question: Why does aluminum foil never get hot to the touch. You can leave it in the oven and touch it directly out of the oven without it feeling hot. Why is this?
comment: Aluminum has a very low specific heat capacity and foil is very thin, these combine together to get it a very low thermal mass (it cannot hold a lot of heat). It also has a very high surface area to volume ratio and a very high thermal conductivity. This means that the heat that it can store can move in and out of it very very fast. When you combine these, Aluminum foil can go from red hot to room temperature in a matter of seconds. The very low thermal mass means that with very thin foil it might be 400 degrees, but putting that heat into your finger only raises your finger by 20 degrees, and results in no burn.
passages: Tin was first replaced by aluminium in 1910, when the first aluminium foil rolling plant, "Dr. Lauber, Neher & Cie." 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 process 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.
   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 ; thinner gauges down to are also commonly used. In the United States, foils are commonly gauged in thousandths of an inch or mils. Standard household foil is typically thick, and heavy duty household foil is typically . 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.
   The thermal contact resistance of certain sandwich kinds of materials that are manufactured by rolling under high temperatures may sometimes be ignored because the decrease in thermal conductivity between them is negligible.
  
  Section::::Factors influencing contact conductance.:Interstitial materials.
   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.
   Section::::Uses.: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.
   In the absence of interstitial materials, as in a vacuum, the contact resistance will be much larger, since flow through the intimate contact points is dominant.
  
  Section::::Factors influencing contact conductance.:Surface roughness, waviness and flatness.
   The effect of microwaving thin metal films can be seen clearly on a Compact Disc or DVD (particularly the factory pressed type). The microwaves induce electric currents in the metal film, which heats up, melting the plastic in the disc and leaving a visible pattern of concentric and radial scars. Similarly, porcelain with thin metal films can also be destroyed or damaged by microwaving. Aluminium foil is thick enough to be used in microwave ovens as a shield against heating parts of food items, if the foil is not badly warped. When wrinkled, aluminium foil is generally unsafe in microwaves, as manipulation of the foil causes sharp bends and gaps that invite sparking. The USDA recommends that aluminium foil used as a partial food shield in microwave cooking cover no more than one quarter of a food object, and be carefully smoothed to eliminate sparking hazards.
   Aluminium foils thicker than are impermeable to oxygen and water. Foils thinner than this become slightly permeable due to minute pinholes caused by the production process.
   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 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.
  
  Section::::Properties.
   As is the case with all metallic items, aluminium foil reacts to being placed in a microwave oven. This is because of the electromagnetic fields of the microwaves inducing electric currents in the foil and high potentials at the sharp points of the foil sheet; if the potential is sufficiently high, it will cause electric arcing to areas with lower potential, even to the air surrounding the sheet. Modern microwave ovens have been designed to prevent damage to the cavity magnetron tube from microwave energy reflection, and aluminium packages designed for microwave heating are available.
  
  Section::::Uses.:Art and decoration.
   A foil is a very thin sheet of metal, usually made by hammering or rolling. Foils are most easily made with malleable metals, such as aluminium, copper, tin, and gold. Foils usually bend under their own weight and can be torn easily. The more malleable a metal, the thinner foil can be made with it. For example, aluminium foil is usually about 1/1000 inch (0.03 mm), whereas gold (more malleable than aluminium) can be made into foil only a few atoms thick, called gold leaf. Extremely thin foil is called metal leaf. Leaf tears very easily and must be picked up with special brushes.
   Thick film resistors may use the same conductive ceramics, but they are mixed with sintered (powdered) glass and a carrier liquid so that the composite can be screen-printed. This composite of glass and conductive ceramic (cermet) material is then fused (baked) in an oven at about 850 °C.
  
  Thick film resistors, when first manufactured, had tolerances of 5%, but standard tolerances have improved to 2% or 1% in the last few decades. Temperature coefficients of thick film resistors are high, typically ±200 or ±250 ppm/K; a 40-kelvin (70 °F) temperature change can change the resistance by 1%.
   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).
   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.
  
  Section::::History.
  
  Section::::History.:Before aluminium foil.
   During the heating phase, the weld region is heated conductively until melted, without substantial displacement of the material. Pressure is maintained either at a minimum to keep the parts and the hot plate in contact or at zero with a preset displacement. The melt surface reaches approximately 20 °C below the temperature of the hot plate. The viscosity of the melted material can be controlled through the temperature of the hot plate and the heating time. The surface of hot plate is often coated with PTFE to stop the molten plastic from sticking, which limits the hot plate temperature to 270 °C.
   Section::::Composition.
   Aluminium foil is widely sold into the consumer market, often in rolls of width and several metres in length. 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.
  
  Section::::Uses.:Insulation.
   Cold foil evolved from hot foil stamping. Hot foil stamping is mostly used offline when foil is required on a preprinted substrate, such as in the manufacture of greeting cards and special occasion ribbons. Hot foil is economical but very slow. The types of graphics that can be applied are usually limited to text and bold images. Hot foil is not usable with heat-sensitive substrates such as polyethylene, vinyl, or shrink film.
  
  Section::::History.:Cold foil evolution.
   Section::::Usage.
   Section::::Welding techniques.:Contact welding.
  
  This is the same as spot welding except that heat is supplied with thermal conduction of the pincher tips instead of electrical conduction. Two plastic parts are brought together where heated tips pinch them, melting and joining the parts in the process.
  
  Section::::Welding techniques.:Hot plate welding.
   A rubber PTC heater is made from a special type of rubber which conducts electricity, but only up to a temperature which is defined at the production of the PTC rubber. Typical design temperatures are between 0–80 °C (32–176 °F). The resistivity of the rubber increases exponentially with temperature for all temperatures up to the design temperature. Hence, it has strong PTC properties for all temperatures and heats up rapidly. Above this temperature the rubber is an electrical isolator and cease to produce heat. This makes the heater self-limiting. The rubber foil is thin and flexible and can be formed to any shape and size.
   When the two bodies come in contact, surface deformation may occur on both bodies. This deformation may either be plastic or elastic, depending on the material properties and the contact pressure. When a surface undergoes plastic deformation, contact resistance is lowered, since the deformation causes the actual contact area to increase
  
  Section::::Factors influencing contact conductance.:Surface cleanliness.
  
  The presence of dust particles, acids, etc., can also influence the contact conductance.
  
  Section::::Measurement of thermal contact conductance.
   In 1911, Bern-based Tobler began wrapping its chocolate bars in aluminium foil, including the unique triangular chocolate bar, Toblerone. By 1912, aluminium foil was being used by Maggi (today a Nestlé brand) to pack soups and stock cubes. 
  
  The first use of foil in the United States was in 1913 for wrapping Life Savers, candy bars, and gum. Processes evolved over time to include the use of print, colour, lacquer, laminate and the embossing of the aluminium.
  
  Section::::Manufacture.
   Steel can wall thicknesses are now 30% thinner and weigh 40% less than 30 years ago, reducing the amounts of raw materials and energy required to make them. They are also up to 40% thinner than aluminium.
  
  Section::::Design and manufacture.:Magnetic properties.
   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.
  