These planters are made with Standard Ceramic Supply's #306 brown stoneware clay. It's an awesome clay body that I've never had a single problem with. I've used it for cone 8 oxidation and cone 10 reduction, and it never warps or cracks, even on these large pieces.
My large planters vary in size from 25-45 pounds, made by assembling wet sections and pulling and shaping them. A 3 piece, 32 pounder takes about 20 minutes. I'll often add another ring to the top after letting it set up a bit over night. Using this method I never have to center more than 15 pounds at a time, which really cuts back on the wear and tear on my wrists. Plus a generally get larger, thinner pots than if I did them in bigger pieces. I'll post some pictures soon showing the process.