I’m sure you’ve all heard the ominous warnings. Don’t store polymer clay in plastic! Polymer clay is incompatible with plastic boxes. Many plastics will melt if touched by polymer clay. You’d think that polymer clay is a toxic sludge that’s going to eat through anything it touches (it’s not). But it does make you wonder. What plastics are okay with polymer clay? And if plastic is so bad for use with polymer clay, then why does it come in a plastic wrapper or clamshell box? Doesn’t the polymer clay eat the plastic packaging? There is no doubt that it’s confusing when trying to figure out which plastics that you can use with polymer clay and which plastics will degrade, melt, or get sticky. So to help figure this out, I dug through our recycle bin and looked through our house to find all sorts of plastics. I then stuck pieces of raw polymer clay onto the plastic and waited a month. Here’s what I found.

How Does Polymer Clay Eat and Soften Plastic?

Polymer clay is PVC powder suspended in a mixture with plasticizers, fillers, binders, pigments, and lubricants. After oven curing, the plasticizers become locked in and fused with the PVC particles, making them (mostly) inert. But uncured polymer clay has free plasticizer. Plasticizer, by the way, is a chemical that makes plastic soft and flexible. If you place a piece of raw polymer clay onto the surface of an incompatible plastic, the plasticizer can actually soften that plastic and cause it to “melt”.

Not All Polymer Clay Acts the Same

I have found that different brands of polymer clay have different chemical compositions and use different plasticizers. So when I do any tests, I do try to use several brands of polymer clay to see how each of the brands reacts. Quite often I find a large difference between brands. This is certainly the case here. For the tests featured below I used the following polymer clays. Each sample of unbaked polymer clay was pressed to the plastic surface and left there for one month.

• Premo – Black
• Fimo Professional – White
• Sculpey III – Red
• Kato Polyclay – Yellow

What are the Types of Plastic?

There are many types of plastic and each is made of a different “soup” of chemicals. Many plastics can be recycled. But how can a recycling center know which type of plastic they have? If you’ll look on the bottom of most modern plastic containers, you’ll see a “recycle symbol” embossed into the plastic. Either within or next to that recycle symbol is a number. That number correlates to the type of plastic that was used to make that container. Polymer clayers can use those numbers to identify the plastic that they have, giving them an idea if that plastic is compatible with polymer clay and can be used to store unbaked clay.

The recycle numbers range from #1 to #7, and they are as follows:

1 PET or PETE – Polyethylene terephthalate

This (typically) clear, glossy plastic is strong, flexible, and used for disposable water bottles, soda, ketchup and condiment bottles, dish soap, liquid soap, and clear plastic of blister packaging and clamshell containers. If it’s a clear bottle in your home, it’s likely PET. It can be tinted as well, such as the green color of 7Up bottles. A variety called CPET will be colored black and are used for those disposable freezer-to-oven baking pans. When spun as a fiber and made into fabric, PET is known as polyester. PET is also the plastic used in making mylar.

Result: No effect.

2 HDPE – High Density Polyethylene

If a bottle in your home isn’t PET, it’s most likely going to be HDPE. This strong, somewhat flexible, and durable plastic is used for any opaque or frosted bottles that you’ll find in your home. It’s also the plastic used to make larger children’s riding toys and play houses (think Little Tikes). It often has a slightly pebbled surface. It’s also used to make the molded table-top for folding banquet tables and chairs. It can be tinted any color, but will always have a sort of frosted or dull look. It will never be clear and glossy like PET. Tyvek is a fabric made from spun HDPE fibers that are pressed into a sheet.

Result: No effect.

3  PVC – Poly Vinyl Chloride

I found one vinyl bottle in my home, and used it for this test. (It was my daughter’s fluoride rinse and I dare not ruin the bottle, so we tested this one as-is!) This is the same plastic that is used to make polymer clay, vinyl shower curtains, mini-blinds, house siding, aquarium tubing, pool toys, flooring, pipes, and yes, even old LP records. Much of the vinyl that you’ll find in your home will not be labelled with a recycle number. In fact, recycling of PVC is typically not done as some vinyl uses lead as an ingredient and it’s not cost effective to remove the lead.

Result: Premo and Fimo had some slight softening. Sculpey III and Kato caused the clear plastic to become slightly frosted.

4 LDPE – Low Density Polyethylene

I have seen some LDPE bottles, but you’ll most often run into this plastic in the form of Ziploc bags, shopping bags, trays, bowls, and various miscellaneous flexible items. LDPE is fairly stretchy and flexible, making it perfect for tubing and snap-on lids.

Result: There was no obvious marring, frosting of the plastic, or melting. But there was a slight rippling of the plastic under the Kato, Sculpey III and Premo, as you can see in the next photo. Fimo Professional, which had the most melting with polystyrene and polycarbonate, left no mark on the LDPE plastic.

5 PP – Polypropylene

This plastic is the one used to create those large stackable storage totes, plastic sweater boxes and shoeboxes, embroidery floss boxes, and pretty much any large plastic container you’ll find in the housewares section in your local discount store. It is used to make some bottles, but is certainly used to make most flip-top lids because it can withstand the repeated bending of the hinge. You’ll find this in plastic document folders, fold-up boxes. It is often left uncolored and will appear translucent, but will always be slightly cloudy. It can also be colored and opaque. It’s generally heat resistant at polymer clay curing temperatures, but becomes brittle below freezing. It’s often difficult to tell the difference between PP and HDPE because they have a similar feel and are used for some of the same applications.

Result: There was a very tiny bit of surface change, barely noticeable, under the Premo, Kato, and Sculpey III. It’s difficult to know if this was due to a reaction with the PP, or if there is perhaps a coating on the surface of the plastic which the clay reacted to.

6 PS and EPS – Polystyrene and Expanded Polystyrene

Polystyrene is a lightweight, clear, rigid, brittle plastic that you’re familiar with as it’s used to make CD and DVD cases. It also makes plastic cutlery, petri dishes, model airplane and train kits, disposable cups, and some clear packaging films. Shrinky Dinks, the shrinkable plastic sheet that you can color and bake in your oven is made from PS. You’ll notice that PS will turn white when it bends or breaks and has a distinctive acrid smell when you do flex it.

Result: All four types of clay melted into the polystyrene plastic. Fimo and Sculpey III had the greatest effect. The polymer clay melted the plastic, causing a clear goop to form around the clay sample. It was impossible to remove the clay as it had fused with the plastic.

Expanded polystyrene is PS in foam form or that’s been “popped” like popcorn, and you’re familiar with it as styrofoam. Expanded PS makes meat and produce trays, disposable coffee cups, some floral foam, insulation, pool floaties, packing materials, packing peanuts, takeout boxes, and in building materials. PS is very susceptible to many common solvents and is actually glued together by the melting action of model cement. E6000 glue and many spray paints will dissolve PS.

Result: All four brands of polymer clay affected the polystyrene foam. The Fimo Professional and Sculpey III actually melted down into the foam, creating a black, sticky, oozy goop. Kato created the least effect, making only a slight mark on the surface.

7 – Other

This catch-all category includes any plastic that isn’t in one of the other categories. It will also include polycarbonate, which is the clear, hard and rigid plastic that is used to make Nalgene bottles, reusable water bottles, and prescription eyeglass lenses. Other plastics in the #7 category include acrylic, nylon, and teflon. To test this, I used an old CD and my favorite water bottle. I really didn’t expect the clay to interact with these plastics, so I was more than a bit miffed to find that my water bottle now has a few spots on it. (The sacrifices of artistic science!)

Result: Both Premo and Fimo Professional created a haze on the surface of the CD. But only the Fimo Professional dissolved and melted the plastic of my water bottle. You can see above how the clay slid down over time, “skating” on the surface of the dissolved plastic.

Summary: Does Polymer Clay Melt Plastic?

Quick answer…yes…and no. The only plastics that actually “melted” and were ruined by contact with uncured polymer clay were polycarbonate and polystyrene. That is recycle numbers 6 and 7. While some of the other plastics did show a very slight mark, none of them were actually made sticky by contact with polymer clay. Feel free to use PET, PETE, HDPE, Vinyl, PVC, LDPE, and PP plastics in contact with unbaked polymer clay. This means you can feel free to use them as work surfaces, to make tools and textures, and as storage containers. Avoid polystyrene and polycarbonate as they will be damaged by contact with polymer clay. In fact, polystyrene will begin to show damage within hours.

It does appear that there is a slight amount of rippling or softening of plastic bag material, so this could mean that plastic tablecloths or some plastic wraps might ripple or have distortion after prolonged contact with unbaked polymer clay. Also know that polymer clay might break down the paint of a printed pattern on a tablecloth as well.

Polycarbonate vs Acrylic with Polymer Clay

Both categorized under recycling number 7, “other”, polycarbonate and acrylic are sometimes confused for one another. You are likely to run into them while using polymer clay, however, so I wanted to talk about them for a moment. Polycarbonate is also known as Lexan or Makralon, and is the impact-resistant plastic that is used in re-usable water bottles (such as Nalgene), and the surface of CDs and DVDs. It’s hard, extremely rigid, and quite clear. But some brands of uncured polymer clay will soften and dissolve it.

Acrylic, at least for our purposes, is cast or extruded into sheets and rods that many of us use for tools with our clay. This material might be more familiar to you as Plexiglas, Lucite, Perspex, or Acrylite. More properly called Poly(methyl methacrylate), or PMMA, this material is very clear, hard, dense, strong, and rigid. It is often used in place of window glass because it’s lighter and less prone to shattering. Although I didn’t test it at the same time as the others, I have tested acrylic and found that uncured polymer clay doesn’t cause problems with PMMA. Feel free to use acrylic tools and storage boxes with your raw clay.

What is the Best Plastic to Store Polymer Clay?

You can use recycle numbers 1, 2, 4, and 5 to store polymer clay. Feel free to use plastic wrap, Ziploc bags, and any commercially available storage boxes that are not polystyrene. You will recognize polystyrene boxes because they are clear and brittle, just like a CD or DVD case. If the plastic is cloudy, like those plastic storage bins used for storing clothes, then it’s perfect. Now I always want to point out that new clays come out all the time, and any plastic can have a coating or paint that clay might react to. It’s always a good idea to check any new containers or clays, just to be sure. Just take a peek after a few days and make sure things are going well.

Hard, clear plastic boxes can be made from polystyrene, polycarbonate, or acrylic. While acrylic is safe to use for storing unbaked polymer clay, the other two are not. And it’s not always easy to tell which that you have. Storage containers made from these plastics often do not have recycle numbers to help you tell the difference. If you’re unsure, the best thing is to do a test in an inconspicuous place. I would use a tiny piece of Fimo Professional. Leave it overnight and see what happens. If there’s no mark by morning, the material is likely acrylic and safe to use with raw polymer clay.

If you do want to store polymer clay in a polystyrene container, then do make sure that you fully wrap the clay with plastic wrap first. Or, you could use a small Ziploc bag.

What About Baking Polymer Clay on Plastic?

One of the things we like to do with polymer clay is to cover things. So we also like to know if a plastic is able to be baked in the oven. Can you cover a plastic container with polymer clay and bake it? In some cases, yes, you can. Here are the temperature limits for each of the plastics.

1 PET or PETE – Polyethylene terephthalate – Melts at 482°F (250°C)
2 HDPE – High Density Polyethylene – Withstands 248°F (120°C) for short periods.
3  PVC – Poly Vinyl Chloride – Some PVC begins to degrade at 284°F (140°C).
4 LDPE – Low Density Polyethylene –  Withstands temps up to 200°F (95°C) for short periods.
5 PP – Polypropylene – Begins to melt at 266°F (130°C).
6 PS and EPS – Polystyrene and Expanded Polystyrene – Begins to soften at 176°F (80°C).

It looks to me that you’d be best to stick with #1 PET or #5 PP. I did not directly test these plastics and relied on data I found online. I have, however, baked plastic and some plastics, such as the BIC Stic pen bake very well. Always do a test bake with an unknown plastic to make sure that it will bake without incident. Also keep in mind that some plastics expand with heat, some shrink, and some slump. As you can well imagine, this could make for quite a disastrous project. So always do a test bake before risking a valued project.

Polymer Clay Dissolving Other Materials in your Studio

I want to say a word about how polymer clay can cause a bit of trouble in your studio if you’re not careful. What prompted me to write this article was when a bottle of Sculpey Clay Softener fell over onto my workbench and I didn’t see it. The cap was off and the liquid oozed its way out. It seeped under a #5 storage basket (unharmed) and settled around the base of my desktop speaker. Sure enough, speakers (and keyboards, too) are made from a dense type of polystyrene. My cute little speaker now has a sort of sticky bottom.

All of your tools, your work surface, and your tools should all be made from polymer clay safe materials. Don’t leave polymer clay sitting on unknown and untested materials for very long. Polymer clay will soften some varnishes, so make sure to keep it off your wooden and painted furniture. Some polymer clay will also cause your nail polish to get dull. If you repeatedly have clay-covered hands and then touch something in your studio, you could end up with a reaction over time. For instance, the housing on an Atlas pasta machine motor is chrome plated polystyrene (so far, so good, however!) Some products that we use with clay come in polystyrene containers, particularly Pearl Ex and the small Inka Gold paste samples. And the bowl of many food processors is polycarbonate. Always try to be as clean as possible when working with polymer clay and know that it can harm other things in your studio.

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