I have seen my mother do this by putting raw duck eggs into salt water. The salt water is made with boiling water. After the salt water cools down, put the duck eggs in and take one out and boil it after a month. If you don't think it's salty enough, you can keep it for a while, but not too long. If you keep it for half a year, the duck eggs will definitely go bad. Also, you don't need to seal the lid when making this. Do you understand?