Extrusion is a handbuilding technique in which a plastic clay body is forced through a shaped opening called a die, producing a length of clay with a uniform cross-section along its whole length, whatever shape the die itself is cut to. It is the standard way to produce coils, tiles, tubing, and pulled handle stock quickly and consistently, rather than shaping each piece by hand.
Wedged clay is loaded into an extruder, a barrel fitted with a die at one end, and forced through under pressure, either by a hand-cranked screw or lever mechanism or, in larger studio and production settings, a pneumatic ram driven by compressed air. The die's shape, a simple circle for a coil, a flat rectangle for a slab or tile blank, a hollow profile for tubing, determines the cross-section of the extruded length; changing dies changes what comes out without changing anything else about the process. The extruded length is cut to whatever size is needed once it clears the die.
A true hollow extrusion, a tube with an enclosed cavity rather than just a U- or C-shaped open channel, needs a two-part die: an outer die cut to the piece's outer profile, plus a separate inner die held in place just behind it on a small bridge or spider. Clay flows around the bridge and re-fuses on the far side of it, closing back into a single continuous wall; the gap between the outer and inner die sets the wall's thickness, so swapping just the inner die for one a different size changes wall thickness without touching the outer profile at all.
Extrusion produces coils and other stock far more uniform in thickness than rolling by hand, which matters for coil building at any real scale, and for handle stock that needs to match across many identical pieces. The clay body itself needs to be plastic and smooth enough to pass through the die without tearing or dragging, which rules out a heavily grogged or coarse body for fine dies with tight, detailed profiles. A pneumatic extruder can generate substantial force and needs to be operated with the same care as other studio equipment capable of injury, particularly around the die opening.