As a physicist, I don’t really get asked for professional advice all that frequently. Our Carpenter Buddy was super helpful with our deck construction project (Thanks Drew!) and our MD Pal gets asked about questionable moles (and more exotic things) all the time. But it’s not often that a social gathering sees someone grappling with the finer points of the Laws of Thermodynamics.
When my scientific expertise is called upon, it’s usually because someone’s child has asked a great “but why?” question that Mum or Dad isn’t entirely sure how to answer. My absolute favourite questions are the ones that make me say: “hmmm … I don’t know! Let’s find out together!” Here’s the most recent one, courtesy of my almost-twelve-year-old nephew, Cameron Redmond. Foodie Cam wants to know: why is foil shinier on one side compared to the other? And, does it matter?
Now the first question is pretty straightforward to answer – it’s shinier on one side compared to the other because of how it’s made. Foil is made from sheets of aluminum that are sent through industrial-sized rollers to squish it nice and thin. With each pass through these rollers, the foil gets thinner and more delicate – a bit like sending your fresh pasta dough through the press to get it nice and thin.
For the final squish through the rollers, the aluminum sheet is folded over on itself, doubled up to make it stronger. The top and bottom surfaces, against the rollers, come out extra shiny. The inside surfaces that are pressed against each other, not so much. When the final sheet is unfolded, you have one slightly shinier surface and one that is a little dull looking.
Now, for the second part of Cam’s question – does it matter? Well, the answer to that is: it depends. If you’re going to use this foil to wrap up a potato for baking in your oven or in the coals of a campfire, then no. In both these scenarios, the potato heats up by a process called convection: the surrounding hot air transfers heat into the potato. The temperature climbs at a rate that has nothing to do with whether your spud is wrapped in a super shiny jacket, a less shiny jacket, or is au naturel – it mostly depends on how big the potato is and the temperature difference between the surrounding air and the spud.
But, if you don’t happen to have easy access to a regular oven or firewood, and you are just craving a baked potato, then the difference between the shiny and dull side of the foil could be a factor as you build your solar oven.
Solar ovens are fantastic devices for harnessing the heat from the Sun to cook. The most important feature is being able to collect a lot of the Sun’s energy and concentrate it into the small space where the food is located. Using the shiny (more reflective) side of aluminum foil, you can direct more sunlight into this central space and get your baked potato ready more quickly!
So, for everyday cooking, shiny or dull doesn’t really matter. But, it does matter if you’re building a solar oven. And, if you’re making a foil hat to block your nemesis from reading your mind, you should definitely keep the shiny side out. You’d look just plain silly otherwise!