Probably carbon steel made from thinner sheets, there is a limit to how thin you can practically make a cast iron pan due to the brittle nature of the material and the casting process. The resulting properties with respect to seasoning are basically the same if the surface finish is consistent.
"The raw material in Ultra Light Original is exactly the same as for our traditional cast iron. The difference is that with the new technique, the pans are less porous. This explains why the pans responds much quicker to changes in the stove’s power effect, and why they heat up and cool down faster than traditional cast iron cookware."
It isn't near-feather-light like some pans I've picked up in the store for sure. It's just light enough to be a noticeable difference from other cast iron pans I've tried, like being much more comfortable to hold one-handed.
Interesting, I'm not sure why you would bother to do that vs. just stamping carbon steel sheet, the resulting product would seem, at least to my guess, be basically interchangeable. I wonder if there is some difference that matters here.
