The short answer is “Probably not and I’m definitely not a scientist!”.
Years ago I did a polymer clay painting that I was really pleased with. I even had it framed and it was on display in an art gallery for several months (possibly over a year). However when I got it back the colours looked strange to me. Sure enough when I took it out of the frame there was a definite colour shift visible where it had been under the mount card. I don’t know if it had been in a window or if it was due to the bright shop lighting, but I was really distressed. I was so distressed that I didn’t take a photograph before trying some remedial actions.
To try and save the picture, I carefully scratched off the surface to blend the hard line where the colour changed. You can still see that the foreground is much more yellow at the bottom. I couldn’t do too much of this as I would have lost all the shadows and details added to the surface. You can see more clearly in the next image where I have layered the front and the back of the picture together (both photos taken in the same lighting right after each other).
Because it takes me days to make these paintings, I decided I didn’t want to do any more of them until I could check that the colours would remain as I intended. I couldn’t find any information about the colour fastness of polymer clay, but I did find an article for checking if water colour paints are colourfast. The suggestion was to paint a swatch and then leave it in a sunny place for a year to see if the colours change.
So I made a sheet with a spectrum of colours and cut it into three strips. I made the line wavy so I could fit the pieces together later. I kept one strip in a drawer, one strip on the wall of a room with north facing window and one strip on a south facing windowsill. I don’t know what brands of clay I used for the painting (especially the browns because I blended several scraps to make the colour), but for the test I used Fimo Professional (which at the time was a new product).
From top to bottom: wall, drawer, windowsill 
From top to bottom: windowsill, drawer, wall 
From top to bottom: windowsill, drawer, wall
I couldn’t see any change for the strip that was on the wall, but for the strip that was on the windowsill there was some slight fading of the yellow where it mixed with the red (it’s now less orange) and the green seems to have faded overall. The colour change is a lot less that that which was suffered by the painting but it occurs to me that the double glazing of my window may have had some UV protection that the shop window didn’t. I think I need to put the “wall” strip in the garden for a year and see how that goes.
Unfortunately my process was a bit too chaotic to make this a nice (and accurate) scientific test. For a start I didn’t write down the dates for when I started and finished and I think there are several other variations that are worth testing. For example:
- Different brands of polymer clay
- Colours mixed with white
- Black polymer clay
- Clay exposed directly to sunlight (outside)
So in conclusion there was definitely some colour fading of the yellow and green. Was it significant enough that I don’t want to make any more paintings? Is it unreasonable to expect no change? Was it less faded than before because it’s a new formula or a different environment or because it’s fully saturated colours?
I don’t know!
I told you I wasn’t a good scientist, but I thought I’d share with you what I have found out so far anyway. Maybe in another year I’ll have some results for how the colour strip fares directly outside. In the meantime please let me know if you’ve done similar tests or if you have better information.
