There’s one more factor that comes into play, and that’s aging. All vinyl mixtures will begin to “age” as the PVC particles swell and absorb free plasticizer within the mixture. This happens regardless of the temperatures the clay has been exposed to. It’s a problem industrially and it’s a problem for us as well! Old clay is harder, much more resistant to flow, and much more likely to fracture when you apply stress quickly. (This is why old polymer clay is often very crumbly. It’s not cured, it’s aged.) Aged polymer clay has absorbed the plasticizer the same way your leftover spaghetti has absorbed all the sauce after a day in the fridge.
They’re All Different
While the age of clay or “curing in trucks” is often blamed for hard clay, that’s not actually what’s causing this wide range of textures. I’m sure that you’ve noticed that it can be quite a “crapshoot” trying to guess what a block of clay’s texture is going to be. Sometimes it will be super soft and sticky. Other times, it’s crumbs. It becomes even worse when you pick up a bar from a brand that’s new to you. It behaves totally differently!
Now that we’ve gone through this conditioning series, you know that the clay’s behavior has to do with the specific ingredients in a particular bar/brand of clay and the physical interactions that can happen inside the material, itself. Change the ingredients, the flow changes. That’s why adding clay softener (plasticizer) or oils (or removing them by leaching) changes the way the clay behaves.
At its simplest, all polymer clay needs to have PVC particles and plasticizer. It makes up a large part of the volume. The plasticizer does not “bake out” of the clay. In fact, it’s plasticizer that fuses the PVC particles with heat and allows them to form a solid mass that’s still flexible. Yes, the more plasticizer that’s in the clay, the more flexible it will be after baking. You can make any clay more flexible by adding more plasticizer to the mix. It’s also why you can end up with a brittle project if you leach away too much plasticizer in an attempt to make soft clay more firm.
Each brand has its own fillers that are used to turn the clay from a plastisol (like liquid clay) to a plastigel (like bar clay). Some brands use little, others use a lot. Brands with a lot of filler, such as Sculpey III, have a more “clay like” texture, bake more matte, and tend to be more brittle. That’s because the fillers “get in the way” of the PVC-plasticizer bond. I’m sure you can see how different ingredients would mean that each manufacturer has to have a full-time chemist on hand to tweak all the formulas!
The manufacturers guard the secrecy of their formulas, so it’s impossible to know the exact formulas of the clay brands. But you can tell certain things by carefully paying attention to the way different brands behave and how they change when you add stuff. Here are some things I’ve noticed.
- Softness (squishability), stickiness, and crumble-ness don’t seem to be at opposite ends of the spectrum. I have some clay that’s very soft, quite sticky, AND it crumbles to bits when I try to condition it. (Blame particles!!)
- Cernit gets stiffer when you apply stress to it. A sheet right out of the pasta machine is almost like board. But it softens as you let it sit on the work surface. But by the next day, it’s crumbly again. Weirdness!
- Unconditioned Fimo Professional will crumble when you apply rapid stress. Once it’s conditioned, the “fracturing” doesn’t happen. This happens to both Pardo and PVClay, too.
- Papa’s Clay and CosClay will fracture/shatter when you flex the bar. Use the pasta machine on these and they crumble. Yet they respond nicely to slow, firm hand pressure.
- Prism & Pro has a TON of plasticizer in it and so it’s very vinyl-like and flexible after baking.
- Premo translucent suffers from aging terribly. Cernit translucent (as currently formulated) does not. Fimo Soft also suffers from aging.
- Adding mica powders can sometimes make the clay super soft, flowing, and sticky. Yes, even though you’re adding a dry powder. Other times, the clay gets super stiff.
- Souffle’s viscosity is a trip. It canes SO weirdly! The microbeads in the Souffle (which cause it to be so light) change how it handles.
- All vinyl is stronger when baked hotter. All. (Lemme repeat that. All.) But most brands have ingredients in them that brown when baked hotter, so we keep the baking temp lower. Kato has more heat stabilizer in it which allows it to bake hotter and therefore it can be stronger. It’s also stiffer by nature.
Try adding things to your clay to change the viscosity. Add mica powders. Talc. Chalks. Pigments. Calcium carbonate. Add oils…see what happens. Try mixing your clay with various softeners or Sculpey Mold Maker. How does that change the viscosity? And try adding Ultralight. See how it affects the way the clay flows. Add CosClay or Papa’s Clay…how does that change the viscosity?
You see, there’s a lot more to a clay’s texture than the basic hard/soft parameters. And once you open your eyes to the way your clay flows, it changes the way you choose your brands and use your clay.
There is no bad brand of clay. There’s only clay that you haven’t yet learned how to use. Each clay has its quirks and just like with people, you will find some quirks that drive you nuts. But learning about the differences means you’ll experience many more creative options. Have fun!