Paper clay is an ordinary clay body with cellulose fibre mixed through it, and the fibre changes what the clay will let you do while it is wet without changing much about what it becomes when fired. For constructed and sculptural work it removes the two most common failures: cracking at joins and being unable to repair anything once it has dried.
What the fibre actually does
Cellulose fibres bridge across the clay like reinforcement, giving the wet body far more tensile strength than the same clay without them. That means thin sections hold, large flat slabs resist warping, and a join between two pieces has fibre running through it rather than relying on slip alone. The clay also takes and gives up water more readily, which is why dry-to-wet joins become possible at all.
Joining dry to wet, which ordinary clay refuses
With a normal body, attaching a wet piece to a dry one guarantees a crack, because the two shrink by different amounts at different times. Paper clay tolerates it: wet the dry piece, use paper clay slip, and the fibre carries the join. This single property is why sculptors and installation makers buy it, and it changes how a piece can be built, in stages rather than in one session.
What is left after the firing
The fibre burns out completely, leaving the clay body behind with slightly more porosity than the same body without it, because each fibre leaves a small void. On a sculptural piece that is irrelevant. On functional ware it means the fired body is a little more absorbent, so a glaze fit matters and the higher end of the firing range is usually the better choice.
Price, and the smell of an open bag
Paper clay bodies cost more than the plain equivalents and are sold in smaller bags, and CTM's list prices its paper clays in 5 kg and 10 kg units rather than the 12.5 kg standard, which is worth noticing before comparing a headline price. An opened bag left wet for weeks will smell, because the cellulose ferments; it is harmless and it goes away, and keeping the bag sealed and cool avoids it.
Questions people ask about paper clay
Is paper clay weaker after firing?
Slightly more porous, because the fibre burns out and leaves voids. The fired strength is close to the same body without fibre.
Can I make my own paper clay?
Yes, by blending soaked paper pulp into slip and drying it back to workable. Commercial bodies are more consistent, which matters if the results have to repeat.
Why does my paper clay smell?
The cellulose is fermenting in the wet bag. It is harmless, it disappears in the firing, and sealing the bag and keeping it cool prevents it.