The Laws of Thermo-culinary Dynamics
By TODD OPPENHEIMER
When thinking about how pots made from clay or metal differ, it’s helpful to begin with their differing capacities for conducting heat. Consider this:
- Clay conducts heat at a rate of .15 to 1.8 Watts for each meter of thickness in the material.
- Iron conducts heat at a rate of 80 Watts per meter.
That is a huge difference. And it means that in a given hour, iron moves 50 times more heat from the oven into the walls of the pot–and then into the food inside–than a clay pot does. Aluminum is even more aggressive, moving heat 100 times faster than clay. And a copper pot is the speediest of them all, transmitting heat 500 times faster than its equivalent in clay.
Once heat is stored in a cooking material, it’s helpful to then think about the vessel’s “thermal capacity” — or its ability to store heat, and move it around within its own material. Thermal capacity is a measure of how many calories it takes (yes, heat is known by its calories too) to raise one gram of a material by one degree. Physicists call this quality “specific heat.” So let’s look at the differences in the specific heat for each of these cooking materials:
- Clay has a specific heat rating of .33
- Iron has a specific heat rating of .11 — about one third that of clay.
- And copper has a specific heat rating of .09 — almost a fourth that of clay.
What all this means is that clay can store up three to four times more heat than iron or copper before it needs to move that heat into food. And once it does, it has the capacity to distribute three to four times more heat as well–but do it slowly. Most important, this slow-motion magic can happen, at least for the first hours of a cook, at 200 degrees or at 400 degrees.
The sum of these dynamics create a kind of “thermal inertia,” says Paolo Carini, who has a Phd in Physics and teaches at the San Francisco Waldorf High School. (Carini and I had so many communications about how all these factors work in a kitchen that I was tempted to nominate his idea to the International Union of Pure and Applied Physics as “Carini’s Law of Thermal Inertia.”)
To test how the inertia plays out in practice, I decided to conduct an experiment (at the suggestion of Harold McGee, the author of “On Food and Cooking: The Science and Lore of the Kitchen).
First, I heated three pans of similar sizes over the same level of flame. One was clay, one cast iron, and one stainless steel with an aluminum underside (an All-Clad). After each one had been heating for 5 minutes, I took its surface temperature (with an infrared thermometer) every minute for three minutes. Then I turned off the burner and continued measuring the temperature at the same intervals for seven more minutes; and then again 10 minutes later. To see how quickly the heat spread, and how evenly, I took my measurements in two locations: at the pan’s center, and around its edges. The differences astounded me.
Of the three of them, the clay pot got the hottest — over 400 degrees — but only at the center. And it took a while. Once I turned the heat off, however, the heat in the clay pot started to spread, raising the temperature of the edges from roughly 195 degrees to slightly more than 200 degrees. After a few minutes, the edges continued increasing in warmth, reaching as high as 212 degrees, while the center slowly cooled. After the first 10 minutes of measurement, the pan’s temperature was nearly uniform.
The iron pot, however, began at roughly 375 degrees in the center and around 250-275 at the edges. As the pot got hotter, that differential remained relatively constant — and it did the same as it cooled. In other words, as the pan rushed to get rid of its heat, each area of the pan cooled at the same rate. So the pan was never able to achieve an even temperature.
The stainless steel pan was the most surprising: It struggled to top 250 degrees, but that temperature quickly spread throughout the pan so there was virtually no difference from center to edge. The low temperature really confused me, until I realized that these two materials (aluminum and steel) diffused their heat so rapidly that, quite possibly, very little could be measured on the surface of the pan. For cooks in a rush who want a consistent heat, what could be better than this? Once the burner was turned off, though, this pan started cooling quite dramatically at the center — so much so that it was soon 25-50 degrees cooler than the edges. Within minutes, the pan rid itself of its heat almost entirely. Easy come, easy go.
After a full 20 minutes, the clay pot remained the warmest of the three — by far (150, 100, and 60 respectively). These differences point to one reason that so many cooks who use clay love it. Once a dish has finished cooking in a clay pot, many delight in taking the pot straight to the table, where it bubbles away invitingly while guests crowd around, giddy about the gorgeous meal awaiting them.
The whole exercise made me imagine the reaction of a steak or a roast, whose benefactor had been fed and raised for months, until it reached perfection. When it was about to meet its ultimate transformation in a pan or pot made of metal instead of clay, it might turn to the vessel, like a self-assured lover, and say, “What’s the rush? Why don’t we take our time and enjoy this?”