Comments on: Staying warm: Cast iron vs. stainless steel

By: Guido

Guido — Fri, 26 Feb 2010 12:33:15 +0000

For a computation of air convection losses on the pot look at the online book “Renewable energy resources” by Twidell & Weir, Example 3.3
http://books.google.com/books?id=CipkYGCrecEC&pg=PA58
Rescaling to the pots of Martin’s experiment it gives 83.5W and 81.5W for cast iron and SS at 100°C. These values should be taken as indicative (say with 5% uncertainty).

The blackbody radiation terms, according to the reported emissivities, are 135W and 10W.
Summing together convection+radiaton, we have total power losses of 218.5W for cast iron and 91.5W for SS.

The thermal capacities added by the pot materials are instead negligible (+13% and +5.5%) compared to the 5 L of water.

The calculated cool-down time constants are 2.4 hours (2h25min) for iron and 5.37 hours (5h22min) for SS, both filled with 5 L of water. The meaning of the time constant is the following: on Martin’s graph draw a line tangent to the curve at time=0 and find the the time this line takes to reach ambient (20°C) temperature.

Hoping to have clarified a bit the matter

By: AnthonyRC

AnthonyRC — Tue, 27 Oct 2009 00:54:59 +0000

Awesome experiment!

I would try the experiment again this way: preheat both pots for say 5 minutes. Then add already boiling water into the pots.

I am thinking that the cast iron will get hotter and stay hotter longer. No science, just guessing.

By: Jeromy

Jeromy — Wed, 01 Jul 2009 22:55:54 +0000

the cast iron post is a more efficient condutor of heat between the air and the pot itself (combination of conduction and convection). This is due to the fact the cast iron surface is rougher than a polished stainless surface, so if you look on a micro level there is a much greater surface area for the air to come in conctact with.

I doubt radiation would be a factor. radiation only comes in to play when the difference in surface temperatures gets closer to 1000K not 100. You could test this by quickly placing a sheet of glass next to the pot and see if you can feel any warmth thru the glass. This will seperate the conduction/convection with the radiation.

By: Sharon

Sharon — Wed, 30 Jul 2008 10:22:48 +0000

I have heard that the quality of cast iron pots n pans these days is not the same quality it has been renowned for in the past, how best would one determine/know they are buying high/good quality iron?

By: Martin Lersch

Martin Lersch — Sun, 22 Jun 2008 17:48:02 +0000

chris: I’ve updated the table now with wall thickness and the surface area of the water that is in contact with the pan. I can’t see how this should explain the large difference observed for keeping water warm.

By: chris

chris — Sat, 19 Jan 2008 14:14:33 +0000

you need to calculate the surface area of the actual cyllinder of water within the pan,(the area of the water that actually touches the pan) not the area of the pan itself. Iron, having greater thermal capacity, as well as conductivity, will draw more heat up out of the water into the top portion of the pot, increasing the effective surface area for heat radiation. To remove these variables, it would be necessairy to have two pots of identical shape and volume, and to fill them both to capacity.

By: Martin

Martin — Tue, 15 Jan 2008 16:33:55 +0000

The surface areas are in fact almost the same. From the table: Surface area (top+sides) 1619 cm2 vs. 1629 cm2.

It’s true that the pans have different mass and different heat capacities, but this is negligible compared to the heat capacity of water.

In conclusion I still think that it’s the difference in emissivity which is the main reason for why the stainless steel pan keeps water warmer.

(I realize now that there is one parameter which I forgot to include in the table – the thickness of the walls/lid – the cast iron pan for sure is much thicker, but I’ll measure them both and report back)

By: chris

chris — Tue, 15 Jan 2008 14:48:50 +0000

the pans are not the same shape. This is the most likely cause of variance in heat transfer times. The wider pan will expose more surface area than the narrow pan, allowing greater heat transfer. The ideal shape to minimize heat loss is a sphere. (least surface area per volume). Also, a large heavy pan acts as a heat-sink, pulling heat out of the water and transfering it to the surrounding air.

By: bob

bob — Wed, 19 Dec 2007 05:09:43 +0000

Cast iron has better thermal conductivity than stainless steel. The surrounding air is cooler than the water so it begins to cool with the passage of time. Since cast iron is a better conductor of heat, it transfers the heat of the water to the surroundings better than stainless steel. If you had put the water into a large styrofoam container it would have stayed warm even longer! Yet if you cook in styrofoam it will disintigrate where it comes into contact with the burner. But the foam will be fine elsewhere. Why? Because styrofoam has poor thermal conductivity. Likewise cooking in a stainless steel pot or pan may scorch or burn the contents in the vicinity of contact with the burner because it too has poor thermal conductivity. That is why an aluminum or copper base is added to a stainless steel pot/pan. Cast iron does have higher heat capacity than stainless steel. Also it is denser increasing this effect. Also it is thicker (more volume, the pot that is) yet again increasing this effect. This extra heat stored in the pot/pan in conjunction with the higher thermal conductivity acts as a buffer to protect against hot spots burning or scorching food in the pan. You could aim a propane torch at the bottom of a cast iron skillet and cook just fine. With a stainless steel pan you’d likely punch a hole through the pan.

By: Rob

Rob — Sun, 04 Mar 2007 19:52:47 +0000

OK -I could see stainless composite keeping heat longer- however us Cast Iron fanatics don’t look for that property so much, if at all. We like the even heat distribution, the lack of a possible carcinogen ( teflon ) – and the no stick surface of a good seasoned pan or pot.

We ditched all our other pots and pans for good old cast iron – except for our pyrex glassware. Thats about as non reactive as you can get.

Rob