Heat Transfer and Kitchen Pans

I know what you look forward to at this time of year: hot apple cider, chestnuts roasting over an open fire, and discussions of heat transfer. Not heat transfer, you say? Too bad, I'm on a tear. For the moment, let’s stick to two metals: cast iron and stainless steel. Cast iron isn’t really steel since it lacks some other added elements but it does contain carbon. Stainless steel isn’t a single material---several different alloys or recipes are used---but those differences don’t make much difference at the level of detail for which we operate. Heat capacity measures the ability to store heat. The higher the heat capacity, the more heat a pan can store. Some measurements of heat capacity for various materials can be found here. The bottom line is that, for our purposes, cast iron and stainless steel have equivalent heat capacities. Based on this mechanism, cast iron and stainless steel pans should provide equal performance---both heating up and cooling down at the same rate. However, heat capacity isn’t the only mechanism at play. The pans will lose heat as the stove pumps heat in. Several heat loss mechanisms are at work. A surprising one is emissivity, a surface property of the material. Shiny surfaces have lower emissivities than do rough surfaces. This reference shows that cast iron’s emissivity is about 9 times higher than that of stainless steel. As a result, the cast iron pan emits more energy to the environment as it is heated.

I ran a simple test to better understand the thermal characteristics of these types of skillets. I used this simple setup to measure temperature. I placed each skillet on a maximum-heat burner for 60 seconds, then removed it and let it cool for another 180 seconds. I recorded temperature every 10 seconds. Here is a plot for temperature of my stainless steel and cast iron skillets:

You can see that the stainless steel skillet heated up faster and reached a much higher temperature after one minute than did the cast iron p skillet an. But this plot is a little misleading because the skillets don’t weigh the same: the stainless steel pa skillet n weighs 30.6 oz while the cast iron skillet comes in at 85.8 oz. Heat capacity is defined per unit weight. Just as a smaller swimming pool can be filled up faster than can a larger one, the heat from the burner produces more temperature rise in the skillet that weighs less. This plot normalizes the stainless steel skillet temperature for weight:

The normalized temperature of the stainless steel skillet is below that of the cast iron. The lower emissivity of the shiny stainless steel means that it puts out less heat. The moral we can draw from this sojourn into physics is that you like your cast iron pan not because of the material with which it is made but because of its weight and texture. You could make a stainless steel pan behave more like cast iron with two treatments: weld a slug of metal onto the bottom, then sandblast the whole thing. Of course, you wouldn’t do that to a fancy, expensive stainless steel pan. Or would I?