Yesterday I cooked the apple bark with water and alum for about an hour. (Making a high concentrated Alum-Solution is not that simple, I have to find another way for doing this.)
And now I have about 200 ml of something that looks a bit like this colours you use to dye your eastereggs with. It has a layer of crystals at the bottom which I’m hoping to dissolve by careful heating.

Next thing I have to do is to precipitate the Yellow and thereby not to muck up the Kitchen.