First and foremost, a large platter requires good throwing and trimming. It must be evenly thick throughout, and well-compressed as it is thrown. A thin area will crack, and a thick area will crack, because these cause uneven drying and shrinking, which the clay cannot tolerate. This factor requires experience and good technique, which every potter possesses in different amounts. But here are four factors that everyone can control right now:
1. Dry the platter on a melamine board. Melamine has a non-absorbent hard plastic surface, with a slightly grainy texture, that does not bond with leather-hard clay, therefore the platter is free to shrink as it dries. The other choices in my studio are wood and drywall, which are useful for other pots but not for large platters. A wood board warps when it becomes damp. A drywall board doesn't warp, but its paper surface becomes spongy when wet. The platters I dried on drywall cracked 100% of the time. That's right, I said 100%. My (unsubstantiated) explanation is that the platters were stuck to the spongy paper surface, they couldn't shrink and therefore the floors split open.
2. Dry the platter on its rim. All pots dry from top to bottom, and a rim dries much faster than a floor because its surfaces are more exposed. So drying a platter upside-down evens out the drying process. When dried right-side-up, the rim will harden and shrink much faster than the floor. When the floor tries to shrink later, the hardened rim does not allow it, and the floor splits open.
3. When the platter is upside-down, support the floor. Before I flip a platter over for trimming, I stack a combination of studio sponges, upholstery foam, and 1/4 inch thick scrubber pads, in the middle of the platter flush with the height of the rim. So when the platter is upside-down, the floor is supported from underneath. Without the support, sometimes the floor of the platter would sink downward while trimming, and these platters were guaranteed to crack later. My (unsubstantiated) explanation is that the flexing caused micro-cracks in the leather-hard clay, which grew into visible cracks later. I leave the supports under the platter for its first few days of drying. As soon as the platter is stiff enough to be picked up, I remove the supports so there is nothing to prevent the platter from shrinking.
4. Cover the drying platter with fabric. My basement studio is very drafty, and uneven drying is a constant battle. But a sheet of fabric is all it takes to slow down and even out the drying process for a platter. Any lightweight fabric works, like an old t-shirt or bedsheet. A sheet of plastic also works, but it slows down the process too much. Using fabric, the platter is ready to fire in 8-9 days. Using plastic, the platter takes 3-4 weeks to dry, unacceptable for a wholesale schedule.
All of these techniques can be summed up thusly: even out the drying process, allow the pot to shrink, and don't allow the pot to flex. I am happy to report that after incorporating all four of these factors in my platter-making process, I have not lost a single platter!