Zen:
I've tried using the Fimo brand colored clays, and the Sculpey brand white clay (comes in a 1 lb box).
With the Sculpey, the small pots I made and used outside without any kind of sealer ended up getting chalky and breaking easily. The "mudmen" type figures I've done, which were painted with acrylic paints, have survived a year so far. The acrylic paint is wearing off, although I'm sure a sealer would help with that.
The few Fimo sculptures I did are ok, although the colors are fading pretty quickly. I didn't do any pots out of it, but if the sculptures are any indiciation, they'd be too fragile.
Now, my biggest problem with the little pots I had made is that I had to bake them several times due to the thickness. The white Sculpey scorches easily, and trust me, you don't want to be around when it does! PHEW... plus the fumes aren't good (move any birds out of the area)... The finished pieces are fragile regardless of what the package says
. I've thought about using the flexible stuff and seeing if its stronger (I think sculpey makes it), but when I figured out how much I'd be spending on the materials it was cheaper to buy a "real" pot....
Now, my experience with it has been in Florida, I have no clue how it would hold up to freezing. I know that frozen raw (unbaked) clay is very very breakable-I'm guessing the baked clay would be worse.
I'm not sure the long term leachability (is that a word?) of the plasticizers... but the Portulacaria cuttings that were in my pots grew just fine (until the pots broke).
If you're gonna try this, my advice is make the pots and then leave them out for a day or two on several pieces of paper-you'll see the oily plasticizers leaching out around the pot... I've found letting this happen firms up the clay and it's easier to handle. I've done this to raw clay (not sculpted) when it was too mushy to work with. My hands heat the clay up fast and the white sculpey got too soft (although it was nice when kneading the fimo brand, which is a lot harder).
You can color the white sculpey with acrylic paint when its raw by kneading paint into the paint (not too much or it gets nasty). You can paint either brand with acrylic paints. What I do, I do a pre-bake at 3/4 the time to cure the piece so I can handle it, then I paint it, and then rebake the proper amount of time. That way the heat sets the acrylic paint too. The prebake lets you paint so you don't distort the object. I bake my pieces on a piece of white ceramic tile, they remove easily once they cool and the tile is easy to handle (and cheap-.88 at home depot).
WARNING: When taking the baked pieces out of the oven, LEAVE THEM TO COOL! You WILL burn yourself and break the pieces if you try to handle them before cooling. Use oven mitts to take the stuff out. Trust me, I've burned fingers and messed up several sculptures. Also, SET A TIMER... set every time in the house and every alarm clock when baking it..... if you forget it, it will scorch/burn and it's really really nasty. Highly recommend not using the kitchen oven, buy yourself a toaster oven (check thrift stores) ...baking outside is best, the stuff stinks. Don't even eyeball the microwave, I've heard some nasty stories about the end result....
If it were me, I'd stick with only doing the mudmen type figures with it.....