How to Harden Polymer Clay: Do´s, Dont´s and Troubleshooting

Polymer Clay is an amazing alternative to regular Air-Dry clay. It is easy to work with and you don´t need a kiln or high temperatures to dry it. Polymer clay also doesn´t tend to crack as easily when it is being cured. In this article, you will learn how to properly harden Polymer clay, the do´s and dont´s and what to do if something went wrong.

So how to harden Polymer Clay? Polymer Clay can only be hardened by cooking it in the oven. Depending on the kind and brand of Polymer Clay the temperature at which the clay has to be baked can range from 210° F to 300° F for up to 30 minutes.

The more in-depth answer is obviously way longer than the answer above.

Let’s start with the basics of polymer clay. Polymer clay is a plastic-based clay. It is basically soft PVC-plastic and it can only be cured by heating it in an oven at the right temperature.

Polymer Clay will not dry out or get hard by leaving it out in the open but when you are storing it then you should keep it away from heat sources and direct sunlight, or your clay will partially cure and therefore become unuseable.

Once you are done with modeling your sculpture you should cure your polymer clay but this is where you can actually do quite some things wrong.

So let´s take a look at the individual steps and how you should cure your Polymer Clay.

How to Harden Polymer Clay Step by Step

The time and the temperature for baking polymer clay are different for most major brands. So I would highly suggest reading the instructions before you bake your clay in your oven.

Step 1: Prepare Your Oven Tray

You can either use baking paper to put your sculpture on or you can use a ceramic baking dish, or something similar.

You should not put your sculpture directly on the blank metal of the tray because your model could end up being burned on that side or it could get stuck to the metal of the tray.

Depending on the size of the model and the complexity you should consider building a “tend” for your sculpture. This “tend” is basically a piece of baking paper that you fold and put over your project.

It should not touch your model and it does not have to completely surround your model on every side. 3 sites are completely enough.

This “tend” will help to keep the heat around the model and so the model will not burn on one side and it will help your model to cure a little better especially when it has some thin parts on it.

Step 2: Heat the Oven to the Right Temperatur

To find the exact right temperature you should read the instructions of the package of your polymer clay.

Here is a table, that shows the temperatures of the most popular polymer clay brands.

Sculpey has several different clays, that have pretty much the same temperature at which to bake them.

Polymer BrandTemperature
FIMO230° F / 110° C
Premo!275° F / 135° C
Sculpey III 275° F / 135° C

Preheat your oven to the right temperature of your clay.

Once the temperature is right simply put the polymer clay in the oven.

Step 3: How Long to Bake the Polymer Clay?

This step is a little harder to give clear instructions on. Generally, the time it takes to bake your clay will change depending on some things.

The bigger your clay model is, the longer it will take to bake and depending on the form and thickness of your project, the baking time can vary as well.

It is also important to remember, that the baking time varies depending on the type and brand of polymer clay, that you are using. So make sure to read the instructions of your clay.

To calculate the right baking time simply take the average baking time of your clay from the instructions. This time will be quite accurate if your sculpture is not thicker than 1/4 inch (6 mm), which is considered the standard sculpture size.

If your sculpture is bigger than 1/4 inch (6mm) then simply divide the thickness of your sculpture by 1/4 inch (6mm) and then multiply the result by the standard baking time that is written in the instruction.

For example, I have a sculpture, that is 1 inch thick at its thickest part.

Not I divide the 1-inch thickness by 1/4 inch which equals 4.

Not I look at the standard baking time of my clay, which is in my case Sculpey III with a baking time of 15 Minutes.

Now I multiply the 4 with the 15 Minutes which equals a baking time of one hour for my sculpture.

Now a one-inch sculpture is really big and I would not recommend making such a big sculpture as a very long baking time could cause the clay to burn on the surface. But you get the idea of how to calculate the baking time of your project.

Step 4: Letting it Cool Down

Important to note is, that polymer clay is not hard right away when you take it out of the oven.

The clay is still soft and it needs to cool down to become hard and fully cured.

So take the sculpture out of the oven and put it somewhere, where it can cool down without being disturbed in any way.

It takes a few hours depending on the size of your sculpture before it is completely cooled down and fully hardened.

Once your sculpture is cooled down completely it is done and ready to be primed, painted or just used as it is.

Why is My Polymer Clay Brittle After Baking?

There are two main reasons for brittle polymer clay. If you bake your polymer clay at a wrong temperature or not long enough, then your Polymer clay could end up being brittle. Also, keep in mind, that polymer clay is only truly cured and hardened when it is cooled down completely after taking it out of the oven.

So the main reason for brittle polymer is when the clay is not cured properly. So before you put your sculpture in the oven make sure, that the oven is preheated to the right temperature. Temperatures can be different depending on the brand and kind of polymer clay, that you are using.

So check the instructions of your Polymer clay and heat the oven to the right temperature.

The second thing, that you should calculate properly is the right baking time. Depending on the thickness of your Sculpture the actual baking time can vary quite a lot.

You can see how to calculate the right baking time above in the article under the third step “how long to bake polymer clay”

The final important thing is, that polymer clay will only harden once it is cooled down. So after you take it out of the oven leave it somewhere undisturbed for a few hours until it is completely cooled down. Then you check how hard it is. And if it is still brittle, then you might have miscalculated the baking time or used the wrong temperature.

Also, not every Polymer clay brands will cure hard, there are some brands, that only cure medium-hard or even slightly brittle. So have a look at the instructions of your clay to see what kind of polymer clay you have.

Can You Bake Polymer Clay With Metal in it?

The short answer is yes, you can. If you have an armature or wires inside your polymer clay, then you can simply bake it in the oven without any problems. The only thing, that you should keep in mind is that polymer clay with metal in it will take a little longer until it is cooled down completely after baking.

So generally, metal will not hurt your polymer clay in any way. For most more complex sculptures you will actually need to use metal armatures.

Simply use the same temperature and baking time, that you would usually use.

You also don´t have to account for shrinkage as polymer clay will not shrink at all.

The only thing, that is a little different from metal inside Polymer clay is the time it takes until the clay is completely cooed down after you take it out of the oven. Because of the metal inside the heat is kept in the sculpture for way longer than usual. So let it cool down properly before you use the sculpture after baking.

Can You Bake Polymer Clay With Aluminum Foil?

The short answer is yes, you can bake polymer clay with aluminum foil. Aluminum foil will not interfere with your polymer clay at all. Just make sure, that you are careful when taking the foil out of the oven because it can be quite hot.

Why Does My Polymer Clay Crack After Baking?

If your polymer clay cracks after baking then you either used water on your clay, which puts a lot of stress on the clay in the oven which in turn will result in cracks or because your polymer clay was mixed with another kind of polymer clay, which could result in an unstable mixture and result in cracks as well.