Cast iron has a hot spot on the stove top due to graphite morphology not the burner. This is something unique to cast iron. I am convinced of this.
To AJ, I am not a metallurgist and have no idea what you are talking about re: graphite morphology. I do know that cast iron simply does not conduct heat as well as aluminum or copper, and you can wind up with a hot spot that corresponds to the burner circle as a result. See, for instance:
http://www.cookingissues.com/2010/02...-iron-cooking/ (already posted photos via SE but this is a fuller treatment)
http://www.nytimes.com/2008/10/08/di...pagewanted=all (they have a photo gallery, author talks about the familiar ring of fire problem with cast iron)
http://chowhound.chow.com/topics/804287 (see his revised shootout numbers and how copper and aluminum were head and shoulders above cast iron in terms of even heat)
http://www.discusscooking.com/forums...l-78265-2.html (not cookware but just the grating, still, gives a good visual on how heat drops off as you get away from the burner, on whatever the grating material is--which is definitely not copper or aluminum!)
If you have an IR gun and some black tape, you can set up your own experiment (I've done this myself with my cookware). Take the temperature of the black tape on the side of anything shiny to reduce errors due to emissivity. Cast iron isn't shiny and doesn't really need tape. I can already tell you the results: cast iron will be slow to even heat out and can't maintain the evenness of heat the way aluminum and copper of equal thickness can, and even when you compare 6mm cast iron to say, 2mm copper, the Cu will STILL win (see tanuki soup's shootout results on induction via chowhound, linked above; too bad he didn't have exact measurements of thickness but we know the PM is 1.8mm copper, 0.2mm stainless, and a thin layer of ferritic material, and the cast iron was undoubtedly much thicker than 2mm). That is not to say cast iron is unusable, just that you may need to move food around a little bit more often. Plus 1/4-inch thick cast iron has the high heat capacity to withstand the sudden insertion of cold meat without losing TOO much heat, something that would require extra-thick aluminum or copper to match (4mm Al, 2.5mm Cu ought to do it). Some people also like cast iron seasoning's quasi-nonstick surface. But in terms of stovetop even heating, Al and Cu are way better than cast iron.
First my apologies for the thread drift. Yes, cast iron would rank #3 behind copper and aluminum. But, cast iron in the grand scheme of things is a phenomenal heat conductor because it contains graphite flakes which conduct heat almost 5x better than copper [I]along[/] the crystal axis but 70x worse than copper across the crystal axis. Sparing you a lot of detail but these flakes are distributed in different sizes and thicknesses. They will be thickest and longest in the center of the pan, maybe slightly off center toward the handle. That's why there's still a hot spot with the cast iron pan on an induction cook top. Didn't that strike anyone as odd? There will always be that hot spot.
Sorry for being so technical. I'm not refuting or arguing anything. Just explaining.
http://www.cookingissues.com/2010/02...-iron-cooking/ So it's not just a graphite flake issue unless that also applies to Al. The simplest explanation, imho, is that they used a too-small induction coil and the 1.6-2mm of Al in all-clad wasn't thick enough to spread the heat much. . This coil/cookware diameter mismatch is what MC warned against.