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If we want to create a strong ceramic piece, we want to align and compress the clay particles. Again, consider the slate and Jell-O. Suppose we want to make a strong driveway out of the slate. We could have a truck dump the whole mess out in front of the house and kind of push it around to make it more or less level.
Clearly we'd be better off to run a steam roller over the pile to align and compress the mass of slate. With the plates aligned and interlocking we'll have a much stronger driveway.
This is exactly analogous to what happens when we roll clay into a slab. The resulting slab is much stronger than a similar thickness of clay cut from a block. When we roll a coil, the clay platelets align lengthwise along the coil. Throwing clay on the wheel, a potter uses his/her fingers or tools to compress and align the clay as (s)he pulls up a cylinder. Extruded clay is compressed by the edges of the die as it's pushed by. Even in casting the particles tend to align and compress as they deposit on the surface of the mold. Whenever we're working with clay we should look for ways to compress the clay to strengthen our forms. This is particularly true when making joints or seams.