It’s well known that the dyes in alcohol inks or alcohol markers will migrate through polymer clay, causing the colors to bleed with time. But have you ever had this happen when using paint, pastels, or other pigmented media? What is the cause when paints bleed on polymer clay, causing a halo or fuzzy edge?
Pigments vs Dyes
Before I get deeper into this, we’ve got to rewind this a bit and talk about the difference between pigments and dyes. Pigments are powdered, crushed materials that give our art materials color. The simplest pigments and the first that our human ancestors used are just dirt or mud. That’s where we get ochre, umber, and sienna. Ground lapis lazuli stone makes ultramarine blue. Modern chemistry has given us lab-created pigments such as Prussian blue. Pigments are particles; therefore, they don’t dissolve in a medium. Instead, they sit on the surface and require a medium or binder, such as paint, to make them stick. (Note: pigments stick to raw polymer clay because the surface is sticky. But they won’t stick to baked clay without a binder.)
Dyes are chemicals that bond to and stain a substrate through a chemical bond. Because the bond is chemical, different dye classes are soluble in either aqueous (water-based) or oil-based solutions. This is why the dyes used in alcohol ink will stain polymer clay, but the dyes in orange soda will not. Read my article here for a more in-depth article about the difference between pigments and dyes.
Lake pigments are a class of pigments that are actually dyes. What? Yes! The powdered dye itself is used as a pigment. When lake pigments are used as pigments, they’re not soluble in the paint they’re used in. A well-known example of a lake pigment is cochineal. It’s a red dye that’s extracted from bugs that infest cactus. When dissolved in water, cochineal will dye fabrics. It’s a lake pigment when the dried dye is mixed with oil and used as paint. Many modern industrially produced (chemically synthesized) pigments are also lake pigments.
Heat + Polymer + Lake Pigments
When a lake pigment is used in acrylic paint, it is not soluble in the water-based medium. The color stays put just fine. But remember that dyes are either soluble in oil or water. So when a lake pigment is not soluble in acrylic paint, it WILL be soluble in oil.
If you apply that same paint over polymer clay and bake it, the plasticizer and oils from the clay seep into the paint. The dye becomes soluble and begins to bleed through the clay. The heat makes it worse and speeds up the process.
For this reason, it’s not uncommon for polymer artists to notice that the paint used to silkscreen a design will come out of the oven having bled into the clay. Also, the color that bleeds might not be the same color as the paint. Craft paint is a mixture, so it could be that only one pigment in the mix is bleeding, as you see in this blue earring. The blue paint has a faint red halo around it.
This isn’t just a problem with paint. Lake pigments can be used as pastels or as dry media as well. Phyllis Cahill shared in Blue Bottle Insiders that Bright Yellow Green pastels bled after baking.
What’s the Solution?
There’s nothing you have done wrong and nothing you can do to prevent this from happening. It’s down to the specific pigment in your art material. To prevent it from happening, choose another paint brand or paint color. Patricia Tamborino from O.Y. Clay tested the paints she had in her studio and here’s what she found.
I suspect the brand of clay might also be a factor. This is why it’s always important to test paints you will use on polymer clay, both for stickiness and color shift and bleeding.