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|This article may be expanded with text translated from the corresponding article in the French Wikipedia. (May 2010)|
A tin can, tin (especially in British English and Canadian English ), steel can, or a can, is a sealed container for the distribution or storage of goods, composed of thin metal. Many cans require opening by cutting the "end" open; others have removable covers. Cans hold diverse contents: foods, beverages, oil, chemicals, etc.
The tin canning process was allegedly developed by Frenchman Philippe de Girard, who came to London and used British merchant Peter Durand as an agent to patent his own idea in 1810. The canning concept, based on experimental food preservation work in glass containers by the French inventor Nicholas Appert the year before. Durand did not pursue food canning himself, but, in 1812, sold his patent to two Englishmen, Bryan Donkin and John Hall, who refined the process and product, and set up the world's first commercial canning factory on Southwark Park Road, London, and by 1813 were producing their first canned goods for the Royal Navy.
Early cans were sealed with lead soldering, which led to lead poisoning. Famously, in the 1845 Arctic expedition of Sir John Franklin, crew members suffered from severe lead poisoning thought to be caused by eating canned food. More recent research suggests the lead poisoning was more likely to have been caused by the internal pipe system on the ships.
Most cans have identical and parallel round tops and bottoms with vertical sides. However, where the small volume to be contained and/or the shape of the contents suggests it, the top and bottom may be rounded-corner rectangles or ovals. Other contents may justify a can that is overall somewhat conical in shape.
The fabrication of most cans results in at least one "rim", a narrow ring whose outside diameter is slightly larger than that of the rest of the can. The flat surfaces of rimmed cans are recessed from the edge of any rim (toward the middle of the can) by about the width of the rim; the inside diameter of a rim, adjacent to this recessed surface, is slightly smaller than the inside diameter of the rest of the can.
Three-piece can construction results in top and bottom rims; in two-piece construction, one piece is a flat top and the other a deep-drawn cup-shaped piece that combines the (at least roughly) cylindrical wall and the round base; the transition between the wall and base is usually somewhat gradual. Such cans have a single rim at the top.
In the mid-20th century, a few milk products were packaged in nearly rimless cans, reflecting different construction; in this case, one flat surface had a hole (for filling the nearly complete can) that was sealed after filling with a quickly solidifying drop of molten solder. Concern arose that the milk contained unsafe levels of lead leached from this solder plug.
No cans currently in wide use are composed primarily or wholly of tin; that term rather reflects the nearly exclusive use in cans, until the second half of the 20th century, of tinplate steel, which combined the physical strength and relatively low price of steel with the corrosion resistance of tin.
Use of aluminium in cans began in 1957. Aluminium is less costly than tin-plated steel but offers the same resistance to corrosion in addition to greater malleability, resulting in ease of manufacture; this gave rise to the two-piece can, where all but the top of the can is simply stamped out of a single piece of aluminium, rather than laboriously constructed from two pieces of steel. Often the top is tin-plated steel and the rest of the can aluminium.
A can usually has a printed paper or plastic label glued to the outside of the curved surface, indicating its contents. Some labels contain additional information, such as recipes, on the reverse side. A label can also be printed directly onto the metal.
In modern times, the majority of food cans in the UK have been lined with a plastic coating containing bisphenol A (BPA). The leaching of BPA into the can's contents is currently (as of 2013) being investigated as a potential health hazard.
Can shapes are usually one of two shapes that maximise the volume and minimise the material, the "soup tin" or the "tuna tin." Walls are often stiffened with ribs, especially on larger cans, to help the can resist dents that can cause seams to split.
Can sizes in the United States have an assortment of designations and sizes. For example, size 7/8 contains one serving of half a cup with an estimated weight of 4 ounces; size 1 "picnic" has two or three servings totalling one and a quarter cups with an estimated weight of 10½ ounces; size 303 has four servings totalling 2 cups weighing 15½ ounces; and size 10 cans, most widely used by food services selling to cafeterias and restaurants, have twenty-five servings totalling 13 cups with an estimated weight of 103½ ounces (size of a roughly 3 pound coffee can). These are all "U.S. customary" cups, and not equivalent to the former Imperial standard of the British Empire or the later Commonwealth.
In the United States, cook books will sometimes reference cans by size. These sizes are currently published by the Can Manufacturers Institute and may be expressed in three-digit numbers, as measured in whole and sixteenths of an inch for the container's nominal outside dimensions: a 307 x 512 would thus measure 3 and 7/16" in diameter by 5 and 3/4" (12/16") in height. Notice that this is not in millimetres. Older can numbers are often expressed as single digits, their contents being calculated for room-temperature water as approximately eleven ounces (#1 "picnic" can), twenty ounces (#2), thirty-two ounces (#3) fifty-eight ounces (#5) and one-hundred-ten ounces (#10 "coffee" can).
In countries and regions that use the metric system of measures, most tins are made in 250, 500, 750 ml (millilitre) and 1 L (litre) sizes (250 ml is approximately 1 cup or 8 ounces). In situations where products from the USA have been repackaged for sale in such countries, it is common to have odd sizes such as 3.8 L (1 USA gallon), 1.9 L (1/2 USA gallon), and 946 ml (USA 2 pints / 1 quart).
In Australia, cans are usually measured by net weight. A standard size tin can is roughly 400g; however, the weight can vary between 385g and 425g, depending on the density of the contents. The smaller half sized can is roughly 200g; it can vary between 170g and 225g.
Rimmed-can construction necessarily has three phases:
Rims are crucial to the joining of the wall to a top or bottom surface. An extremely tight fit between the pieces must be accomplished to prevent leakage; the process of accomplishing this radically deforms small areas of the parts. Part of the tube that forms the wall is bent, almost at its end, turning outward through 90 degrees, and then bent further, toward the middle of the tube, until it is parallel to the rest of the tube, a total bend of 180 degrees.
The outer edge of the flat piece is bent against this toward the middle of the tubular wall, until parallel with the wall, turning inward through 90 degrees. The edge of bent portion is bent further through another 90 degrees, inward now toward the axis of the tube and parallel to the main portion of the flat piece, making a total bend of 180 degrees. It is bent far enough inward that its circular edge is now slightly smaller in diameter than the edge of the tube. Bending it yet further, until it is parallel with the tube's axis, gives it a total bend of 270 degrees. Outward from the axis of the tube, the first surface is the unbent portion of the tube.
Slightly further out is a narrow portion of the top, including its edge. The outward-bent portion of the tube, including its edge, is slightly further out. Furthest out is the 90-degree-bent portion of the flat surface.
The combined interacting forces, as the portion of the flat surface adjacent to the interior of the tube is indented toward the middle of the tube and then outward forward the axis of the tube, and the other bent portions of the flat piece and the tube are all forced toward the axis of the tube, drives these five thicknesses of metal against each other from inside and out, forming a "dry" joint so tight that welding or solder is not needed to strengthen or seal it.
The first tin cans were heavy-weight containers that required ingenuity to open, using knives, chisels, or even stones. Not until about 50 years later, after can manufacturers started using thinner metal sheets, were any dedicated tin openers developed.
While beverage cans or fluid cans such as soup merely need to be punctured to remove the product, solid or semisolid contents require access which is generally gained by removing one end of the can. Although this can be accomplished by brute force using something like a great, heavy knife, specialized and more convenient can openers have been devised and marketed.
Some cans, such as those used for sardines, have a lid that is specially scored so that the metal can be broken out by the leverage of winding it around a slotted church key. Until the mid-20th century the lids on some sardine tins were attached by soldering and the winding key was used to tear the solder joint apart by brute force.
The advent of pull tabs in beverage cans spread to the canning of various food products, such as pet food or nuts (and non-food products such as motor oil and tennis balls), allowing the convenience of opening without need for any tools or implements.
Steel from cans and other sources is the most recycled packaging material. Around 65% of steel cans are recycled. In the United States, 63% of steel cans are recycled, compared to 52% of aluminium cans.
While much recycling of tins occurs once the can hits the smelters other methods are also used. For instance some people use two tin cans to form the basis of a camp or survival stove top upon which small meals are cooked.
Although tin is corrosion resistant, acidic food like fruits and vegetables can cause corrosion of the tin layer. Nausea, vomiting, and diarrhea have been reported after ingesting canned food containing 200 mg/kg of tin. A 2002 study showed that 99.5% of 1200 tested cans contained below the UK regulatory limit of 200 mg/kg of tin, an improvement over most previous studies largely attributed to the increased use of fully lacquered cans for acidic foods, and concluded that the results do not raise any long term food safety concerns for consumers. The two non-compliant products were voluntarily recalled.
Evidence of tin impurities can be indicated by color, as in the case of pears, but lack of color change does not guarantee that a food is not tainted with tin.
Bisphenol-A is a toxic chemical compound present in many tin can plastic linings and transferred to canned food. The inside of the can is coated with an epoxy coating, in an attempt to prevent food or beverage from coming into contact with the can. In September 2010, Canada became the first country to declare BPA a toxic substance. In the European Union and Canada, BPA use is banned in baby bottles. The longer and warmer the food is stored in a can, the more BPA will leach out into it. The U.S. Food and Drug Administration (FDA) currently has BPA under review. Campbell's Soup announced that it will eliminate BPA from the linings of its soup cans.
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