You should add limestone or fossils that you could ake with plants and wood.
Well, I can layer, just when, for instance i layer say 2 pixels of TNT, if I put too many together (I haven't found the exact number) then it layers more. I made a small bomb with 2 pixels TNT and one Firw and in some parts of it it was layered the 3 pixels i chose, in some four or five. A part of it was six as well. It is very unpredictable.