plastic forming
Anyone who has shaped modelling clay knows the feel of a plastic body: soft enough to squeeze into a new shape, stiff enough to hold that shape when you let go. Plastic forming is the whole family of ceramic shaping methods that work this way — you take a damp, deformable clay-based body and mould it by hand or machine into the shape you want. It is the oldest ceramic technology there is, the potter's craft, and it still shapes an enormous fraction of the world's tableware, tiles, and clay building products.
What makes it possible is clay plasticity, and the mechanism is worth seeing clearly. Clay minerals like kaolinite are tiny flat plates. Add the right amount of water — usually somewhere around 15 to 25 weight percent — and thin films of water coat the plates. Those films let the plates slide past one another when you push (so the body deforms and flows), yet surface tension pulls the plates back together so the body sticks and holds its new shape (so it does not just fall apart). Too little water and the body is crumbly; too much and it slumps into mud. The plastic window in between is where throwing, jiggering, ram pressing, and roller forming all operate.
Plastic forming is cheap, fast, and forgiving, which is why it dominates traditional whiteware: plates and cups by jiggering, tiles and hollow shapes by pressing, artware by throwing on a wheel. Its limits are set by the water it needs — a wet plastic body must then be dried carefully, and it shrinks as the water leaves, so drying cracks and warping are the constant enemies of the method. It works beautifully for clay-bearing bodies but poorly for the non-plastic advanced ceramic powders (pure alumina, zirconia) that have no natural plasticity, which is why those are pressed or cast instead.
A ball of plastic porcelain body, at about 20 weight percent water, is thrown on a potter's wheel: the wet plates of clay slide as the potter pulls the wall up, then hold the new shape when the hands lift away. The pot is then dried slowly and fired — but it shrinks as the water leaves, so drying must be gentle to avoid cracks.
Damp clay bodies deform and hold their shape thanks to water films between clay plates — the basis of throwing, jiggering, and pressing.
Plastic forming needs natural plasticity, so it suits clay-bearing bodies but not the non-plastic advanced ceramic powders. And because it starts wet, drying shrinkage and drying cracks — handled in the drying field — are its built-in follow-on risk.