How to tell if Polymer clay is cured? and where did I go wrong?

So I was firing some polymer clay charms in my toaster oven and I noticed that the translucent clay was brown/black! the translucent clay was ON the pan but another part of translucent clay that was not touching the pan was fine. Where did I go wrong? I had an oven thermometer and the temperature was always around 260-275 degrees (I use Primo and the instructions said 275 degrees) I took it out and noticed some of my thinner creations were really hard and the thicker ones felt kind of squishy/bouncy. How do you tell if its completely cured? and is it ok to refire it?

1 Answer

  • 8 years ago
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    Several questions there, and several things it's important to understand (and do) when baking polymer clay--and you may not have known them all.

    For example, you were using an oven thermometer (very good) but many ovens have hot spots (even worse with small ovens like toaster ovens) so it's important to have the thermometer exactly next to the clay (and the clay not too near the oven sides or the top or bottom coils).

    The thickness of the clay item can make a difference too (or any "projecting" areas), as well as total size of the clay item (i.e., miniatures take much less time).

    (Btw, I'm presuming you *preheated* well before putting the clay in the oven.)

    The brand and line of polymer clay makes a difference too.

    And the color Translucent in any brand/line will also darken more easily than other "colors" in that brand/line (as well as any brands/lines that contain a lot of translucent clay in them, which isn't always discernible).

    Another thing you probably had going on was the "baking surface" you used.

    Some materials just absorb and hold heat more than others (bare metal and ceramic, for example), so any clay touching them can get hotter than the heated air in the oven, and darken.

    The easiest baking surface to use (which will also avoid shiny spots that will appear on the clay when in contact with very smooth surfaces during baking) is just a sheet of plain white paper (on top of a baking pan or something else stiff and flat to move it in and out of the oven), but there are many other possibilities too.

    As for the flexibility and "hardness" of polymer clay, all brands and lines of polymer clay will soften in the heat of baking and even till they're completely cool, and be flexible if they're at all thin.

    Most brands/lines will still be a *bit* soft on the surface (compared to earthen clay that's been fired in a kiln anyway) since polymer clay is after all a plastic. They will also be flexible if they're not thick and rounded shapes, but very strong because of that.

    A few brands/lines will be harder on the surface and not flexible even if thin after baking and cooling, but they sacrifice strength for that and will be brittle and easily broken if flexed or stressed.

    You can't really tell when polymer clay has been thoroughly cured & all the way to the center, just by looking at it or feeling it (unless it's *very* undercured) which is why it's important to understand how to bake it and to use an oven thermometer.

    Since polymer clay is a thermosetting plastic, it polymerizes gradually with time and temperature being interdependent. It also will darken at too high a temp though (even for a few seconds, etc) so unless you don't care about the darkening, you'll have to do various things to control the temp at all times in all areas and also be aware of the brand/line you're using.

    It is okay to rebake polymer clay and in fact will only get stronger and stronger...the polymerization will just pick up where it left off previously once the interior of the clay becomes heated. (There used to be a concern from some of the dollmakers who used some brands/lines of polymer clay that their heads were crumbling/falling apart over time from rebaking, but that turned out to be other factors.)

    You can read ALL about baking polymer clay, baking surfaces to use, how to avoid darkening, and much more in detail on the Baking page of my polymer clay "encyclopedia" site though:

    And you may also be interested in the Characteristics of Clays page, especially under the category called "Strength--Flexibility, Rigidity":

    My answer to these previous questions have some of that info in summary, and were written later so have some more updated info since the formulations for the various brands/lines has changed a number of times:;_ylt=Ah3Tm...;_ylt=Appnn...

    As for making polymer clay "charms," you may also be interested in my answers to these previous questions:

    (And for any other info about using polymer clay, or making things with it, techniques, etc, check out the rest of my site from the Table of Contents page:

    ...scroll all the way down go to any of the pages listed for the lessons/info/etc, click on their names from inside the alphabetical navigation bar on the left


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