Why does aluminum foil have a shiny side and a dull side?

The shiny and dull sides of aluminum foil come about because of how it is made. Aluminum foil is made by rolling sheets of aluminum between pairs of polished steel rollers. The process is repeated over and over until the aluminum is squeezed thin enough.

The catch is that aluminum foil for kitchen use is so thin that it's hard to work without it tearing, so the last rolling step (to get down to 0.0005-inch thickness) is done passing two foil sheets together facing each other. The sides that face the rollers get very shiny (the rollers are very smooth and harder than aluminum), while the sides that face each other don't get that polished look and come out sort of dull or matte.

Some people worry about which side to use for what. The shiny side reflects light and heat a little better than the dull side, but in practice for most applications involving the transfer of heat (i.e. cooking, refrigerating) it makes no significant difference whether the shiny side is in or out.

Although aluminum is the most common metal in the earth's crust, the original chemical means of extracting aluminum from minerals was tedious, and the metal commanded high prices, so foil wrapping was initially made of tin. (Thomas Edison's first records were based not on grooves pressed into a plastic disc; instead he used a thin sheet of tinfoil covering a cylinder.)

In 1886 a new and vastly more efficient process was discovered independently in the United States and in France based on the use of electricity. The price of aluminum dropped, and aluminum foil became economically reasonable to produce. It rapidly displaced tinfoil, initially largely because food wrapped in tinfoil often got a sort of metallic taste. In addition, aluminum is more flexible than tin. These days it's all but impossible to get actual tinfoil, but some people still use the term when referring to aluminum foil.

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. Include your initials and hometown.