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Does your cast iron have an enamel or coating of some sort? These coatings can melt or crack above 400 F. If your skillet is simply iron, than it should be safe to go above 400 F.
Nancy is a trusted home cook.
Agree with Sam Corber.
And, follow directions of manufacturer. And/or contact them.
I'd guess the directions are for seasoning the pan, yes?
Last, FYI, Lodge reportedly says its pans can stand up to 500F. see link:
I had heard tales of cast iron cracking at very high temperatures if there were any flaws in the casting. However, I have a small cast iron barbecue from Lodge that handles direct contact with hardwood charcoal at hellacious temperatures with no problem- I don't think you have much to worry about.