Cookware made from cast iron has a reputation for keeping food warm for a long time. Is that really true? Best way to find out is by an experiment. I decided to compare a cast iron pot with one of stainless steel. These are the pots I used:
For the first experiment I filled them each with 2,5 L of water, put the lids on and brought both to the boil and let them boil for a minute so the pot itself would be warm throughout. Then both were placed on cork plates and left to cool. The temperature probe was carefully inserted under the lid in order to reduce the heat loss, and removed once the temperature had stabilized. For the second experiment 5 L of water were used. The measured temperatures are shown in the graph.
Contrary to what I had expected, the stainless steel pot keeps water warmer! After approximately 1,5 hours there is a 10 °C difference between the two. As expected, when using 5 L of water, it stays warm longer. Physical data for the two pots are given in the following table:
|Cast iron||Stainless steel|
|Volume||6 L||6 L|
|Diameter||27,9 cm||25,0 cm|
|Height||11,5 cm||14,5 cm|
|1619 cm2||1629 cm2|
in contact with 5 L water
|1301 cm2||1286 cm2|
|Weight||6,1 kg||2,3 kg|
|Wall thickness||~4 mm||<1 mm|
|Heat capacity of pan||2,8 kJ/K||1,2 kJ/K|
|Thermal conductivity||80 Wm-1K-1||16 Wm-1K-1|
|Thermal diffusivity||22 x 10-6 m2/s||4.3 x 10-6 m2/s|
The heat capacity of the cast iron pot is more than double that of the stainless steel pot. But this is negligible compared to the heat capacity of water: 10.5 kJ/K (2,5 L) and 20,9 kJ/K (5,0 L). Also, there is only a small difference in their surface area which cannot explain the large difference in temperature loss observed.
This leaves me with two eplanations:
My guess is that the difference in emissivity is more important (but please correct me if I’m wrong). With an infrared thermometer, one should therefore be able to measure a difference between pots of cast iron and polished stainless steel (even though they’re at the same temperature!) due to the difference in emissivity. Any one who can do the experiment and report back?
Conclusion: There are many good reasons to use cast iron, but keeping food warm is not one of them!