Quality Testing

A question that keeps coming up about the homemade polymer clay recipe I posted awhile back is whether hardened objects are waterproof. Although I've answered this question in the negative a bunch of times, I have also indicated that applying a finish (e.g., polyurethane) to the items would increase their water-resistant quality. Then I had an idea about how to use the clay that required hardened objects to be more water-resistant than I had previously demanded. So, I went back to my answer about finishes did some testing to see if they performed adequately to my purposes. The result of my tests, following, provides a better answer to the question of water-resistance than I've been able to give to-date. Since so many people have asked, I thought I'd detail how I got to that answer.

Methods

The test items were disks that I made years ago when I first began working with homemade polymer clay. Because of their age, I knew they were completely dry. This is important because finishing hardened objects before they're completely dry will cause the finish to crack. I glued the disks to wooden skewers and painted them with simple designs in order to tell them apart. I finished one with a combination of polyurethane (undercoat) and acrylic (topcoat), and the other two with several coats of acrylic alone. The acrylic I used was Aleen's Spray Acrylic Sealer. 

I stuck two of the disks into pots and placed one in the shower and one on my fire escape. I gave the third one to a friend to stick in his yard. They stayed in their respective locations for several weeks (the ones in the shower and on the fire escape are still there).

Test Case 1

Finish: Polyurethane undercoat wit acrylic topcoat. Location: Shower.

Test Case 2

Finish: Acrylic. Location: Fire escape.

Test Case 3

Finish: Acrylic. Location: Yard (not pictured; shown here in potted plant).

Results

Test Case 1, the poly-acrylic-shower disk, fared quite well. The front shows no signs of water infiltration.

The back however, shows mild water-wear along the top edge. It should be noted that this wear hasn't worsened over time, and may be due to the polyurethane undercoat not being fully dry before the acrylic top-coat was applied. It should also be noted that I applied the acrylic topcoat because the polyurethane was wearing at that edge. 

Test Case 2, the acrylic-fire escape disk, shows mild cracking of the paint beneath the finish, probably due to it not being fully dry when I applied the acrylic. As in the case of Disk 1, the cracking occurred early and hasn't worsened. 

Test Case 3, the acrylic-yard disk, fared least well, with severe cracking all over the front and back of it. This is probably due to its having been placed in a yard and thus subjected to more severe weather conditions than the other two disks.

Conclusion

Several coats of acrylic finish will enable fully hardened and dried (if they're painted) items made with homemade polymer clay to survive in moisture-rich environments for some time. Those placed outdoors will fare best in semi-protected areas such as fire escapes, patios, decks, etc. 

Until next time --

Linda Purty Bird

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Homemade Polymer Clay* Questions and Answers

Awhile back I posted a recipe for polymer clay. I recently viewed the post and saw all the questions that readers had asked in their comments. Rather than answer each individually, I thought I'd post a follow-up that answers the most frequently asked among them. So, here goes:

Can you add color while the clay is wet?

Yes, you can add acrylic paint or even food coloring, but for even tinting throughout, you'll want to do it while the mixture is still liquid. If you want a swirly effect, then add it to the dough. Be prepared for color-stained fingers. And also note that adding a lot of colorant will alter the recipe and affect how the clay performs.

Does the clay shrink as it dries and how much?

It does, about 15-20%.

Can you bake the clay to speed drying?

Absolutely! I usually dry my items in the oven at about 150°F. How long to keep them in depends on their thickness. Also, if you’re making flat shapes, watch them for curling. Periodically flip them over to keep that to a minimum.

Will dried items stand up to water?

Dried items are water resistant but not waterproof, so if they get a little wet it’s OK, but don’t submerge them in water or even subject them to wet environments. They’ll certainly do better if they’re sealed with polyurethane or acrylic or the like.

Can dried items be used outdoors?

I haven’t tried but wouldn’t risk it. Dried items are water resistant but not waterproof. A sealant will help but I still wouldn’t expect them to last very long, especially in wet conditions.

Can dried items be used for food?

No. While the clay does plasticize during the curing process, it won't stand up to extended exposure to moisture.

Can the clay be used to make shapes with cookie-cutters?

Absolutely! Sprinkle a bit of cornstarch on the surface and roll it out on wax paper, or even in a pasta machine. Flip your shapes over throughout the drying process to keep them flat (they tend to curl).

Can you use glue other than Elmer’s Glue-All?

I haven’t tried but any PVA (polyvinyl acetate) glue should work, with, I would expect, somewhat different but probably OK results.

Cracking?

Shouldn’t be a problem. If it is, your clay might be too dry. Be sure to keep it in an airtight container between uses and only take out what you intend to use immediately.

Stickiness?

Cook the mixture for longer or use a bit less glue. Once out of the pot stickiness is harder to fix but sometimes adding more cornstarch helps. Altitude and likely also humidity will affect how the recipe performs.

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* Not homemade Sculpey or Fimo. The polymer base of those products is polyvinyl chloride (PVC), while for this one it is polyvinyl acetate (PVA). Both PVC and PVA are polymers, and both this clay and commercial polymer clay “plasticize” during a curing process. This clay therefore has many of the characteristic properties of commercial polymer clay (elasticity, pliability, flexibility, durability), but it doesn’t have all of them (particularly insolubility).

Until next time --

Linda Purty Bird