Next time you finish a soda, flip the can over. Instead of a flat bottom, you'll see a smooth dome curving up into the can, surrounded by a narrow ring it stands on. A flat bottom would seem simpler, and it would even hold a little more soda. So why the curve?
The short answer is strength. A sealed soda can is full of pressurized gas that pushes outward on every surface. A curved, domed shape handles that push far better than a flat sheet of metal, which would tend to bulge. The dome keeps the bottom stiff and stable while the can itself stays remarkably thin and light.
Fizz Under Pressure
Soft drinks followed beer into cans in the 1930s, even though sodas are more acidic and carry somewhat higher pressures. Today, a filled can usually holds pressurized gas, which actually makes it stiff enough to handle easily. An empty, opened can, by comparison, crumples with a squeeze.
Engineers who design tanks and other containers that hold pressure use the same idea. Pressure vessels are usually built from rounded shapes like spheres and cylinders, often capped with domed or dished ends, because curved surfaces are far more stable under pressure.
Thin Where It Can Be, Strong Where It Must Be
Modern aluminum cans start as a flat disk punched from stiff sheet metal. It's pressed into a cup about three inches wide, then stretched and thinned in a process called ironing. The bottom is shaped at the same time.
The finished can isn't the same thickness everywhere. The walls are thinner than the top and bottom, where stiffness is needed most. That's one reason cans are so light: in 2011, an empty U.S. can weighed about half an ounce, roughly 34 cans to the pound.
Even the metal is chosen carefully. The body is made from an aluminum alloy that can be drawn easily, while the lid uses a harder alloy. The standard U.S. can is about 4.83 inches tall and narrows from 2.6 inches across at its widest to 2.13 inches at the lid.
Cans Weren't Always Like This
Early cans were simple cylinders with flat tops and bottoms. Beer first went on sale in cans in 1935 in Richmond, Virginia, and drinkers needed a tool nicknamed a church key to punch holes in the top. Even then, cans had real advantages over bottles: flat-top cans packed more compactly and weighed less, and shoppers didn't have to pay a bottle deposit.
The pull-tab arrived in the 1960s, invented by Ermal Fraze of Dayton, Ohio. The first soft drinks sold in all-aluminum cans were R.C. Cola and Diet-Rite Cola, in 1964. Later, the stay-on tab solved the problem of sharp, discarded pull tabs littering the ground.
Why Every Gram Counts
Saving metal matters because of the sheer numbers. About 370 billion drink cans are made worldwide every year. Australian manufacturers, for example, trimmed the average can from about 16.55 grams in 1992 to about 14.9 grams by 2001, and aluminum is one of the most cost-effective materials to recycle.
So that little dome on the bottom of your can is a small feat of engineering. It helps a featherweight shell of aluminum hold back the fizz, stand up straight, and make it safely all the way to your fridge.
