Getting those bubbles
My best bet has been to use a mix of baking powder and borax rather than the bubble powder. I am careful to not sprinkle too much on the piece and am careful to not have any large piles anywhere which will result in huge bubbles. It’s all in the experimentation–hard to be exact in my description of how much to use. My tries w/ enamel powder were never satisfactory, it stays granular and doesn’t melt. I intend to try fine frit next. Good luck w/ your experimentation.