I recently bought a set of 3 cast iron skillets that had a "Made in China" sticker on them. After washing and using them a few times,the black finish started coming off on the towel I dried them with. The inside of the pans started to look brown where the finish had come off. Does this sound normal for cast iron? I have an old cast iron skillet that I picked up in a thrift store that is not doing this. Do you think it is safe to use the newer skillets?