Also can anyone explain the name to me? Vermi- is usually to do with worms, but I thought vermiculite was mineral...Thanks!
Both absorb and hold water, but I don't think that they would both add softness to the clay. The vermiculite is much softer on Moh's scale of hardness than perlite (which starts out as volcanic glass). I would stick with the vermiculite.
I would also suggest Perlite for this situation. You should be able to get your hands on coarse Perlite at your local Gardening supply store, if not let me know and I'll see what I can do to change that. You should be able to also find a medium grade at your local building supply center as loose fill insulation. Not speaking from experience but I would assume you would want the finer particle of the medium grade Perlite, often referred to as STP (Silicone Treated Perlite). Good luck on your project and I hope this helps.
