The pan was completely cool in the first frame, and had been sitting on my gas stove for a few minutes in the second. You can see that just the heat from the pilot light had a measurable effect. I turned on the stove as I took the second photo, and the rest were snapped at 15-second intervals.
We can see that the center of the pan actually takes quite a bit longer to get warm than the parts directly under the flame. I have the kind of burner pictured at right, but I imagine the effect would be the same on all but the fanciest gas burners.
The images above make it seem like the pan gets to a reasonably even temperature, but things aren’t always what they seem. All the images are scaled to the same temperature range, where black is 70° Fahrenheit and white is about 365. Once the pan gets hotter than that, everything looks the same. I adjusted the images to automatically scale the coloring so that we can see the full range of temperatures:
Even after two minutes, the pan’s temperature is still pretty inconsistent. The hottest parts are 560-580 degrees F, the center is about 470, and the edges are only 350 or so.
Some recipes recommend preheating your skillet for 5 minutes or more, and it appears that that would result in a more even cooking temperature. But does an even temperature really matter? Cast iron is not a good conductor of heat, which is why the pan has “hotspots.” But the reason that cast iron is such a popular cooking material isn’t its conductivity (or lack thereof), it’s because of its large heat capacity. In other words, the iron skillet is so heavy that throwing a diced onion into it isn’t going to change its temperature too much.
Think of the skillet as a heat battery. The stove charges up the skillet with heat, and the food discharges this energy by heating up and vaporizing water. The longer you charge the skillet, the more energy it has to transfer to your food. Because of the stored heat in the skillet, you can get a lot of energy into your food very quickly; often faster than the stove could give off on its own. This is why cast iron skillets are ideal for searing steak, where the goal is to cook the meat at a very high temperature for just a couple minutes.
So what did we learn from these fancy thermal images? When you preheat your skillet, your food will cook more evenly and quickly. If you’re searing or sautéing in cast iron, don’t even bother unless the pan is smoking (over 500°F). And if you want some info on seasoning and caring for your beloved cast iron, check out these articles.