I've been doing some more thinking and 'sperimentin with scorchprints and induction. Scorchprints can be useful, but they're certainly not perfect. They're snapshots, and as such can vary by time and temperature. And the browned flour and any pattern that emerges needs to be understood for what it is. I'll try to post photos, but CH is not supporting Windows 7 any longer, so I may have to add them later in subsequent posts.
Here's what I learned:
1. Basically, bread flour will not brown below about 300F, and browns weakly and extremely slowly up to about 350F. Above 400, it browns quite quickly. So, e.g., if a pan shows only central browning at 400F, the rest of the pan can easily be very near 375F and it will look like it is much cooler than it is.
2. Even good conductors like aluminum require time to move heat. Moving heat *up* is relatively short and fast; moving it laterally outward (and up the walls in clad) is a much greater distance, and takes longer before the system reaches equilibrium. So, it is possible to overwhelm *any* pan's lateral heat-spreading capacity and create a small circular scorch. This is not especially useful, as the first three worst-case photos show. Bear in mind that cast iron is SO terrible, that it burned in only 34 seconds!
3. We can consider the conductive layers in disk-base and clad pans to be *pipes* for purposes of lateral heat transfer; a thinner layer is a smaller pipe than a thicker layer. Clad's core is like a longer, (usually) smaller pipe with more total surface area; a disk is like a shorter, (usually) bigger pipe with less total surface area. Consequently, since two pans of these different constructions will move heat at different rates before reaching equilibrium, the same heat delivered for the same time may show somewhat misleading scorchprints. One may be adequately preheated, and the other may not.
4. Just as heat can overwhelm a pan's heat-spreading ability in an orthogonal (vertical) direction, heat can temporarily overwhelm the pipe laterally and accumulate. On an 1800W induction hotplate set to only 20/100 (392W), my clad pan reached a central equilibrium temperature of 305-309F and never browned its flour even after 30 minutes. My thick disk pan on the same setting reached a center equilibrium temperature of 425F. Photos 4 and 5 therefore show what? A difference in evenness?
5. On a 3500W commercial induction hob, a 50% setting (10 of 20) created a different picture: the disk-base pan at equilibrium had a center temperature of 379F (Photo 6), whereas the clad pan's center settled at 315F and did not brown.
6. This suggested preheating might be useful. So I first tried preheating both pans in a 300F oven, and *then* running at 20/100 on the 1800W. The center temperatures of the pans initially spiked 10-20F above what they hit without preheating, but also both settled back to what they were without preheating. Again, one showed browned flour and the other did not.
7. Then I tried merely setting both pans on the 3500W unit set to a *temperature* of 400F. This appliance heats until a single IR sensor reads the set temp near the center of the pan and then it beeps. The disk pan indicated preheat after 1:14; the clad pan beeped at 0:46, the difference I attribute to the "small pipe" effect--I suppose you could call it a temporary hotspot.
8. Then something very interesting happened. At equilibrium, the disk pan's center stabilized at exactly 400F and the clad pan at 394F. That part's unsurprising. But then I took floor periphery temperatures with a contact thermometer. It turns out that the average Delta T for the disk pan's floor was 37.5F, whereas the Delta T for the clad pan was slightly LESS, 35F. By this measure, the clad pan's floor was actually more even.
9. If you cook only on the pan floor, and see no role for heated sidewalls, you can wisely choose disk for induction. But if you want to be able to utilize a skillet's sidewalls for actual cooking, a look at the sidewall temperatures is useful. So I also took the temperatures of the sidewalls of both pans, at the rim and midway between the floor and rim. The clad pan midway averaged 314F and 302F at the rim. The disk pan's sidewalls, being only stainless, registered only 161F at the midpoint and 139F at the rim.
10. All of the above follows logically from the fact that a 28cm disk pan has about a 24cm-diameter cooking surface, whereas a 28cm clad pan has about 35cm. While induced heat drops off substantially as it is conducted that great distance, you may still find it useful for cooking.
11. One way to think about the choice--and scorchprints--is that a disk contains and accumulates heat in the floor at equilibrium, whereas clad continues to move it laterally a greater distance. In so doing, thick clad tends to moderate heat to the point where it can actually better limit max temperatures. This difference, together with the way flour browns, can skew the appearance of scorchprints. Photo 7.
12. The difference in the way the two induction appliances performed with the same pans was unsettling. The VMP at setting 20/100 should be close to the 3500W unit at 10, but it wasn't. The temperature settings were 'way off with one, but not the other. And with the VMP, the wattage draw cycled wildly (every second it jumped around, like 610-37-156-6.4-520) with the clad pan, whereas it stayed rock solid at 392W with the disk pan. This makes it hard to generalize, and would explain different cooks' experiences, even with the same pans.
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