Did you use any kind of chemical on it?
I had the same thick, really baked on crust on the big one......see to the left of the food.....
The kettle's at least 50 years old & I think it may not have been deep cleaned ever......as I was scraping, I thought I was actually peeling off the porcelain, that it was 'fused' to the crud.....
Here's what that looked like:
Same 'ridge'..................
This is what it ended out like :
If that's peeled off porcelain, it'll be a first for me....don't ever remember that happening....