Sharpmaker, I guess the disconnect on this issue is: If you can't lower the heat output of your heat source to the point at which the thermal conductivity dampens it to the required level, you'll want WORSE thermal conductivity. This is entirely dependent on the equipment and process involved though...of course, this scenario is the only* reason why you'd want a diffuser for cooking (vs defrosting) in the first place.Induction is a good alternative to this though.*I suppose pots/pans that heat very unevenly could also be a reason, but I doubt many folks here have that issue.
I would not phrase it that way. You almost NEVER want WORSE thermal conductivity. If you wanted to keep the sentence structure of what you wrote, I would rephrase it like so: "If you can't lower the heat output of your heat source to the point at which the thermal conductivity dampens it to the required level, and you can't change your cookware, then you will want to buffer it with an underlayer of material. How thick that underlayer of material has to be to achieve even heating depends on the thermal characteristics of the material--the more thermally conductive the material is, the thinner that layer has to be."
My points were to address MC which framed it this way: ideally you would want a burner diameter to be pretty close to the diameter of the bottom of your cookware and for your cookware. (The middle of the pan is less of a problem since heat doesn't have anywhere to go but into the food anyway; it's the edges of the pan that keep losing heat and thus need heat replenishment continuously). But what if your largest burner isn't very large?
You have these options:
1) get a new cooktop with larger burners
2) don't use larger-diameter cookware on undersized burners
3) get more thermally-conductive cookware
4) get thicker-bottomed cookware
5) expand the size of the burner via plates/sheets/slabs/whatever of metal
1) and 2) aren't always realistic, so the real debate is on 3) vs 4) vs 5), and many people don't want to replace cookware so it's really just a discussion of 5).
Regarding 5), MC and I diverge on the value of thermal conductivity here. MC does not seem to care much about thermal conductivity and diffusivity, as they even recommend thick steel. MC greatly stresses how thickness is FAR more important than thermal qualities of the material going so far as to put steel in the same sentence as aluminum, which is crazy. It would be only a slight exaggeration to say that in MC's fantasy world, you should just find the lowest-density material available and use that as your heat diffuser, regardless of how good or bad of a thermal conductor it is. That way you maximize thickness without increasing weight to the point where you can't lift the plate up anymore.
MC isn't entirely wrong, but the truth is more complex. Thermal conductivity and diffusivity DO MATTER when it comes to even heating. This has been proven repeatedly, such as at
http://www.seriouseats.com/2010/02/everything-you-need-to-know-about-cast-iron-cooking.html (The cast iron was about 6mm thick and was very uneven-heating compared to the All-clad stainless pan which is roughly 3mm thick, of which roughly 2mm is aluminum and the rest stainless steel.) For a list of thermal conductivity numbers see
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
The fact of the matter is, the better thermally something is, the thinner you can get away with making the slab and still achieve the same degree of even heating. The thermally worse stuff needs thicker thicknesses to achieve the same degree of even heating.
MC says use plates of aluminum 0.5 to 1.5 inches (12.7mm to 38.1mm) thick. IMHO that is overkill for most people. Let's put this in context. All-Clad (and its clones like Cuisinart MCP, Calphalon, etc.) have only 1.6-2mm of aluminum. MC is basically saying with a straight face that in order to have even heating, you need an aluminum layer 6.35-24 times thicker than what you find in tri-ply clad, IN ADDITION TO what you already have in your pans themselves. (Either that, or aluminum is even worse than I thought about warping, and you need at least 0.5 inches thickness to guard against aluminum slabs warping over time. Which actually sounds about right, now that I think about it.)
What I'm telling you is that 1/8th thick copper (~3.2mm copper) is good enough for most realistic scenarios (i.e., putting your largest pan on your largest burner, not putting your largest pan on your smallest burner). A 1/8th-inch (3.2mm) thick copper slab is roughly equivalent in heat-spreading power to 4mm of aluminum due to copper's superior thermal conductivity and diffusivity. And you add that 3.2mm of copper to whatever you ALREADY HAVE in your cookware, such as the 1.6-2mm of aluminum in All-Clad. But beyond sheer heat-spreading power, copper is simply more efficient at transferring heat, so you have less pre-heating wait time and wasted energy (heat that goes into your kitchen rather than your pot); plus copper is harder than aluminum so it can take more abuse before getting dinged up or warping.
While the conclusions are correct in the serious eats article, I disagree with the reasoning. The hot spot I am more than convinced is metallurgical in nature. But, cast iron shines in an oven and I don't think there is any dispute of it's even heating properties in the oven. The diffuser is mimicking the oven I believe on a cast iron pot, albeit poorly as Sharpmaker has described.
-AJ
ANY metal is going to heat evenly in an oven because heat is bombarding the material from all directions. There is nothing special about cast iron in that respect; a copper, aluminum, or stainless steel dutch oven will also heat evenly. (The biggest advantage of cast iron dutch ovens inside of ovens is the heavy lid sealing in moisture, whereas you may need to weigh down the lid with something if you use, say, a flimsy aluminum dutch oven.) A stove is different, where you have a hot heating element circle at one extreme end of the cookware (the bottom end). A square or circle heat diffuser is essentially stretching that little heating element circle and making it into a solid square or circle. This has the effect of evening out the heat on the bottom.