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Measuring practical heat conductivity - Cast iron, aluminum, copper


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Measuring practical heat conductivity - Cast iron, aluminum, copper

athanasius | Oct 3, 2010 05:20 PM

A couple months ago, I posted that I had stopped using most cast iron mostly because of its uneven heating. I still sear steaks in a large skillet and occasionally fry an egg or two, but that's about it:

In response to one of the comments, I stated that I had observed greater than 100 degree F differences in the surface temperature of cast iron, even pre-heated. A few commenters asked me for details, since they were surprised by the magnitude of the heat difference. At the time, I had just observed differences informally.

Anyhow, I ran a few quick tests today, and below are the results. I have to admit that I was a bit shocked how poorly cast iron performed, which was even worse that I expected. I've always been aware of its poor conductivity, but finding up to 200-degree differences between center and outer bottom of a preheated pan is pretty crazy. It does explain why I've found it impossible to cook just about anything evenly in it over the years, except when stirring constantly.

I'm not trying to convince anyone not to use cast iron (for years I babied my stuff and used it almost every day), but I do think those who use it need to be aware of what they're up against. Obviously moving food around in a pan (particularly liquids) will even the temperature out a bit, but cast iron still seems a pretty poor choice for most general purpose cooking, if you're looking for evenness. (I'm not disputing its good uses for browning, searing, non-stick cooking, etc.)

I tested four pans primarily, all roughly the same thickness:
(1) a large vintage tin-lined copper saute pan, about 9.5" in diameter
(2) an aluminum griddle with non-stick coating, about 12" in diameter
(3) two well-seasoned standard 10" Lodge skillets (I have two, many years different in age, and I wanted to prove that I didn't just have one dud pan)

I turned my gas stove to low and pre-heated each pan in turn, observing the changes over 10-15 minutes with an infrared thermometer. I waited until the pan temperatures were reasonably stable. I kept the burner at the exact same setting each time, just changing pans.

The results for the stable temperatures after 10-15 minutes shocked me a bit:
(1) Copper: center ~225F, edges ~190-200F
(2) Aluminum: center ~280F, edges ~215-230F
(3) Cast iron: center ~400F, edges ~200-300F

When I say "edges," I mean near the edge of the bottom of the pan, not counting the sides. Notably, for the copper pan, even the top lip of the side was less than 50 degrees off from the center of the pan, whereas the top of the cast iron sides were off by more than 250 degrees, even after 10-15 minutes of preheating. (I did a brief test on a copper saucier, and saw a similar pattern -- even though most of the pan was sloped above the burner, the top of the sides was rarely more than 50 degrees off from the center of pan on the burner.)

Note that the cast iron was incredibly uneven, with the bottom of a 10" pan varying by 200 degrees even after well preheated, and even though the pan was centered on the burner, the temperature near the sides varied by almost 100 degrees from one side to another. There were also plenty of hot spots all around the pan where temperatures varied by 50 degrees within an inch or two. (Both skillets showed similar unevenness.)

These differences remained relatively constant while the pans were preheating. Copper was consistently about 30 degrees hotter in center than sides, aluminum 50-60 or so, cast iron, 100-200. I had expected that preheating on low would give cast iron the best chance, but it still displayed variance of hundreds of degrees.

I did brief trials over medium and high heat, but I didn't try to heat to equilibrium, because I don't like to preheat empty pans to over 400F except for searing purposes. In general, I observed similar patterns in terms of the differentials between center and sides at higher temperatures.

I did try heating the cast iron to quite hot, since I do that for searing. Over high heat, the temperature differential was slightly better at first, I think because the burner flares out more on high heat. But it still was always greater than 100 degrees different across the bottom of the pan. And once the center got above 500F or so, I started seeing 150-200 degree differences again for the edges. At 700F in the center, the outer edges were even below 450-550F, but I expected that.

I briefly tried putting my cast iron dutch oven on the stovetop, as well as a cast iron griddle, and I saw similar patterns, so it isn't just two dud skillets.

Now I know why I could never get my pancakes to be done evenly on any cast iron pan/griddle, no matter what I tried. One could always preheat in the oven to obtain a consistent temperature at first, but once returned to the stovetop, you'd only have a few minutes of relatively even heating.

I'd be interested in hearing in others have tried measuring temperatures in their pans and if they've seen similar results.

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