Does Polymer Clay Melt Plastic?

Recycle number 5 is safe for use with polymer clay.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.

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

Recycled PET testing with polymer clay samples.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.

Interesting Fact: The PET bottle was patented in 1973 by Nathaniel Wyeth, brother of painter Andrew Wyeth and son of artist and illustrator N. C. Wyeth.

2 HDPE – High Density Polyethylene

HDPE is another plastic which is fully compatible with polymer clay.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

PVC bottle tested with polymer clay samples.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

Polymer clay will not melt or eat into LDPE plastic.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.

There is slight marring with LDPE.

5 PP – Polypropylene

polypropylene polymer clayThis 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

CD cases are polystyrene and are not compatible with polymer clay.

Polystyrene, recycle number 6, is incompatible with polymer clay and will melt upon contact.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.

Polystyrene is a plastic that is not compatible with polymer clay.

Polystyrene is a plastic that is not compatible with polymer clay.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.

Polystyrene is a plastic that is not compatible with polymer clay.

7 – Other

CD surfaces are polycarbonate and will be damaged by contact with polymer clay.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!)

Uncured polymer clay, when left in contact with a CD, will react with the plastic of the CD.

Polymer clay will react with polycarbonate and break down the plastic.

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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66 thoughts on “Does Polymer Clay Melt Plastic?”

  1. When I make sheets or canes that I don’t use up right away, I put them on pieces of parchment paper, cut to the same size as a piece of heavy box board (eg. cereal boxes), which I cut to fit inside a GOOD QUALITY ziplock bag. The box board keeps things stiff, so that the bag doesn’t slump when I have thin sheets, and I can stack several bags inside my storage drawers. So far, things seems to be okay using this.

    1. So the cardboard is inside the ziploc bag? And the clay is stuck to parchment paper? When people say parchment paper, it can mean so many things. Some of them are coated and won’t leach the clay. Others are terribly absorbent and will ruin the clay if it’s stored on it. I suppose I need to find the right kind.

  2. I love reading your research articles. The amount of work you put into them is phenomenal, and I always learn so much as I read them! That said, I wish I’d had this article back in August/September. I’ve been making fairy doors to sell, and I decided that transparencies (like the ones used on those old projectors and more recently to make clear greeting cards, which is what i originally purchased them for) would be a great way to store the wood-grain sheets I make for the doors. I made up several sheets of many colors and stuck them to the transparencies, then placed them in a box. They were fine for a while, and then one day all of them were completely stuck to the sheets. The sheets themselves didn’t seem melted or distorted in any way, but I sure couldn’t pry off the clay, no matter what I did. What did eventually come off was brittle – totally unusable. I had to throw away at least two pounds of clay and two dozen sheets, which made me very sad – not just for the expense, but for the work that went into what was now unusable. I never even thought to do any sort of test to see if anything bad would happen, either to the clay or the sheets. I’ve learned a hard lesson, and I appreciate what I’ve learned from this article. My clay sheets are now safely wrapped up in food-grade plastic wrap, and I don’t make as many in advance now. Thank you so much, Ginger!

    1. Oh no! That’s so unfortunate! I wouldn’t have thought the transparencies would be a problem, but perhaps they were polystyrene or something similar. How frustrating to have all that work ruined!

  3. Thank you for the detailed research and time you took on this “experiment” and for selflessly writing up such a comprehensive, thorough post! This was very helpful!

  4. I am relatively new to polymer clay and am somewhat concerned about the safety of this medium. If it can melt plastic, how safe is it for our skin?

  5. Oven control knobs — I must not have cleaned my hands before I put my beads into my polymer clay dedicated toaster oven. Just one time doing that kind of mistake and the numbers were all melted off said knobs.

    In all fairness, this happened with the older formula (pre 2007-2008) polymer clays.

  6. Thanks for doing the research. It’s good to knwo what will work and what to avoid. 20 years ago, I knew polymer clays would melt the minitrays in the storage box I used, so I wrapped them in plastic wrap. Now it seems I should just have built a storage container for mushroom boxes!

    1. Ha! You know…you could make a bigger box with slots for the edges of the mushroom boxes to fit into. So it would be like drawers sliding in and out. There are so many ways to go with this.

  7. Excellent article, Ginger. Thank you for doing the research and sharing. For storing my clay canes, I use the small 3 or 4-drawer storage units and always line them, bottom and sides, with waxed paper. In 13+ years, I haven’t had any deformed drawers. I also use the waxed paper to separate layers of canes or slices that I’m saving.

    1. That’s funny, but I just can’t stand waxed paper. There’s something about it. I think because my mother used to use it when all the cool moms used ziploc bags. Aren’t we humans such odd creatures?

  8. interesting stuff.. the worriesome part was that the fluoride rinse was in #3, you might check to see if that particular manufacturer of your rinse uses bisphenol A as the plasticizer in their bottles as this most probably would be leaching into the rinse and could have detrimental effects by acting as an estrogen mimetic on those who use it. You might want to check that out. Many manufactures of #3 plastics have switched plasticizers to bisphenol S or others and unfortunately the latter chemicals have not been subject to much research.

  9. Unbelievable timing! I had JUST looked up if clay was ok with plastic #5. I had some clear egg cartons that were perfect to keep small pieces of some colors I mixed & rolled into balls to store. i did find out that #5 is cool with polymer. This article was the perfect additional reading to educate me on the entire subject! Thanks, Ginger!
    xo Dawn Gaye

  10. Fredrica Van Sant

    An excellent and timely piece – and I add my thanks. Well, I can’t use “I’m a newbie” anymore, but I feel dumb…I thought that the plastic ruined the clay, not the reverse! I guess I am safe though, I use some regular plastic off a roll and then put it in a zip lock bag as well as keep the original plastic wrap intact if possible. But I was not sure this was right and wondered about the plastics I was using. I may have been borrowing trouble, as recently I did some conditioning ahead and stored it as above and then in a plastic container with a closed cover for about 10 days. The regular clay was okay but the Primo translucent seemed to lose a lot of moisture.
    Your article provided a lot of useful information and great explanations…a greatbighuge thanks!!!!!

    1. Premo translucent seems to be quite finicky and I suspect (but have not had it confirmed) that the plasticizer is volatile and will evaporate. So it does actually seem to dry out when stored in certain ways. You’re not imagining that.

  11. Wow. The plastic in prescription eyeglasses reacts with polymer! Important safety tip. I never take my glasses off, but if I set them on my bench and they got clay on them they’d be ruined, optically. Yikes!

  12. Thank you for your awesome research. I have been looking for better ways to store my clay and now I have some new ideas.

  13. Hi Ginger. I really appreciate your attention to detail. I enjoy the background information as much as the detail on how it relates to polymer clay. This was a very useful article. Thank You!

    1. I’m glad you liked it. I sort of felt that I was giving too much non-polymer info. But it’s just words and besides, it was interesting to me. And if it’s interesting to me, then it might be interesting to others, so I went ahead and included it.

  14. Fantastic article Ginger! I have always wondered which plastics are ok to use with polymer clay. I keep clay in zip lock bags and plastic lunch boxes and have never had a problem with melting. It’s interesting that food processor bowls are effected, because I have used a mini one occasionally to chop up clay. I have always cleaned it well afterwards and never noticed a change in the plastic.

    1. I think it depends on what the bowl is made of and how long the clay stays in contact. If you’re cleaning it out right away, all that might happen is a touch of frostiness.

    1. Thanks for sharing that. I amended the article a bit, because I realized that even though the clay doesn’t actually make plastic wrap (and tablecloths) sticky, it does make them bumpy, and that’s not a good thing. I know that it will also make plastic sheet protectors bumpy, too, even though I do use them and the clay is fine over time. But after you take the clay out, the plastic IS ripply.

  15. Thank you for helping all of us with your in-depth research and sharing of it, Ginger! I’m relatively new to polymer clay so I really appreciate all your information and have quickly learned that your guidance is priceless! ❤️

  16. Wonderfully thorough!! Thanks so much for all your hard work. Very good information and reference material.

  17. Hi Ginger! I love the way you put your articles together. I was an engineer in a former life and I love the questions you ask and they way you pursue the answers. Truly fascinating. Thank you!

  18. Great article Ginger-you’re research is extremely detailed and genuinely appreciated by many people! I recommend your blog to all my students. Keep up the fantastic work and scientific study of polymer clay. ❤️❤️❤️

  19. Jeri Staley-Earnst

    Wow, thank you for finally clarifying this for me. The research detail and technique leave no questions for me. Please keep up your good work! -Jeri

  20. The detail you put into your research astounds me, Ginger. Awesome and helpful article!

    Your conclusions back up what I read some time ago: the softer plastics should be fine, but the harder ones could be trouble. A pretest is always wise. 🙂

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