Scorchprints: the lunar landscape
In the previous scorchprint thread, the Testmaster employed a Nambutetsu (Japanese cast iron from near Morioka) footed cauldron on an LG induction cooktop burner. The material to be scorched was King Arthur All Purpose flour, inartfully sprinkled by hand onto the dry surface of the interior of the cauldron, then smoothed, not entirely successfully, with the bottom of a large soup ladle. See http://chowhound.chow.com/topics/7544...
Caroline1 suggested that the flour might have spread more evenly had the Testmaster sprayed the interior of the cauldron with PAM first, then discarded the excess flour after sprinkling it over the oiled surface. kaleokahu wondered about how a flatter bottomed piece of cookware might have fared in the same test. The Testmaster has performed a further experiment to address both of those suggestions.
The pan in this second test is a Kuhn-Rikon frypan/sauté pan, which was not purchased as part of the Kuhn-Rikon Duromatic Duo set, but which is, nevertheless, identical to one of the two pans in that set. http://www.kuhnrikon.com/products/pre... A close up photo of the underside of our copy of the pan -- with a ruler to show its diameter at the base -- is the first image accompanying this post. The third image is a photo from above of the naked interior of the pan.
The pan in this test has a completely different construction from the Nambutetsu cauldron used in the prior test. The body of the Kuhn-Rikon is composed entirely of a single layer of heavy gauge NONmagnetic 18/10 stainless steel. A magnet is not attracted at all to the sidewall of this pan. Fused (or bonded) to the base of the pan is a very thin profile aluminum disk. The aluminum layer appears to be at most 1/16" (1.5 mm) thick, and could be a bit thinner than that. Fused (or bonded) to the bottom of the aluminum disk, in turn, is an even thinner disk of magnetic stainless steel of the same diameter; that bottom disk is the only magnetic material in the pan. Despite the predominance of materials that are poor heat conductors and the thinness of the heat-distributing aluminum disk, we have found the pan to be a very satisfactory tool on both induction and noninduction energy sources for a variety of cooking tasks, and I have recommended it as a fine starter pan in a few Chowhound threads.
Because the body of the pan is nonmagnetic, when used on an induction hob all heat that gets to the inside of the pan is generated in the thin disk at the very bottom and must make its way to the interior by heat conduction through the aluminum disk and then through a thick layer of 18/10 stainless steel, which is a poor heat conductor.
The second image in the series is of the induction burner on which the test was conducted. The ruler shows that the burner's diameter closely matches the diameter of the base of the Kuhn-Rikon.
Caroline1, no can of PAM has ever entered our home in the three decades plus that we have lived here. Lacking PAM, the Testmaster dribbled a few drops of Saffola oil into the bottom of the Kuhn-Rikon pan, then, using a paper towel, spread the oil around the bottom of the pan and soaked up all but a very thin film of oil. Flour then was sprinkled as evenly as possible all around the pan's interior, then the pan shaken and jiggled to get flour to every corner of the floor of the pan before the excess was poured out and discarded. Even with this procedure, there was some unevenness to the coating, as may be seen in the fourth of the accompanying photos, taken before the burner was turned on.
Do you see the lunar landscape of the topic line? That really is a photo of the floured Kuhn-Rikon and not a photo of the far side of the moon during a lunar eclipse on earth.
The induction burner was then turned on to 4. (four-dot), roughly the midpoint of the burner's levels. The fifth image shows the beginning of the scorching process about three minutes into the burn. The identical "map of the surface of the moon" shows as when the flour was unscorched. As was the case with the Nambutetsu cauldron, there is scorching all the way across the bottom of the pan, and the variations in the degree of scorching are almost exactly inversely proportional to the very localized thickness of the layer of flour. The "white stripe" from 7:00 to 1:00 is entirely specular reflectons from the on-camera flash. As if it required any proof, we have proved once more that King Arthur flour is a very good insulator.
The sixth image shows the "final" scorch, after about seven or eight minutes. The dimensions of the aluminum disk on the other side of the thick layer of stainless steel are sharply and distinctly outlined, and the scorching is uniform across the entire diameter. The area towards 4:00 in the photo appears to be less scorched than the area around 10:00, but the difference was much less pronounced in person than it appears in the photo, and may be more (though not entirely: see below) an artifact of the angle of incidence of the on-camera flash (see the specular highlights in the 4:00 region) than of differences of degree of scorching.
The seventh and final photo was taken from a different angle and a slightly greater elevation above the pan and more faithfully than the sixth photo represents the in-person appearance. In the final photo, what had been the 4:00 position of the fourth, fifth, and sixth photos now appears at 1:00, and (as in the earlier photos) that area is somewhat less darkly scorched than the area diametrically opposite on the floor of the pan. However, again specular highlights in the (new) 4:00 position show that it probably is the angle of light incidence from the flash that makes the portion of the scorch around 3:00 look lighter, relative to the darker-appearing scorch in the area opposite it at 9:00.
It is not possible with the data available to us at this time to evaluate whether the difference in degree of scorching between the 7:00 and the 1:00 positions in the final image trace to differences in the intensity of the magnetic field from the induction inverter or to differences between the rate of heat conduction from the bottom steel disk through the intermediate aluminum disk through the stainless floor of the pan.
One more data point.
kaleokahu: "What's the thickness of the Al disc? And is the pan fully clad or disc-bottomed?"
From the fourth paragraph of my post above:
"The body of the Kuhn-Rikon is composed entirely of a single layer of heavy gauge NONmagnetic 18/10 stainless steel. A magnet is not attracted at all to the sidewall of this pan. Fused (or bonded) to the base of the pan is a very thin profile aluminum disk. The aluminum layer appears to be at most 1/16" (1.5 mm) thick, and could be a bit thinner than that."
Politeness: So this K-R pan has, bottom to top, a 3-layer sandwich, SS+Al+SS(nonmagnetic). That tells me that: (a) the induced friction is probably happening just in the thin bottom layer, leaving it for conduction to get the heat upward into the food; and (b) therefore the Al disc IS in position to moderate and even the heat that reaches the nonmagnetic SS body of the pan. Still, I am less confident that a thin --1-1.5mm--AL layer is doing a whole lot.
In this second test, you have convinced me your 7" hob is likely to produce even heat in a 7" pan. Such an ideal match-up is what we all OUGHT to be doing, but alas many of us have 10" and 12" rounds (and ovals) that we must use from time to time on a 7 inch (or a 6" if the 7" is occupied) hob.
It would be interesting to see: (a) if a 10-incher on the same hob would also halo to the extent of the disc; or (b) if the same pan on a 6" coil would be as even.
kaleokahu: "That tells me that: (a) the induced friction is probably happening just in the thin bottom layer, leaving it for conduction to get the heat upward into the food ..."
From the fifth paragraph of my initial post in this thread:
"Because the body of the pan is nonmagnetic, when used on an induction hob all heat that gets to the inside of the pan is generated in the thin disk at the very bottom and must make its way to the interior by heat conduction through the aluminum disk and then through a thick layer of 18/10 stainless steel, which is a poor heat conductor."
kaleokahu: " ... alas many of us have 10" and 12" rounds (and ovals) that we must use from time to time ..."
That is what the 11" 3200w burner of the cooktop was designed for. That, and the triple-burner bridge on the left side of the cooktop that can be configured as a 7" x 16" (approximately) oval element.
The very sharp and well-defined edge of the scorched area, exactly coincident with the diameter and edge of the disks on the bottom of the pan, make it clear that, within the time frame it took for the heat to be conducted through the thickness of the nonmagnetic stainless steel floor of the pan to scorch the flour, no appreciable amount of heat got conducted laterally through the pan's stainless steel body up the curvature to the walls of the pan.
kaleokahu: "I know you consider this a virtue."
Not necessarily. I would not choose a Mazda Miata (MX-5) to haul furniture around or to take the family on a cross-country vacation, nor would I choose a Kenworth or a Peterbilt to go the supermarket for a bottle of milk. Different cooking tasks demand different kinds of pot/pan construction, and no type is best for all kinds of cooking.
Politeness: "Different cooking tasks demand different kinds of pot/pan construction, and no type is best for all kinds of cooking."
Yes, of course. But with certain exceptions, there are generalizations, and margins of difference. I choose to be limited by things other than the source of heat. You accept a source, and the choice algorithm takes you where you need to go. Different courses for different horses. There's always a work-around, and the margins... they're small, all things considered.
O.k., I am going to run some numbers here, and show my work as I go. For simplicity, I'll round the numbers. Feel free to challenge any of the assumptions or insert your own numbers.
Let us say the disk at the bottom of the Kuhn-Rikon pan used in this test (see the first photo in this series) has a diameter of 7" and therefore a radius (r) of 3.5". Close enough. Then (using 3.14 as a value for pi) its surface area is pi · r² = 38.5 sq.in., rounded. In this experiment, the induction energy source heated that area -- all of it, as you can discern from the scorchprint -- to somewhere around 350° F. to 400° F. , no higher. (I can explain how I know what the temperature was, if you wish, but I'll save the space for now.)
Let us say that, instead of the induction burner, I had placed the same pan onto a gas burner, which has 24 gas jets arranged in a ring, and the flame of each of those gas jets touches the bottom of the disk in a perfect circle ½" in diameter; the radius of each of those circles, then, is ¼". Then the total area of contact between the gas jets and the disk is 24·pi·r² = 4.7 sq.in., rounded: about one-eighth the area in which the induction energy source induced temperatures of 350° to 400°. The temperature of the flame at those 24 circles of the gas jets is (according to a favorite site of one of the posters here) is about 3,500° F.
http://www.brooklyncoppercookware.com... The increased temperature of the gas heat source makes up for its much smaller area of physical contact.
No wonder it is useful, even necessary, when cooking on a gas hob, to use cookware that is fabricated from highly conductive materials. Otherwise there would be hotspot islands of 3,000+° separated by room temperature areas of the pan's surface. If you cook on gas, copper and aluminum are not merely your friends, they are your co-dependents.
Now, let us take the tangent to consider a hypothetical variation of the flour scorching test that I conducted a week ago on the Iwachu Nambutetsu cauldron. http://chowhound.chow.com/topics/7544... Suppose that, instead of an induction burner, I had rested the cauldron on the coil of a conventional resistive electric cooktop or range. Recall that that cauldron sits above the surface of the hob, resting on just three small feet that may be seen here: http://www.chow.com/photos/318813 As we know, cast iron has a thermal conductivity of only about 80 W/mK (See http://www.brooklyncoppercookware.com... ); but even cast iron has much, much higher heat conductivity than room air; air, after all, is commonly used as an insulator in architectural construction, as between the panes of energy-conserving windows. So the transmission of heat from an electric coil burner to the cauldron would have proceeded primarily through the feet in contact with the burner coil, but it would have progressed much more slowly than it did with an induction hob, having to work its way through a severe bottleneck (to mix a metaphor); and very likely it also would have produced three localized hotspots at the points where the three feet meet the base of the cauldron; and not much heat would spread elsewhere in the cauldron. That cauldron and an electric coil cooktop would not be a match made in heaven.
I have no doubt that the aluminum layer of the Kuhn-Rikon pan in the later test of this thread MAY have had some supplemental smoothing effect on the heat transmitted from the steel disk below it to the thick stainless bottom of the pan. But, after all, the induction energy source generated heat ab initio in a very large area (the 38.5 sq.in. of the magnetic steel disk below the aluminum disk), and at a lower temperature differentials from the prior temperature of the pan compared to the differential between the prior temperature of the pan and a 3,500° flame. Therefore, it COULD be that the aluminum layer of the Kuhn-Rikon was more like a speed bump than like a valuable accessory to the even heat distribution of the Kuhn-Rikon pan on the induction burner.
That suggests that a further experiment would be in order to test an induction-compatible pot that entirely lacks any highly conductive layer of copper or aluminum. One such test -- the Nambutetsu cauldron -- has already been conducted and posted here. But what about a test of a stainless steel pot -- the thermal conductivity of stainless steel is only about one-fifth that of cast iron -- that lacks a highly conductive disk or (if clad) layer of copper or aluminum?
In fact, I use one such pot with regularity: our 20 cm 1.9 qt. Mauviel Induc'Inox (apparently re-christened M'Cook) Windsor pan, the construction of which comprises three steel layers, of which the outer layers are stainless steel and the inner layers are "just" steel. http://www.lacuisineus.com/catalog/in... That pan reacts very nimbly to an induction energy source and heats very evenly; I frequently make breakfast oatmeal in it in the winter months. Maybe some time in 2011 I will get around to performing a scorchprint test with that pan.
Politeness: "No wonder it is useful, even necessary, when cooking on a gas hob, to use cookware that is fabricated from highly conductive materials. Otherwise there would be hotspot islands of 3,000+° separated by room temperature areas of the pan's surface."
Thanks for your area calculations of a hypothetical gas hob. You make certain assumptions here that I find unrealistic. One is that each "jet" of gas will only heat a spot 1/2 inch in diameter. While there clearly is a drop-off of heat the further one gets from the hottest tip of the flame(s), the circle of flame you hypothesize is not strictly limited to the area licked by visible blue flame, or to the flame at all--hot combustion gases (not unlike the far induction field) are acting to heat a much larger area than you conclude. In fact, these combustion gases also flow around and up the sides of the cooking vessel, heating in 3 dimensions. I learned this painful lesson in my speed-heating and -cooling experiments, where I could hold the thermometer above the pan only after digging out my welding gloves.
Another assumption is that pans of poor conductivity (i.e., those which necessitate the "co-dependency", LOL) would somehow COOK at their hotspots at 3,000 degrees F leaving the unflamed areas at "room temperature". This is clearly not the case even with non-codependent cast iron or steel. Perhaps you could get a serious heat differential in plutonium cookware, but even in cast iron there is only a 100-200F difference between the hottest of hotspots and the edge--significantly less than the differential between the edge and room.
Your second hypothetical (a footed cauldron on resistive electric coil) is also exceedingly unrealistic. Who--besides the unfortunate few who try wok-on-ring cooking this way--would suspend a cooking vessel over a resistive hob, or count on three tiny points of hob contact to conduct real cooking heat? This hypothetical makes as little sense as wok cooking on induction.
"Therefore, it COULD be that the aluminum layer of the Kuhn-Rikon was more like a speed bump than like a valuable accessory to the even heat distribution of the Kuhn-Rikon pan on the induction burner."
Well, what we know is that the heat made it to the flour in your K-R pan strictly via conduction. Every scorched particle of flour got that way because the heat traveled ONLY via conduction through the aluminum and nonmagnetic steel layers to get there. With respect to you and your camera's optics, I DO already see a difference in the scorchprints between the CI cauldron and your Al disc pan. While both were even, the disc pan's print looks more even to me, both dead-center and further out. And, I think if you had used the kind of flat, in-contact, CI skillet that >90% of cooks use, instead of your footed cauldron, we'dve seen a bigger difference.
But I await your results with your experiment with steel-only. Use the same hob, the same 4. heat setting, and a pan with a 7" flat bottom to keep it apples-to-apples, hmm?