These days, usually aluminum, with about 1% manganese and 1% magnesium. The extra metals give the aluminum more strength, and make it easier to form. Other metals are in the alloy in much smaller quantities, including copper, gallium, iron, silicon, titanium, vanadium, and zinc. Some of these are added deliberately to improve the alloy, while others are present only because they would be difficult to remove from the raw materials used to make the alloy.
The lids of the can are made from a different alloy, which contains up to 5% magnesium. This alloy is more expensive, and not as strong as the alloy used for the body of the can, but it is more easily formed into the complex easy-open top. This is why the top of the can is so much smaller in diameter than the rest of the can – it makes it cheaper, and a smaller lid is stronger, so the weaker more expensive alloy is not a problem. The bottom of the can is also made smaller, so the cans will stack.
The cans are lacquered with plastic both to prevent the acids in the contents from attacking the aluminum, and to keep the outside of the can from reacting with things it contacts in shipping and storage.
Food grade petroleum jelly and food grade waxes are used as lubricants when the cans are constructed and crimped closed.
The cans are designed to hold a maximum pressure of 95 pounds per square inch. This is much more than the contents of the can are usually under, and provides some margin for heat and shock in shipping.