boilsover

Carbon steel pans are poorly conductive, so I'm not surprised your glass cooktop failed at polymerizing the oil on the sidewalls. Here's a handy chart for smoke and flash points: http://www.centrafoods.com/blog/edible-oil-smoke-flash-points-temperature-chart Note that at 550F, you're flirting with a housefire if you choose coconut oil. 450F should be high enough. I would just continue cooking in it. For a few uses, scrub it out with an oil/kosher salt slurry, wipe it out, and fire that sucker until it smokes. It'll eventually season.

Hi. Beusho: Your photo of your first attempt looks to me like you started with too much oil. There shouldn't be any pooled oil at all--thin wipes are best, as in a sloppy job of wiping completely clean. If you are getting runnels or "tears", there's still too much oil. Was this done on a radiant or induction top? Gas would work better, IMHO, because you can tilt and rotate the pan in a higher flame. Otherwise, the oven is your friend. Are you heating the pan past the oil's smoke point? Unlike seasoning cast iron and "seasoning" aluminum and SS, you want to do this that hot. Unless you feel something sticky or tactile left over, just keep putting layers down. You should ultimately get a more-or-less even, grungy brown-black appearance.

There is oxalic acid in potato skins. As in Barkeeper's Friend... It is a strange instruction, but I guess BKF isn't universal. Demeyere instructs to boil milk in their tall asparagus steamer, yet this has been their method for so long no one at the company can tell me, why milk?

Thank you. Yes, I think describing cooking on this appliance as ad hoc is accurate. But I suggest that is also the case with most induction units, and for that matter most appliances and pans in general. The idea of precise control is mostly illusory. But I have not yet played with a Control Freak. Why not just use gas? Very good question. I think the only meaningful answer (aside from personal preference and perceived convenience) is repeatability. Once you determine setting X is ideal for Y pan with Z portions, you just set X again. Gas requires judging by flame height or gross dial position, and repeating that is never 100% certain. I did not measure the time needed to heat a skim of oil to frying temperature, just 2L for deep frying. Doing that right requires a flat contact probe that can be immersed in oil. Shooting IR through a thin oil film over a shiny bottom brings its own issues with emissivity. The niche is for cooks who want to use copper/aluminum with induction, and that may be the best performance combination. For example, one of the most revered lines of clad was Sitram Catering. It wouldn't work at all on 24kHz, but it certainly would on this unit. Many restaurants might switch to induction if they could continue to use cheap aluminum cookware. And if the necessity for highest heat falls away, there may be some real energy savings.

I. Introduction This article reviews the 3500W all-metal commercial induction single-hob hotplate by Panasonic, which I believe is the first “all-metal” unit to hit the U.S. market. Where appropriate, it is also compared with another commercial single-hob, the 1800W Vollrath Mirage Pro Model 59500P. Some background is in order. Heretofore, induction appliances would only “work” with cookware which is ferromagnetic. Bare and enameled cast iron, carbon steel, enameled steel and some stainless steels were semi-dependable for choices, and the cookware industry has worked hard to make most of its lines induction compatible. But alas, not all cookware, past and present, has worked; copper and aluminum don’t, at least without a separate interface disk or it’s own ferromagnetic base layer. The reason why non-ferromagnetic cookware hasn’t worked on induction is technical, but it relates to the magnetic field and what’s called the “skin depth” of the pan’s outermost material. With copper or aluminum, the field will not excite the metals’ molecules to the extent that their friction will generate useful heat to cook food. And the way the appliances come equipped, unless the appliance detects something sufficiently large and ferromagnetic, they will not produce any field at all. Therefore, to the consternation of many cooks, pro and amateur, older (and in the opinion of some, better) cookware needs to be retired and replaced if/when they wish to switch to an induction appliance. Some cooks don’t mind, but others who, like me, have invested heavily in copper and are habituated to it and aluminum, would forego induction altogether rather than discard our cookware. But what we’ve really meant—all along--when we say or write that only ferromagnetic cookware will “work” on induction is that the frequency chosen for our appliances (20-24kHz) will not usefully excite other metals. If that frequency is increased to, say, 90-110kHz , then suddenly the impossible happens: aluminum and copper, with absolutely no ferromagnetic content, will heat in a way that is eminently useful in the kitchen. While Panasonic has made dual-frequency induction hotplates available in Japan for several years now, they didn’t make it available here until recently (My unit indicates it was manufactured in early 2016!). I speculate the reason for the delay relates to the detection circuitry and the switches that determine the frequency at which the field will operate. The introduction of all-metal induction in USA is especially interesting because it allows a direct comparison of cookware of all (metal) types. For instance, cookware nerds have long debated how copper cookware on a gas compares with disk-based stainless on induction. While the veil has not completely lifted (for that we would need extremely precise gas energy metering), we now have the ability to measure and compare copper, aluminum, clad and disk-based on the same induction hob. II. Dimensions, Weight & Clearances The Panasonic, being a true commercial appliance, is considerably larger than most consumer and crossover hotplates. It stands 6 inches tall overall, and on relatively tall (1.25”) feet, so that there is space for ample air circulation under the unit. It is 20.25 inches deep overall, including a standoff ventilation panel in back, and the angled control panel in front. It is 15” wide, and weighs in at a hefty 30.25 pounds. Suffice it to say, the Panasonic is not practically portable. The KY-MK3500’s Ceran pan surface is 14.25 inches wide by 14.5 inches deep, almost 43% larger in area than the VMP’s glass. Panasonic tells me they have no recommended maximum pan diameter or weight, but the tape tells me that a 15” diameter pan would not overhang the unit’s top (Compare the VMP, which can accept a maximum pan base of 10 7/8”). Common sense tells me that—unless the glass is well-braced underneath in many places, 25-30 pounds of total weight might be pushing it. For those who might consider outfitting their home kitchens with one or more of these units, in addition to having 20 amp 240v (NEMA #6-20R receptacles) electrical circuits for each appliance, 39 1/2 inches of overhead clearance is required to combustible material (31 ½” to incombustibles) and 2 inches to the back and sides (0” to incombustibles). The overhead clearance requirement and the tall 6” unit height call for no (or only very high) cabinetry and careful design of a “well” or lower countertop/table that will lower the Ceran surface to a comfortable cooking height. In other words, a tall pot on this unit on a regular-height counter might be a problem for a lot of cooks. III. Features A. Display The KY-Mk3500 has an angled 8-key spillproof keypad and red LED numerical display. The keys are large, raised and their markings are legible. All but the four Up/Down keys have their own inset indicator lights, which indicate power, mode and memory operation. The numerical display is large and bright. The numerical display area is divided between time (XX:XX) to the user’s left and power/temp to the user’s right. If the timer or program features are activated, the numerical display shows both the set time and the power/temperature. There is also a small “Hot Surface” LED icon on the panel. The Panasonic also actually uses the Ceran surface as a display of sorts. That is, there is a lighted circle just outside the faint positioning circle, which glows red whenever the unit is operating, awaiting a pan, or the Ceran is hot. Panasonic also claims that this display also changes brightness with the set power level, implying that the operator can judge the heat setting by a glance. Thus this display serves three purposes: (a) pan positioning; (b) burn safety; and (c) intensity. B. Safety Features As one would expect, there are a variety of safety features built into this appliance. In most cases, these features are controlled by detection circuits, some fixed, some defeatable/variable. This being a commercial unit, Panasonic has set the unit’s defaults with commercial users’ convenience in mind. If consumers want the full spectrum of safety settings, they need to vary these defaults. For instance, if a home cook wants to make sure the unit powers off if the pan is removed and not replaced within 3 minutes, they have to manually vary a default. Likewise if the operator wants the power to automatically shut off after 2 hours of no changes. But others, like the basic “Is there a pan there?” detection and overheat shutoff, are there no matter what and cannot be defeated. C. Settings & Programming The KY-MK3500 features both power and temperature settings. For “regular” induction, there are 20 power settings, which range from 50 watts to 3500 watts. For non-ferromagnetic pans, there are 18 power settings, which range from 60 watts to 2400 watts. The display shows these settings in numerals 1-20 and 1-18 respectively. When the power is toggled on, the unit defaults to Setting 14 in both frequencies. The temperature settings are the same in both modes, with 22 selectable temperatures from 285F (140C) to 500F (260C). Other than for the very lowest temperature setting, each setting increase results in a 10F temperature increase. Usefully, the display shows the set temperature, not 1-22; and until the set temperature is reached, the display indicates “Preheat”. The unit beeps when it reaches the set temperature. The Panasonic measures pan temperature using an IR sensor beneath the glass; this sensor sits about 1 inch outside the centerpoint of the painted positioning markings, yet inside of the induction coil. The timer operation is fast and intuitive. Once the power or temperature is set and operating, the operator merely keys the timer’s dedicated up/down buttons, and the timer display area activates. Timer settings are in any 30-second interval between 30 seconds and 9 ½ hours, and the display will show remaining time. The beeps at the end of cooking are loud. There are nine available memory programs, which can be set for either power or temperature, along with time. Programming entails pressing and holding the Program mode button, selecting the program (1-9), then picking and setting the power or temperature, then setting the timer, and finally pressing and holding the Program button again. After that, to use any of the entered programs, you simply press the Program button, select which program, and the unit will run that program within 3 seconds. In addition to Heat-Time programmability, the KY-MK3500 also provides the ability to vary 9 of the unit’s default settings: (1) Decreasing the power level granularity from 20 to 10; (2) Changing the temperature display to Celsius; (3) Enabling a long cook time shutoff safety feature; (4) Enabling the main power auto shutoff feature; (5) Disabling the glowing circle; (6) Lowering or disabling the auditory beep signals’ volume; (7) Customizing the timer finish beep; (8) Customizing the Preheat notification beep; and (9) Customizing the interval for filter cleanings. D. Maintenance The KY-MK3500 has a plastic air intake filter which can be removed and cleaned. This is not dishwashable. This filter is merely a plastic grate with ¼” square holes, so it is questionable what exactly —besides greasy dust bunnies—will be filtered. Panasonic recommends the filter be cleaned once a week. Besides that, the Ceran surface and stainless housing clean just like other appliances. IV. Acceptable Cookware Panasonic claims the unit will accept cast iron, enameled iron, stainless steel, copper, and aluminum with two provisos. First, very thin aluminum and copper may “move” on the appliance. And second, thin aluminum pans may “deform”. Panasonic does not address carbon steel pans, but I verified that they do indeed work. They also warn of the obvious fact that glass and ceramics will not work. Buyers are also warned against using cookware of specific cookware bottom shapes: round, footed, thin, and domed. Trying to use these, Panasonic warns, may disable safety features and reduce or eliminate pan heating. As far as minimum pan diameter goes, Panasonic claims the KY-MK3500 needs 5” diameter in ferromagnetic pans, and 6” in copper or aluminum ones. My own tests have shown that in fact the unit will function with a cast iron fondue pot, the base of which is only 4 1/8” in diameter, and also works with a copper saucepan, the base of which is almost exactly 5” in diameter. Obviously, the field will be most active at the very edges of such small pans, but they do function. V. Evaluation in Use I can say that not only does the Panasonic KY-MK3500 “work” with copper and aluminum pans, but that it works very well with them. Thermally, thick gauge conductive material pans perform in close emulation of the same pans on gas, even though there are no combustion gasses flowing up and around the pan. I found this startling. Nevertheless, a searching comparison between copper and ferromagnetic pans on this unit isn’t as straightforward as one might expect. The Panasonic is capable of dumping a full 3500 watts into ferromagnetic pans, but is limited to 2400 watts for aluminum and copper. Despite copper’s and aluminum’s superiorities in conductivity, that extra 1100 watts is going to win every speed-boil race. I initially thought I could handicap such a race simply by using the temperature setting and comparing the times required to achieve a “preheat” in a pans of cold water. Alas, no—the Panasonic’s IR function signified the copper pan was preheated to 350F before the water even reached 70F! Obviously, the entire thermal system of cold food in a cold pan needs to come to equilibrium before the Panasonic’s temperature readout becomes meaningful. A. Temperature Settings Unfortunately, with every pan I tried, the temperature settings were wildly inaccurate for measuring the temperature of the food. I heated 2 liters of peanut oil in a variety of pots, disk-base, enameled cast iron enameled steel, and copper. I thought it might be useful to see how close to 350F and 375F the settings were for deep frying. The oil in a Le Creuset 5.5Q Dutch oven set to 350F never made it past 285F, and it took 40:00 to get there. I kept bumping up the setting until I found that the setting for 420F will hold the oil at 346F. A disk-based pot didn’t hit 365F until the temperature setting was boosted to 400F. The only pan which came remotely close to being true to the settings was a 2mm silvered copper oven, which heated its oil to 327F when the Panasonic was set for 350F, and 380F when set for 410F. The temperature function was a lot closer to true when simply preheating an empty pan. With a setting of 350F, all the shiny stainless pans heated to just a few degrees higher (about 353-357F) and held there. This is useful for judging the Leidenfrost Point (which is the heat at which you can oil your SS and have it cook relatively nonstick) and potentially for “seasoning” carbon steel, SS and aluminum, but not much else, since it doesn’t translate to actual food temperature. There’s also the issue of the temperature settings *starting* at 285F, so holding a lower temperature for, e.g., tempering chocolate or a sous vide bath, or even a simmer would be by-guess-by-golly just like any other hob—your only resort is lots of experience with lower *power* settings. With heat-tarnished copper, a 350F setting resulted in a wide swinging between 353F and 365F, which I attribute to the copper shedding heat far faster than the other constructions, once the circuit stops the power at temperature. Then, when the circuit cycles the power back on, the copper is so responsive that it quickly overshoots the setting. Aluminum, on the other hand, *undershot*, the 350F setting, registering a cycle of 332-340F. I conclude that the IR sensor is set for some particular emissivity, probably for that of stainless steel. If true, the Panasonic, even though it automatically switches frequencies, does not compensate for the different emissivities of copper and aluminum. And even if Panasonic added dedicated aluminum and copper IR sensors, there is enough difference between dirty and polished that the added cost would be wasted. Bottom line here: the temperature setting mode is of extremely low utility, and should not be trusted. B. Power Mode – Pan Material Comparisons Given the differences in power setting granularity and maximum power between the two frequencies, it is difficult to assess what X watts into the pot means in, say, a copper-versus-clad or –disk showdown. What is clear, however, is that Setting X under disk and clad seems “hotter” than the same setting under copper and aluminum. I will need to precisely calibrate the Panasonic for wattage anyway for the hyperconductivity project, so I will obtain a higher-powered watt meter to determine the wattage of every power setting for both frequencies. Until then, however, the only way I can fairly handicap a race is to apply a reduction figure to the ferromagnetic setting (2400W being 69% of 3500W). Given that we know the wattage at the maximum settings, we can infer that Setting 14 (actually 13.8) on the 20-step ferromagnetic range iis approximately the same heat output as the maximum setting (18) for copper/aluminum. The boil times for 4 liters of 50F water in 10” diameter pots shocked me. The 10” x 3mm tinned copper pot’s water reached 211F in 36:41. Not an especially fast time at 2400 watts. The 10” disk-based pressure cooker bottom? Well, it didn’t make it—it took an hour to get to 208F and then hung there. So that left me wondering if the Panasonic engineers simply decided that 2400 watts was enough for copper and aluminum. I have a theory why the copper pot boiled and the SS one didn’t under the same power, but getting into that’s for another time. C. Evenness Comparisons The wires which generate the induction field are wound in a circular pattern; when energized, they create a torus-shaped magnetic field. The wound coil is constructed with an empty hole at its center. As matters of physics, the magnetic field’s intensity drops off extremely fast as a function of the distance from the coil; a few millimeters above the Ceran, the field is so weak no meaningful heat will be generated. This means that most induction cooktops heat *only* the very bottom of pans, and in a distinct 2-dimensional “doughnut” shape. All of the above can result in a pan having a cooler central spot, a hotter ring directly over the coil, and a cooler periphery outside the coil. It is left to the cookware to try to even out these thermal discontinuities when cooking. Some materials and pan constructions are better at this than others: the successful constructions utilize more highly-conductive metals such as aluminum and copper, but unless the material is very thick, there can be a ring-shaped hotspot that can scorch food. Until the Panasonic arrived to market, hotspot comparisons between ferromagnetic and aluminum/copper pans depended largely on comparing induction’s flat, more discrete heat ring with gas’s more diffuse, 3-dimensional one. Dodgeball-style debate ensued, with few clear conclusions. But now, for the first time, equally-powered flat heat rings in two different frequencies allow us to directly compare evenness in ferromagnetic and aluminum/copper cookware. The simplest and easiest way to assess cookware evenness is the “scorchprint”, which does not require infrared or other advanced thermal imaging equipment. I’ve posted on how to conduct scorchprinting elsewhere, but basically a pan is evenly dusted with flour; heat is applied to the pan bottom. As the flour is toasted, any hotspots visually emerge, giving the viewer a useful general idea of evenness. I will later post the photos of scorchprints I made of 4 different pans run using the Panasonic KY-MK3500: (1) a Demeyere 28cm Proline 5* clad frypan; (2) a Fissler Original Profi disk-base 28cm frypan; a 6mm aluminum omelet pan; and (4) a 32cm x 3.2mm Dehillerin sauté. To make it a fair race, I heated all the pans at 2400W until they reached 450F, and then backed off the power setting to maintain 450F. I did this in order not to compromise my saute’s tin lining. As you will see, both the clad Demeyere and the disk-based Fissler did print the typical brown doughnut, with a cooler center and periphery. By far the most even was the thick, all-aluminum pan, which actually was even over its entirety—even including the walls. The copper sauté was also quite even, although its larger size and mass really dissipated heat; once 450F was dialed in, no more browning happened, even after 30 minutes. I conclude that the straightgauge pans were far more effective at shunting heat to their peripheries and walls (and also to some extent into the air) than the clad and disk-based pans. The latter accumulated their heat with most of it staying in the center of the pans. Eventually, even the “doughnut hole” blended into the scorch ring because the walls were not bleeding sufficient heat away from the floor. This was especially pronounced in the Fissler, the high wall and rim areas of which never exceeded 125F. The aluminum pan, in contrast varied less than 30F everywhere on the pan. D. Other Considerations The Panasonic’s fan noise at the cook’s position was noticeable at 63 dBA, higher than with the VMP’s 57 dBA. These levels are characterized as “normal conversation” and “quiet street”, respectively. Interestingly, I found two other, potentially more important differences. First, the Panasonic’s fan stays on, even after the unit is powered off, whereas the VMP’s fan shuts off immediately when the hob is turned off. Second, the Panasonic’s fan steps down from the louder speed to a much quieter (47 dBA, characterized as “quiet home”) level until the Ceran is cool to sustained touch, at which point it shuts off completely. I think the Panasonic’s ability to continue to vent and cool itself is a great feature, especially since a cook could leave a large, full, hot pan on the glass. The glowing circle is useless for gauging heat setting or intensity. And while it works to indicate a hot surface, it remains lit long after you can hold your hand in place dead center. VI. Summary and Lessons The Panasonic KY-MK3500 is a solid unit, well-conceived and rugged. It is extremely easy to use. It works well with both the common 24kHz frequency used with ferromagnetic cookware, and the 90kHz frequency chosen here for copper and aluminum. It effectively and automatically switches between the two. In my opinion, it points the way to expanding the worldwide induction appliance market to include dual frequencies. It also obviates the need to: (a) junk otherwise excellent cookware merely to have induction; and (b) retrofit designs to bond on ferromagnetic outer layers. In fact, in my opinion, my tests indicate that, in a dual-frequency world, adding ferromagnetic bottoms may well be a drag on pans’ performance. I also consider the Panasonic Met-All to be ground-breaking in what it can tell us about *pans*, because all metallic pans are now commensurable on induction. Clearly (to me anyway), watt-for-watt, the copper and aluminum pans performed better than did the clad and disk-based pans on this unit. Boil times were faster, there was less propensity to scorch, and the conductive-sidewall pans definitely added more heat to the pans’ contents. We may ultimately find that 90kHz fields save energy compared to 24kHz fields, much as copper and aluminum require less heat on gas and electric coil. In terms of heat transfer, the copper and aluminum pans came close to emulating the same pans on gas. And at 2400W/3500W it has the power of a full size appliance in a relatively small tabletop package. The Panasonic is far from perfect, however. It can’t really be considered portable. There are far too few temperature settings, and what few it has are not accurate or consistent in terms of judging pan contents and attaining the same temperature in different pans (and even the same pan unless clean). The luminous ring could easily have been made a useful indicator of intensity, but wasn’t. And it lacks things that should be obvious, including a through-the-glass “button” contact thermocouple, more power granularity, an analog-style control knob, and capacity to accept an external thermocouple probe for PID control. Most importantly for me, the Panasonic KY-MK3500 portends more good things to come. Retail price remains $1,700-$2,400, but I jumped on it at $611, and I’ve seen it elsewhere for as low as $1,200. The manual can be found here: ftp://ftp.panasonic.com/commercialfoo... Photo Credit: Panasonic Corporation

Meyer Corporation.

You are correct that welding clad is quite technical. However, it is not layer separation that is the issue. Weld overpenetration has caused corrosion beneath the weld, and that corrosion can cause catastrophic failure of the welds--obviously a safety issue. As is under-penetration. When the layer being welded to is <0.5mm thick, the process needs to be very precisely controlled, probably something only industrial robotics can do dependably. There have been cookware recalls because of this problem set, including a very costly one for the world's largest producer. The owner of that company has vowed never to offer welded handles again.

Maybe it's like the Searz-All blog that was orphaned?

I'll give MAAS a try, thanks. I noticed from your link that MAAS has a separate product to prolong the polish. Have you also tried that? My experience with many copper polishes is that the Flitz polish alone achieves such a microfine polish, it makes the metal very resistant to tarnish. Not of much use for working pans, but the rarely-used pieces seem to stay bright a long time.

You favor the MAAS metal polish over Flitz? If it's better, I'll buy some.

This is really just a broken French omelet. In fact, if you use the "jerk" technique, you can fill them just like an unbroken omelet, too. IMO, the low-and-stir prep can be very, very good,; it's just different. I'm totally ecumenical when it comes to scrambled eggs. Heck, I sometimes even like to cook them by breaking the eggs straight into the hot pan and stirring/scraping minimally, so they stay a little streaky. Anyone here do eggs scrambled in their shells?

Have you tried TJ's Delice de Bourgogne? Surprisingly good for mass-produced and containerized. Now we need to convince them to do a Brillat Savarin! Let us know how the cheese keepers work out for you.

Only glass and Cambros for me...

Or a weed grow operation.

Looked promising until I checked the dimensions. This thing is 14" wide and my wrap drawer is only 13.25". We have a countertop moratorium, so no go. Otherwise I woulda bit...

Even at that price, it's still $500 too expensive

Ah, I was remembering d5, which Franz scored a relatively poor 109.7 on gas. The d7 did much better at 82.3, but still less even than the Proline. On electric, it was much closer. There's slightly more than 2mm of aluminum in d7, whereas Proline contains approximately 4mm.

The hefty part I get. The heat-distribution part not so much. My impression from the truly thoughtful reviews (e.g., Centurylife.org) is that the good 'ol All-Clad triply is the best clad line they offer in terms of moving heat. The original selling point of d7 was not that it was even, but that it retained heat, e.g., "Holds heat like cast iron, only 30% lighter!" They claimed d5 was even, but the concept of interleaving thin alternating layers of poorly-conductive steel just isn't all that persuasive to me. I think Centurylife's testing and FLIR imagery bears this out.

Blinis.

Let's try it a different way, then. I would not buy a 700 or 900W unit if you were hoping to SV in a 30Q cooler.

Well, up to a point, yes. However, as the volume of bathwater and thermal mass of the food contents go up, a lower powered unit would have to work longer and harder to keep up. This is especially so when using uninsulated containers. I was in a retail store recently that was selling an immersion "stick" circulator that was unknown to me. What struck me was that its instructions warned not to use the unit in a container holding more than X of water, and X wasn't a lot.

Ah, you've given up the secret! The BB&B cashwrap clerks HATE the fact that they have to tell customer after customer that their coupon doesn't apply to X. Some hate it so much they will give you a discount anyway in simple acts of retribution against their corporate overlords. To maximize the chances of success, I have learned to respond to the bad news with something like: "Oh, that's OK. I bet you get so tired of having to tell people that. I made a special trip here to use this coupon. Isn't there something you or your manager can do to help me out?'" This has worked twice for me with replacing Breville "Smart" Ovens that CATO-d. I think one time the clerk upgraded me to the Pro without any upcharge.

Actually, IME they usually exclude Breville.

Yes, the interface disks work--after a fashion. Using them emulates sitting the pot on an AGA, solid top or placque. The loss in efficiency is an overblown disadvantage, IMO. The big problem is that the interface plates don't play well with induction hobs' electronics--you run a real risk of overheating and CATO-ing the appliance. And of course you're reduced to moving the pan in order to quickly reduce the heat, as with a solid top.

I'll ask, but my own guess is the power limitation at the 90kHz (non-ferromagnetic) frequency has to do with safety.