Why is my polymer clay still soft or brittle after baking?

Polymer clay cures by a chemical change through its full thickness, not by drying, and the two things that stop it happening are an oven that is cooler than the dial claims and a cure that was cut short. Both are entirely recoverable, which is the good news that most people do not know.

Safety

Step by step

1. Let it cool completely before you judge it.Polymer clay is genuinely soft and bendy while hot and reaches full hardness as it cools. A very large share of reported failures are pieces tested straight out of the oven.
2. Do the flex test on a thin section.A correctly cured thin piece bends a little and springs back. An under-cured piece snaps cleanly with a dry crumbly edge and no flex at all. A thick piece should resist a firm fingernail.
3. Suspect the oven before you suspect the clay.Put an oven thermometer at shelf level and preheat. If the actual temperature is below the dial, the piece never reached cure temperature, which is by far the most common cause of a soft result.
4. Re-bake it.Put the piece back in at the correct verified temperature for the full time again. This works, it is normal practice, and repeated curing at the right temperature does not weaken polymer clay.
5. Increase the time for thickness, never the temperature.Cure time scales with the thickest section, because heat has to reach the middle. A thick piece may need substantially longer than the packet time, which is written for a thin one.
6. Distinguish brittle from under-cured.Brittle with a chalky crumbly break is under-cured. Brittle with a hard, glassy break and slight discolouration is over-cured or scorched. The first is fixable, the second is not.
7. Check for a thin cross-section.Very thin polymer clay is fragile even when perfectly cured. A two-millimetre earring or a thin projecting limb will break in normal use whatever you do — thicken it, or add a wire armature inside it.
8. Consider whether the clay itself was old or contaminated.Very old clay that has partly cured in the packet, or clay mixed with too much softener or with a non-compatible additive, can cure poorly. If a whole batch behaves oddly, test a piece of fresh clay from another packet.

Tips

• Cure a test piece of the same thickness alongside anything important. Breaking a test piece tells you whether the cure worked without destroying the thing you made.
• An armature of wire or foil inside a thin or projecting shape adds enormous strength and reduces the amount of clay that has to cure through.
• Some brands are naturally more flexible when cured than others. Flexibility in a thin piece is not the same as under-curing — check the crumbly break, not the bend.

Common mistakes

• Turning the temperature up to force a cure — It scorches the surface, discolours the piece and releases irritant fumes, and it does nothing at all to speed the cure at the centre. Time is the variable; temperature is fixed by the packet.
• Judging a piece while it is warm — It is meant to be soft while hot. Testing it warm produces a false failure and leads people to re-bake at higher temperatures and ruin work that was already fine.
• Assuming under-cured clay is scrap — Re-baking works and is standard practice. Very little polymer clay is genuinely unrecoverable unless it has been scorched.
• Making very thin items and blaming the cure — A thin cross-section is mechanically weak regardless of cure. Thickening a piece by a millimetre or adding an internal armature changes the durability far more than any baking adjustment.

If it doesn't work

Questions people ask