Thanks for posting that, I made a few a couple of weeks ago and you've given me the solution to the pipe splitting.They are a very simple (but brilliant) design. They are basically a pipe of kraft paper, closed at 1 end. Filled with fine grain BP & then crushed flat (various methods can be used from a mangle to a hammer). This reduces the BP to half powder & half grains). The pipe(s) are then wrapped in a damp cloth & left to become damp & pliable. This is essential for the next step.
I've used BP and also alternating coloured flame mixtures with some success.
Adding some small robust stars could be the next fun experimnent.
Simon