Why polymer clay doesn’t harden when baked?

Polymer clay melting clockWhy your polymer clay creation is still soft and pliable after baking and what you can do to fix it.

If you’ve ever spent hours lovingly making a beautiful polyclay project only to be disappointed when it didn’t harden in the oven, this is for you.

My first encounter with this problem was with a wall for a cottage I was making. I carefully slid the delicately carved stonework into the oven, turned up the temperature, set an alarm on Alexa and went off for a cuppa.

At the appointed time, I lifted the baking tray out of the hot oven, burning myself in the process (it was one of those days!) and let the clay cool before examining it. You could almost feel my disappointment as it drooped when I lifted it and was still soft to the touch. Anything but the sturdy, stone, cottage wall I wanted!

how to prevent polymer clay being soft after baking

My polymer clay wall, pre-back. It shouldn’t do this when baked.

Why clay doesn’t harden

The most likely reason for my polymer clay not hardening was the oven being too cool.

Although I’d set the oven dial to the right temperature, I subsequently found that what the knobs are set to and the actual temperature on the shelves vary wildly.

I also found that locations in ovens also had a big impact. I knew things on the top shelf of the oven would get hotter than on lower shelves but it turns out that even moving them around on the same shelf can result in differences.

Now admittedly our oven is an old range cooker that’s seen better days and that didn’t help but talking to friends, it seems the temperature setting on all ovens can be way off from what you think it’ll be.

Another cause of disappointing soft bakes is misjudging the time.

Most brands, recommend around 15 minutes and anything under that is likely to have problems. (If you haven’t twigged, that’s the reason for the polymer-clay-Dali-style clock above).

Finally, the thickness of clay creation can result in unwanted flexibility.  Expect long or wide areas of thin clay — around 2mm — to bend, it’s normal.

In my case, although I’ve made walls for other houses that have been this thin, the project I was working on at the time had much thicker walls and shouldn’t have had this problem.

To recap, before we get to solutions, the chief reasons why polymer clay isn’t hard after baking are:

  • The oven temperature set to low
  • The oven not being hot enough when you put the clay in
  • The oven’s temperature dial isn’t correct
  • It’s not baked for long enough
  • Handling it too soon after baking, let it cool first!
  • The piece is too thin (such as when working with slices) or needs longer in the oven given its thickness
Polymer clay after baking

The polymer clay project I was working on, when baked correctly.

How to make your clay harden

If you’ve already got a poly clay sculpture that’s been baked but hasn’t hardened, there’s an easy solution.

Bake it again.

As long as you don’t go over the recommended temperature guides — typically 110 to 135 celsius or 230 – 275 ° F in old money and for longer (see below) — it won’t burn but should firm up on the second bake.

If you haven’t baked it yet, the answer is even simpler.

Get a temperature gauge for your oven — I use this one — so you can make sure it’s hot enough to cook your clay perfectly. I also keep my projects in the oven a little longer, usually 5 to 15 minutes more than what’s recommended depending on how big and in particular thick the model creation!

And, before you start baking, don’t forget to preheat the oven! It takes some time for the oven to reach the right temperature — if you start baking when it’s still cold, your clay won’t get enough heat. So, whether you’re baking for the first or second time, let the oven warm up first.

> FYI: Different brands of polymer clay — Super Sculpey, Fimo etc — cure at different temperatures. And even ranges from the same manufacturer can vary. Always read the instructions on the packaging and never go below what they say.

Conclusion: how to harden polymer clay

If polymer clay isn’t baked at the right heat and for long enough, it will remain soft after baking. Preheat your oven before baking, get yourself an oven thermostat to confirm your oven temperature and then let your project cook for longer than the instructions, double the time is a good margin.

Once I’d got the temperature of my oven sorted, my cottage had solid unyielding walls – a veritable castle for its miniature inhabitants.


Extra: If you mix brands of polymer clay in a single piece, Fimo with Kato for example, use the highest temperature and longest cook time recommendation of the clays being combined.

Update: Updated 2023, with Bing AI created melting clock image.

Founder of ModelRailwayEngineer, Andy Leaning

Andy is a lifelong modeler, writer, and founder of modelrailwayengineer.com. He has been building model railways, dioramas, and miniatures for over 20 years. His passion for model making and railways began when he was a child, building his first layout at the age of seven.
Andy’s particular passion is making scenery and structures in 4mm scale, which he sells commercially. He is particularly interested in modelling the railways of South West England during the late Victorian/early Edwardian era, although he also enjoys making sci-fi and fantasy figures and dioramas. His website has won several awards, and he is a member of MERG (Model Railway Electronics Group) and the 009 Society.
When not making models, Andy lives in Surrey with his wife and teenage son. Other interests include history, science fiction, photography, and programming. Read more about Andy.

Afflliate disclosure:The links on this page may take you to carefully selected businesses, such as Hornby, Amazon, eBay and Scale Model Scenery, where you can purchase the product under affiliate programmes. This means I receive a small commission on any orders placed although the price you pay does not change. You can read my full affiliate policy here. I also sell my my own ready to use, pre-made and painted buildings and terrain features. browse the range.

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