Contrary to what you have probably heard repeated by others, the reason we need to condition polymer clay before use has nothing to do with strength. (If your polymer clay projects snap or crumble after baking, you have a baking problem.) Similarly, conditioning polymer clay has nothing to do with “activating long chains” or mixing ingredients that have separated. Polymer clay is not bread dough that needs to be activated to be strong. That’s just a myth. The purpose of conditioning polymer clay is to make it more workable. That’s it. But, as you’ve probably guessed by now, there is more to the subject. Here’s what you need to know about why we condition polymer clay, how to do it, and why you need to understand the brand of polymer clay that you’re using. Here we go!

Conditioning Polymer Clay – Why?

Polymer clay is a putty-like mixture of uncured vinyl plastisol mixed with clay-like fillers that allow us to model it into a shape and then cure it into a solid by baking it in the oven. This mixture consists of many particles of various sizes (PVC, fillers such as kaolin or talc, pigments) and liquids (plasticizer, lubricants, waxes, oils, stabilizers). Because this mixture is a suspension (actually a plastigel), the particles do not settle out. They stay mixed. But that doesn’t mean that particles don’t interact; they do! The physics of this is a confusing romp through the world of rheology (the study of flow), physics, and forces. I’m not going to go into it (it’s a deep rabbit hole). But the important nugget to know is that depending on the sizes of the particles of a mixture they can interact in surprising ways.

When polymer clay sits for a long time, it begins to gel up, and the particles start to sort of settle and nestle together. Kind of holding hands (hey, it’s physics). Therefore, the clay will be more stiff and unwilling to move quickly (kinda like me in the morning.) So we condition the polymer clay to disturb those hand-holding particles. This makes the polymer clay more elastic, smoother to work with, and more able to stick to itself. In other words, we condition our polymer clay to make it more workable.

Conditioning Myths

The world is full of myths, and the world of polymer clay is no different. This is exacerbated by people feeling pressured to create marketing content on YouTube or Instagram when they don’t yet understand the medium. Misunderstandings explode, and myths are born. Here are some big ones about conditioning polymer clay.

MYTH: Conditioning Polymer Clay Distributes Plasticizers

Our beloved clay is mixed at the factory, extruded and cut into bars, and packaged. It does not become unmixed as it sits on the shelf. This makes sense, of course, because we’ve never seen a bar where the color has settled out, or the liquid has risen to the top. And plasticizer? Well, plasticizer is a small molecule, and it readily migrates through the entire clay mass. It doesn’t become unmixed. It naturally distributes itself evenly (through the process of diffusion). And we even use this property when we leach our polymer clay by blotting away some of the plasticizer with paper.

MYTH: Conditioning Involves Mixing

We are, of course, conditioning our polymer clay by mixing colors. And we often condition our clay by repeatedly sheeting and folding with a pasta machine which will, of course, mix the clay. But that makes many people believe that mixing is a necessary part of the conditioning process. It is not. You can condition clay (make it workable) entirely by massaging it without ever mixing anything.

MYTH: Unconditioned Polymer Clay is Weak

When someone seeks help to understand why their polymer clay project broke after baking, people will usually ask if they conditioned the clay well enough. This is a huge myth. If you slice directly from a block of polymer clay and bake the slice, it will still be strong when baked properly. Conditioning is irrelevant when it comes to strength. Here’s a video from Hobbyrian showing this.

Be aware, however, that poorly conditioned clay won’t stick to itself well. So poor conditioning can be the cause when breakage happens where pieces were joined, as when you attach arms to the body of a bear. But conditioning is not to blame if the arm, itself, breaks.

MYTH: Conditioning Activates Long Chains

This myth sounds so sciency. We know that polymers have long chains of carbon molecules (that are polymerized), so it’s easy to think that conditioning is part of this process. No, not at all. Polymer clay is made from PVC (polyvinyl chloride), which is a polymer of vinyl chloride. The polymerization happened long before the polymer clay was made. Polymer clay does not polymerize. It’s already a polymer. There are no long chains to activate.

And while we’re at it, conditioning is not a chemical process. It isn’t like an epoxy putty where chemicals must mix. And it’s not like making bread where you must knead it to activate the gluten, allowing the bread to be stretchier. Conditioning polymer clay is about disrupting the inter-particle forces so that the clay mass is more workable. Nothing is activated.

Why is Polymer Clay Crumbly and Hard?

Nobody likes crumbly, hard bars of polymer clay. It’s frustrating, messy, and time-consuming to make them workable. But you know what? It’s perfectly normal, especially with some brands. It doesn’t mean anything’s wrong.

It’s easy to assume that a crumbly bar of clay was partially cured during improper storage in warehouses, transport in hot trucks, or display under store lights. Nope. All of these are myths that get repeated. (Before I get comments and mail, let me clarify. Yes, you can improperly store your clay. It can cure in your car or shed or black mailbox in Texas. But we’ve got to stop blaming hot days and UPS trucks. That’s just not happening.)

No, I’m sorry to say, crumbly polymer clay is not only common but normal. And it’s also easy to fix. But first, you need to know why it’s happening so that you don’t create more work for yourself.

Aging Polymer Clay

When you eat leftover pasta, it always seems like that the sauce disappeared in the fridge overnight, right? Of course, what’s happened is that the pasta continues to absorb liquid from the sauce. Polymer clay is the same way. When polymer clay is freshly made, it’s quite soft, sticky, and has a surplus of plasticizer. Over time, the PVC particles continue to absorb the plasticizer, making the mixture feel drier. All polymer clay does this to an extent, but some brands and colors (such as Premo translucent and Effect and the Fimo Soft line) tend to do it more readily than other brands.

The solution to conditioning aged polymer clay is the same as when you fix your day-old spaghetti. Add more sauce. And in the case of polymer clay, the “sauce” or “juice” is plasticizer. This is sold as polymer clay softener. Two well-known brands are Cernit Magic Mix and Sculpey Clay Softener. You can also add small amounts of oil (though large amounts will weaken the clay, just as adding too much water to pasta leftovers would make them flavorless). Liquid clay works, too. Liquid clay is just “clay sauce.” (Fun fact: Kato Liquid Polyclay was first known as “Kato Sauce.”)

While all clay brands can age, this tends to be more common with Premo and Kato Polyclay.

Fracturing and Shattering Polymer Clay Brands

Remember how I said that the particles in polymer clay sort of settle and get locked together when it’s left undisturbed for a while? It becomes more “gelled” and less able to flex without breaking apart. You’ve probably experienced this, yourself. If you take a slice of clay from a block of Fimo Professional and put it through your pasta machine, it will shatter or fracture into a million crumbs. This is because the sudden, intense force of the rollers makes the clay move too quickly, and it breaks apart. Take that same slice in your hands and apply firm pressure, slowly, and the same clay will soften and become workable without breaking apart. (If you’re new to the various brands of polymer clay, read about them here.)

It’s very easy to assume that all crumbly clay is dry and needs to have clay softener added. But some clay brands are prone to this type of “fracturing” and “shattering,” even when they’re perfectly “moist.” These clay brands tend to fracture or shatter when they’re moved too quickly.

• Fimo Professional
• Cernit Number One (when aged)
• Cernit Pearl (when aged a bit)
• Pardo Professional Art Clay
• CosClay
• Papa’s Clay
• PVClay

To condition these brands, they need to be disturbed and “moved” slowly, or they will shatter and crumble. Pasta machine conditioning for these brands takes patience because there will be many crumbs before the clay conditions enough to start to stick together. But soon, the mass will begin to hold together, and soon you’ll have a nice, smooth, workable sheet.

Be Careful with Adding Softener

Don’t add softener to these brands until you’ve worked the clay enough to determine if it’s just breaking apart due to the brand’s tendency or if it’s truly aged. If your bar is fresh from an online supplier, it is most likely NOT aged and does not need to have softener added. Adding softener when it’s not needed will result in a sticky clay that’s difficult to work with.

Polymer Clay Conditioning Tips

I’m not going to re-invent the wheel here and tell you how to condition polymer clay. There is plenty of information out there about it. Here’s how to condition super stiff, fracturing brands like Pardo Art Clay. You can use a pasta machine, a roller, your hands, or a Never Knead (modified arbor press). But here are some tips that might help you.

Can the Clay Be Saved?

If you have an old bar of clay, here’s a quick trick to see if it can be worked enough to be saved. Take some of the crumbles and press them together with your fingers. Can you make a ball? If you can press the pieces and mash them enough to make them stick together, then the clay is not “partially cured,” and it can be saved.

Try Heating It

Just like an aging body, things move better when they’re warm. Try warming your clay with body heat to make it easier to work. Pockets, bras, or even just sitting on a block can make things much easier. Some people use a heating pad or a pet bed to slightly warm their clay to make it easier to work. If you’re working in a cold climate, this is probably one of the first things to try. But don’t make it hot, or you might cure it. Bath temperature is fine.

Let Aged Clay Marinate

If your clay is crumbly because it has aged, adding clay softener is the solution. Solid Clay softeners such as Fimo Quick Mix, Cernit Soft Mix, and Sculpey Clay Softener can be mixed into the clay. A little goes a long way. Keep in mind that all those crumbles of hard clay will need to be completely mixed with the softener to make a uniform mixture. This might take a LONG time.

Another strategy is to mix your crumbled clay with some liquid clay softener in a bag. Set it aside for a week or so and let time do your work for you. The plasticizer will soak in and soften the hard crumbs.

If you use a pasta machine, be aware that the sheet will likely break apart and delaminate as you work it. If this happens, mutter under your breath (it helps) and keep going until the sheet is uniform.

Beat It Up

If the clay is one of the brands that fractures, it will not need to have a softener added (well, not usually anyway). But it will need to be beaten into submission. Put the crumbles into a bag and then beat the bag with a hammer. This will help “disturb” the clay mass until it starts sticking together. Soon the crumbles will hold together, and you can finish conditioning by hand or with a roller or pasta machine.

You could also use a clay-dedicated food processor for this. They’re often super cheap at thrift stores. Make sure to clean out the bowl when you’re done, however, as the clay will degrade the plastic if it sits in there for long.

Get it Thin as Fast as Possible

When conditioning your polymer clay in the pasta machine, it will be more effective if you get it thin as soon as possible. Rather than doing 30 passes on the thickest setting, keep going thinner without folding. Go down to a 6 or so, and you’ll feel the clay relax and be less stiff. You can then fold it and bring it back to a thicker setting. Make sure to avoid trapping air.

How Much Conditioning is Enough?

How do you know when you’ve conditioned your polymer clay enough? There is no definitive answer aside from “until it behaves itself.” If the clay sticks to itself, can be blended without seams, and can be made smooth, it’s conditioned enough. There is no magical number of pasta machine passes. And it’s a myth that the sheet must be perfectly smooth and won’t crack at the edges. As you can see in the sheet below, it will still crack on the edges!

Softer brands (such as Souffle and Sculpey III) will need very little conditioning and perhaps might not need any at all. If the clay is workable, it’s fine. The newer a block is, the less conditioning it typically needs. Old blocks from your stash or purchased from eBay or on a destash group may very well need some serious work.

Please don’t condition your clay beyond the point where it becomes easy to work. There is no advantage, and you’ll just wear yourself out.  Some clay brands can become overworked and sticky if they’re conditioned too much. If this happens, don’t worry. Just set it aside and come back in a few hours. (You can’t irreversibly ruin clay by conditioning it too much. That myth is making the rounds lately. Ignore it.)

Can Polymer Clay Become Unconditioned?

If you let your clay sit for a while, does it become unconditioned? Yes, it will! Some clay brands, such as Cernit Number One, Papa’s Clay, and Fimo Professional can start to stiffen within the day. Other brands will stay soft longer.

Is there any advantage to conditioning your clay in advance? Yes, if you can do it the same day or the day before. But there is no advantage to pre-conditioning clay that you will use in the more distant future. The clay will not stay conditioned. I’ve read of people preconditioning their clay and storing it in sheet form in 3-ring binders. This strategy will not save much, if any, time if you’re waiting more than a few days to use this clay.

Want to Know More?

Want to know more about the chemistry behind conditioning polymer clay (in layman’s terms)? There’s an article series in Blue Bottle Insiders that goes into more detail. Join Blue Bottle Insiders by purchasing a membership here.