If you’ve spent time and effort making the perfect polymer clay miniature, the last thing you want is for it to still be soft once it’s cured. This has happened to me a couple of times but after a bit of experimenting I found out what the issue was.
My polymer clay wasn’t hardening was because the oven temperature was too low. Once I used a separate oven thermometer, I could accurately see the temperature of the oven. Another reason for polymer clay not to harden is that it hasn’t been baked for long enough.
Should polymer clay be hard when it comes out of the oven?
Polymer clay will be slightly soft immediately after it comes out of the oven, even if it has been baked at the correct temperature and for the correct amount of time. It will only harden fully once it’s cooled and therefore fully cured. Even then, it will still have a slight bend to it, unlike traditional kiln fired clay which is rock-hard after curing. Thin pieces of clay in particular will bend relatively easily, even when fully cured.
Most polymer clay models won’t be rock-hard when they come out of the oven, but this is no bad thing as it gives them flexibility to cope with knocks and bumps that would chip or smash kiln-fires clay pieces.
What makes polymer clay harden?
Baking polymer clay in the oven at the correct temperature is the only thing that will harden the clay, unlike air dry clay, polymer clay won’t harden at room temperature. One of the main ingredients of polymer clay is plastic and once this plastic reaches the correct temperature, it will blend together and harden. If you don’t bake your clay for long enough, or at the right temperature, the plastic particles won’t have had sufficient time or heat to harden properly, making the clay weak and susceptible to breaking and crumbling.
5 Things That Affect The Hardness Of Polymer Clay
1. How long the polymer clay is baked for
This is the most important thing that will affect how hard a cured piece of clay is. If polymer clay is not baked for long enough then it will easily break, crumble or snap. It’s actually not very easy to tell if a piece of polymer clay is under-baked or not, which is why it’s best to bake it for at least 15 minuets longer than the baking instructions on the packet. Most polymer clay miniatures will be baked in around 30 minutes.
Very large pieces of polymer clay can be baked for even longer, but for miniatures, an extra 15 minutes or so should be fine.
A word of warning though, baking certain colours of polymer clay (particularly lighter shades) for a longer time can cause them to darken. To avoid this, you can cover your polymer clay with tin foil to protect it from direct heat.
2. The oven temperature
The right oven temperature is a crucial factor for baking polymer clay correctly. If the temperature is too low, it won’t fully bake and could be too soft, crumble or break once it’s cured. If the oven temperature is too hot, it could burn your polymer clay models. For more information on how to bake polymer clay miniatures without burning them, read this blog post.
The only way to really know the correct temperature of your oven is to use an oven thermometer. These can cost as little as a few pounds and they sit on your oven shelf, helping you to spot any hotspots, fluctuations or inconsistencies in temperature in your oven. The front of our oven is about 10° hotter than the back of our oven for example. I found that turning the fan function off and placing the polymer clay in the middle of the oven works best for me.
The best way to bake polymer clay is to follow the instructions on the packet for the temperature and then I bake my models for about 15 minutes longer than the instructions recommend.
Most brands of polymer clay recommend baking at the following temperatures:
- 275F
- 135C
- Gas mark 1
Remember, you can’t burn polymer clay if you leave it in the oven for too long, it will only burn if it’s baked at too high a temperature.
3. Room temperature once cured
Can the temperature of a space after curing really affect the hardness of polymer clay? Yes, a fully baked and cured polymer clay model can still be flexible if it’s stored or displayed in warm conditions, such as conservatory or in a car. In cooler conditions, the polymer clay will feel much harder. This is because the main ingredient in polymer clay is plastic which can easily soften and harden depending on the room temperature.
4. The brand of polymer clay
The brand of polymer clay you use will affect how hard your finished creation is. The strongest polymer clays when cured are:
- Kato Polyclay
- Cernit
- Premo
- SuperSculpey Firm
I’ve found that Kato Polyclay is the hardest polymer clay once it’s cured. So if you’re looking for a clay for a project that will need to withstand wear and tear, go for Kato Polyclay.
Some brands of clay such as Sculpey III, original Super Sculpey, original Sculpey and non-branded polymer clays tend to be more brittle after being baked and cured. If you’re working on a miniature which you know will get a lot of wear and tear (such as pieces for a kids’ dollhouse) or which will have thin protruding pieces then avoid these brands.
5. Room or hand temperature when curing the clay
Although this affects the clay pre-baking, it’s worth mentioning that the heat in the room your working in and the heat from your hands can really impact on the hardness of raw polymer clay.
I have naturally very warm hands so I find some types of soft clay almost impossible to work with and prefer the harder raw clays such as Fimo Professional or Sculpey Premo. I tested out a range of polymer clays when I started modelling miniatures to find out which was the best option for me and these two came out on top. You can read the full blog post here The Best Polymer Clay For Making Miniature Models.
Conclusion
In a nutshell, if your polymer clay models aren’t hardening you should:
- Check the oven temperature is correct by using an oven thermometer.
- Bake your piece at least 15 minutes longer than recommended on the packet instructions.
- Choose a harder brand of clay such as Kato Polyclay, Cernit, Premo or SuperSculpey Firm.
Further Questions
Can you bake polymer clay twice?
Yes, you can re-bake a piece of polymer clay if it’s not hardened after the first bake cycle, in fact you can bake polymer clay as many times as you like. This won’t burn the clay as you can only burn a piece of clay if the oven temperature is too high.
Can you over bake polymer clay?
No, in fact, baking polymer clay for longer actually makes it stronger. The longer polymer clay is baked for, the more time the plastics in the clay have to polymerise (which basically means fuse together).
How to tell when polymer clay is done baking?
You can’t tell by looking at polymer clay if it’s done baking or not. It will look exactly the same as when you put it in the oven, except some brands may have a slightly more matt finish. The only way to know that your polymer clay is done baking is to use an oven thermometer and bake the clay at the correct temperature for at least 15 minutes longer than the recommended baking time on the polymer clay packet. For miniatures this is likely to be about 30 minutes at 275F, 135C or Gas mark 1.