Are they cracking before set up or after? after half dry? I make jumbo balls ( orniments) and I dont have a pressure method...it might be the recipe your using...not enough this or that or too much moisture etc...also if its humid, that makes a difference. Mine use to not harden all the way if humdity was up ...it would take a long time or they would kinda bend or move then crack. But as practice has taken over and a thousand baths for everyone who knows me....no crackin and nice hard balls:D