Not a problem. They barely elevate the base and it will still be well within the induction fields.

FYI, those rings are often referred to variously as "casting rings," "fire rings," smoke rings" and "heat rings." They were an adaptation for back in the days of wood-burning and coal-fired stoves. Use those terms in a search string and you'll find out as much about old cast-iron cookware as you might care to know, and maybe more

Anyway, the rings should not make any difference to induction. One thing, though, is that the description you give sounds like you might have some rather old pans. You probably want to avoid plonking those down cold and immediately cranking the burner to max. That risks cracking the old pans. This is pretty much true of any kind of stove but particularly Induction because heats so much more quickly. Start with lower heat and ramp up to lead-meltingly-hot temps if that's where you want to go. Here's a link to a thread we had a few years ago:

http://ths.gardenweb.com/discussions/2298452/cracked-cast-iron-griddle-on-induction

Thank you sooo much for the info. I am still looking at all the different brands of induction cooktops, trying to decide which one to buy! We are making a trip to Tennessee and I wanted to go to the Lodge Outlet Store but wasn't sure if I needed to buy some new skillets or not. Thanks again.

The only thing I would be careful about is dragging them around on the glass. It could scratch.

One of the reasons why you're not supposed to use them is that the air between the pan and the glass can get very hot. This can lead to accidents and burns when it escapes, and it can on one of those it never happens but it really did type situations lead to cracking of the glass. If you're cooking a couple of eggs it shouldn't be a problem, but for tasks that are a lot longer and hotter there might be an issue. Oh. And in some cases the doohickey that's supposed to sense the pan might not register it and let the element turn on.

If you are swinging by the Lodge store, take a look at their rolled steel skillets. I personally prefer them over the cast iron. They are a little lighter and have less heat retention, which means that they adjust more quickly to changes in burner temperature. Of course, this would be a downside, if your burners are to wimpy. But I can't imagine that being a problem, if you are planning on getting a modern induction cook top.

And once you season the pans properly (they do common pre-seasoned), they are perfectly non-stick. Just the way you would expect it. Clean the same as with cast iron; i.e. no soap, and use kosher salt if you need to scrub. Always dry completely and coat with a little bit of vegetable oil before storing.

Also, please use dish soap. If a pan is actually seasoned, you'd need oven cleaner to remove the seasoning. If it comes off with soap, it's just gross goo, not seasoning.

I never use dish soap on my cast iron pans.

Both oil and water are great solvents. You can wash with water, but you'll need to add a detergent to have any effect on fatty residue.

Alternatively, you can wash with oil. This doesn't make sense for "normal" pots, but works well for pots where you want to leave thin film of clean oil to prevent rusting. Oil is actually really great for washing of dirt.

Here is what I do for my rolled steel cookware: I scrub with water and kosher salt. This removes the bulk of the residue and works primarily because salt is abrasive and absorbent. It's the same idea as scrubbing with sand (an additive in some dishwashing powders), but it has the advantage that it washes out perfectly without leaving gritty bits of sand.

I dry the pan with a paper towel, this a) removes (most of) the water, and b) almost all of the remaining cooking grease. I then add a little fresh vegetable oil, scrub, and then wipe off all but a thin film. The remaining film of oil is from this cleaning step and not from cooking.

Finally, I set the skillet on the burner and heat it up. This evaporates any water that might be remaining. And it'll over time also ensure that seasoning gets restored, if it was damaged (e.g. by cooking acidic foods).

Sounds more complicated than it is. Takes less than a minute to clean a skillet perfectly and to leave it with a very thin protective film.

I am sure there are other ways to treat your cookware. But this one works for me. It's fast and easy, it leaves my skillets very clean, it prevents rusting, and if I accidentally damaged the seasoning, it'll build up again.

The only comments I would add to the above are:

(a) The pan-present sensor is likely to be the induction coil itself being used to test the sufficiency of induction susceptible material above it. This is done by electronic means. It is likely that with this method that the minimum diameter pan that is acceptable on a given hob can vary depending on its base material. An actual magnetic pan sensor is not needed. The only sensor in each of the hobs of my failed Kenmore (Electrolux) cooktop is a thermal sensor to detect overheat conditions.

(b) Heat conduction back into the Ceram can be reduced by using silicone pads cut from cookie sheets to space off the pan from the glass.

kas

Sorry to hear that your Kenmore failed. Is that the one you bought back in 2007?

Yes. A power board evidently failed, taking a filter board with it. At present a Frigidaire (Electrolux) unit is replacing it. I intend to go through the Kenmore's electronics and see if the failure can be repaired (when I have time). Replacing the boards would have cost more than the new cooktop.

kas