Why does aluminum foil have different sides

Physics in everyday life: which side of the aluminum foil should be on the outside?

Surveys among friends and acquaintances show that practically everyone favors a certain method of using aluminum foil, but usually cannot explain why. Most common answer: My mother taught me that.

In fact, the two sides of aluminum foil have slightly different physical properties. But this is by no means the reason why the film is produced with two different surfaces. This is purely due to production engineering. In the last rolling pass, two foils are processed at the same time for reasons of efficiency. Only the two outer sides come into contact with the polished rollers and are therefore very smooth and shiny. The two inner sides, however, remain matt. The aluminum foils produced in this way for the kitchen are generally between 0.01 and 0.015 millimeters thick - i.e. 10 to 15 micrometers.

To finally answer the initial question: For everyday use, it hardly matters how you use the two sides of the aluminum foil. It's just a matter of taste. But let me tell the purists what subtle differences there are. Glossy surfaces reflect light and heat better than matt ones. So if you want to wrap a dish in aluminum foil because you want it to stay warm longer - for example a kebab from the snack bar around the corner that you want to take home with you - then it is theoretically better to turn the reflective side of the foil inwards use so that the warmth of the food is reflected back. Conversely, if something needs to be cooled, the shiny side should be on the outside, because then heat radiation that hits the film from the outside is better reflected. Those interested in science may use their aluminum foil correctly in each individual case according to these rules.

It is balm for the soul of the knower. But at temperatures below 500 degrees Celsius, the differences in the radiation properties between the two sides of the aluminum foil are ultimately irrelevant. This also applied to the emergency money printed on aluminum foil in Germany in 1923.

Alternatives to aluminum foil

If possible, however, you should avoid using aluminum foil. Their production is very energy-intensive and disposal is not that easy. In addition, components of the film can come off if acidic or salty foods are wrapped in it. It is therefore advisable to look for alternatives. Aluminum foil is becoming increasingly popular, especially when grilling. Leaves (for example from rhubarb, banana or chard) can be used as a substitute for film. Parchment or sandwich paper are also ideal. Reusable cans, dishes or greaseproof paper help in everyday use. A bio-film made of cotton is still relatively new on the market. The fabric is treated with resin, oils and waxes and can be reused. If aluminum foil is unavoidable, you can fall back on recycled aluminum foil, which was manufactured in a way that conserves energy and resources.