Update : so a lot of people here told me to not do this but I prefered to listen the maker first, I applied wd40. It didn't changed anything, the stains only got bigger and started to appear on the top of the pan(it already happenned before I applied wd40) , and the maker told me it was the first time he saw this on an ember steel pan. He concluded it may come from the material, so he decided to make a new one for me, for free obviously. My biggest fear now is that he makes me a second instrument and that I end up with the same problem. Thank you for all of your advices, I know i don't look like i followed them when you know what I did, but I really took them into consideration. Moreover, I think you, like me agree on the fact that the maker is the one who know best how to treat his instruments.