This is a very decent and expensive wok bought in the early 90s by my parents and they say it is a perfect work. Yes we 've used it for more than 20 years with no complaints at all but when I fried an egg with it yesterday it develops a lot of black **** like this (see picture below), I could hardly clean it at all with all my efforts, and the wok is stuck with this black thing. This is the first time - We used to clean the wok very easily and painlessly.
1. Why does it happen?
2. How to clean out the black thing?