effective, due to different and complementary electromagnetic permeability properties, and common practical shielding implementations utilise both an inner high-frequency reflective material such as aluminium, preferably bonded (via annealing or electroplating, done to avoid capacitance between separated layers), to a more substantial structural ferromagnetic shell, usually mild steel (in specialized applications, more expensive, less structurally useful and less workable materials may be preferred.) Despite the relative low mass density of aluminium, this design is usually both lighter and more effective than an equivalently absorptive design utilizing aluminium alone (although with poorer heat dissipative properties, typically accommodated by improved ventilation, which itself needs careful consideration in order to preserve the desired shielding effectiveness).[citation needed]
...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.[citation needed]
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.[citation needed]
Foil is used by organic/petroleum geochemists for protecting rock samples taken from the fields and in the labs 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.[citation needed]
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 to 2.0 micrometres thick. It is virtually the same material that the BBC used on Coles ribbons, with the exception that they also hand beat the leaf even thinner. They did this by sandwiching the ribbon between toilet paper and beating with a ball-peen hammer. This "cold forges" the leaf. The aluminium leaf was then annealed for an hour in an oven to restore flexibility. Corrugations must also be imparted into the ribbon: Coles used 25 per inch (1 mm cycle). RCA 44BX has 19 corrugations 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 to 1.5 micrometres (0.00005 inch) thick. The Nady ribbon plus AEA both state that they use 2 micrometre aluminium ribbon in their microphones.[citation needed]
...although many aluminium laminates are not recycled due to difficulties in separating the components and low yield of aluminium metal.[citation needed]
