Everyday Science: Why do Metal Pans Warp in the Oven?

The old sayingcookieblog “You can never be too rich or too thin” certainly doesn’t apply to baking. Overdo it with the butter and sugar, and your cookies will definitely be too rich. You’ll also regret it if your metal baking pans or sheets are too thin. Simple physics and the basic properties of metal mean that when it comes to baking implements, thicker is better.

Many materials, including metal, expand when they get hot. That’s because heat is energy, and when it’s applied to matter, that energy affects the atoms that make up the material. Those atoms are already moving to some degree — much more for gases and liquids, much less for metal and other solids. Heat makes those atoms move more quickly, and the more they move, the more space they take up and the material expands.

When you’re making an object you know is going to expand due to heat, you need to take steps to prevent warping and cracking. That’s why sidewalks are pre-made with grooves between panels. The extra space allows the concrete to expand, preventing stress cracks. The problem is simple to solve because you’re dealing with a single flat surface.

In the case of your metal baking sheets and pans, multiple surfaces need to expand — the bottom of the pan or sheet and the four raised edges along its perimeter. As they heat up, they don’t all expand at the same rate. The bottom will expand slightly more than the raised lip. This creates stress at the folded edges. If the pan or sheet is too thin, the stress may cause it to buckle or twist. You can tell your metal cookie sheet is under stress when you hear it popping and crackling in the oven.

If a pan buckles or twists enough times, it could crack. We could then analyze the crack’s fracture surface to determine if it was a stress-induced fracture. We might even see fatigue striations on the crack surface.

Thicker metal baking sheets and pans are often better able to withstand the stresses of heat expansion. Temperature variations in the oven itself, and temperature differences between exposed sections of sheet and those covered by food may also make the sheet pop and bend. However, once the temperature equalizes, the pan should be fine.

Of course, while most metal baking pans have the raised edges to contain foods, cookie sheets may only have one or two edges — or even none at all. In that case, with fewer surfaces needing room to expand, you can probably afford to choose a cookie sheet that’s a bit thinner. But you may still want to think about dialing back on the butter and sugar!

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