Where you put it in?
Of course in the cauldron on the right side, beside the inventory shelves. Don't say you didn't find that one yet. If you didn't, you find it at the place with the crocodile, in the tree hole. Can't miss it
k:
Edit: I see you've found it. Well, if you try to put items in the cauldron that are critical to a plot, that's where Zantia says I don't want to lose that. That should tell you that you're putting a wrong thing in it.