Slab building makes angular and constructed forms from flat sheets of clay. The two things that decide whether a slab piece survives are the evenness of the slabs and the quality of the joins, and both are settled before the piece looks like anything.
Rolling even slabs
Two battens of equal thickness either side of the clay with a rolling pin run across them, or a slab roller if the volume justifies one. Evenness is not cosmetic: a slab that is thick in one place and thin in another dries at two speeds and tears where they meet. Rolling in several directions rather than one also reduces the tendency of a slab to warp as it dries.
Joining
Score both faces with a serrated kidney, wet them with slip made from the same clay, press until slip squeezes out, and clean the seam. A coil of soft clay pressed into the inside corner and smoothed reinforces the join and, more importantly, removes the sharp internal angle where stress concentrates. Almost every cracked slab box cracked at an unreinforced internal corner.
Assembling at the right stiffness
Soft leather hard: firm enough for a wall to stand without slumping, soft enough that the joins will bond. Slabs assembled too soft sag, and slabs assembled too dry will not join properly and come apart in the drying. Cutting all the parts and letting them stiffen together on a board is how most people manage the timing.
Drying a slab piece
Slowly and under cover, because the flat faces dry much faster than the joined corners. A box dried in open air warps and splits at the seams; the same box under loose plastic for the first day or two dries evenly. This is the stage where the work is most often lost, well after the making feels finished.
Questions people ask about slab building pottery
Why do my slab boxes crack at the corners?
Usually an unreinforced internal angle where stress concentrates. A soft coil pressed into the inside corner and smoothed prevents it.
How stiff should slabs be when I join them?
Soft leather hard: firm enough to stand up, soft enough to bond properly.
Why does slab work warp?
Uneven thickness and uneven drying. Roll in several directions and dry the piece slowly under loose plastic.