Tin can

Three tin cans of varying sizes; the one at the front is opened with a pull tab.

A tin can, tin (especially in British English), steel can, or a can, is an air-tight container for the distribution or storage of goods, composed of thin metal, and requiring cutting or tearing of the metal as the means of opening. Cans hold diverse contents, but the overwhelming majority preserve food by canning.

"Tin" cans are made of tinplate (tin-coated steel) but may also be made of aluminium or other metals.

History

The tin can was patented in 1810 by the English inventor Peter Durand, based on experimental work by the Frenchman Nicolas Appert. He did not produce any food cans himself, but sold his patent to two other Englishmen, Bryan Donkin and John Hall, who set up a commercial canning factory, and by 1813 were producing their first canned goods for the British Army.

Early cans were sealed with lead soldering, which has led to lead poisoning. Famously, in the 1845 Arctic expedition of Sir John Franklin, crew members suffered from severe lead poisoning after three years of eating canned food.

In 1901, the American Can Company was founded which, at the time, produced 90% of United States tin cans.^[1]

Description

A selection of tins

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 "rim"; in two-piece construction, one piece is a flat top and the other a 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-20^th 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.

Materials

No cans currently in wide use are composed primarily or wholly of tin; that term rather reflects the near-exclusive use in cans, until the second half of the 20^th 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.^[2] 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. A label can also be printed directly onto the metal.

In modern times, the majority of food cans in the UK^[3] have been lined with a plastic coating containing bisphenol A (BPA). The leeching of BPA into the can's contents is currently (as of early 2010) being investigated as a potential health hazard.

Standard sizes

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 totaling 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).^[4]

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 3.89 L (1 USA gallon), 1.89 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.

Fabrication of cans

Rimmed-can construction necessarily has three phases:

1. Joining the bottom and wall (or forming the cup-shaped piece, for a two-piece can)
2. Filling the can with content
3. Joining the wall and top.

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 away from 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.

Manufacturing information on the bottom of cans.

Inside of a tin can.

Opening cans

The first tin cans were heavy-weight containers that required ingenuity to open, using knives, chisels or even rocks. Not until cans started using thinner metal about 50 years later were any dedicated can openers developed.

While beverage cans or cans of liquids such as soup merely need to be punctured to remove the product, solid or semisolid contents require access which is generally gained by removing the top (or bottom) of the can. Although this can be accomplished by brute force using something like a large, heavy knife, many more convenient can openers have been devised and marketed.

Some cans, such as those used for sardines, have a lid which is specially scored so that the metal can be broken apart by the leverage of winding it around a slotted church key.

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 oil cans and tennis balls), allowing the convenience of opening without need for any tools or implements.

A simple butterfly can opener.

A can opener.

Detail on a can opener.

Recycling

Steel from cans and other sources is the most recycled packaging material.^[5] Around 65% of steel cans are recycled.^[6] In the US, 63% of steel cans are recycled, compared to 52% of aluminium cans.^[7]

Dissolution of the tin coating into the food

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.^[8] A study showed that 99.5% of tested cans contain below 200 mg/kg of tin.^[9]

See also

• Beverage can
• Can opener
• Canister
• Oil can
• Recycle
• Packaging
• Tin can wall

References

Further reading

• Nicolas Appert
• History of the Tin Can on About.com
• Brody, A. L., and Marsh, K, S., "Encyclopedia of Packaging Technology", John Wiley & Sons, 1997, ISBN 0-471-06397-5
• Soroka, W, "Fundamentals of Packaging Technology", Institute of Packaging Professionals (IoPP), 2002, ISBN 1-930268-25-4

External links

• Steeluniversity Packaging Module
• Steel industry fact sheet on food cans
• Standard U.S. can sizes at GourmetSleuth