Each brand of polymer clay has it's own benefits and limitations when it comes to workability, strength, flexibility, firmness, and working time. Personal experience and what the clay is being used to create is the best determining factor when it comes to choosing a brand of clay.
If properly stored (out of direct sunlight and away from heat sources), polymer clay has an indefinite shelf life. You also want to avoid certain types of plastic when storing your clay. Rigid plastic containers can leach the plasticizers from the clay making it brittle. I store my opened clay in plastic sandwich bags which I then put in another container. I keep my colors in separate bags to keep them from accidentally mixing and to keep the clay clean. One of the great things about polymer clay is the fact that it's relatively inexpensive and you don't need a lot of expensive tools to work with it. This makes it a great medium for beginners and children and gives crafters a chance to experiment and find out if this is the right medium for them.
Now on to the different brands of clay.
CERNIT is a clay that is often used by dollmakers. Cernit offers a range of flesh tone colors and the clay has a lifelike translucency after baking. It's a very firm clay and can be somewhat difficult to condition, but once it's baked it's quite strong and flexible. You can find more than 60 different colors of Cernit polymer clay
FIMO Classic, originally simply called FIMO, is the oldest and probably most recognized brand of polymer clay. FIMO Classic is very popular among caners who appreciate the strength and resiliency of the clay. Once baked it offers a good degree of flexibility too. FIMO Classic is available in 24 colors and bakes with a matte finish.
FIMO Soft is one of the easiest clays to condition and also offer the least amount of blade drag when slicing. It remains flexible and strong after baking and has one of the longest open times for working the clay. It also offers superior shelf life for millefiori canes. FIMO Soft is available in more than 50 colors and bakes with a matte finish.
Kato Polyclay is a newer brand of clay and was created by the collaboration of polymer clay artist Donna
Kato and Van Aken International, a manufacturer
of modeling compounds. It can be very stiff and takes longer than other clays to condition, but it's very strong and flexible after balking. While most brands of polymer clay are generally baked at 275°F, Kato
polyclay can be cured up to 300°F for 30 minutes per 1/4" (5 mm)
of thickness. It is available in 17 colors and bakes with a light sheen.
Pardo is the newest clay available and is easy to condition and very strong after baking. It came to the US from Germany and has only been on the market for a few years. It's made from beeswax and other natural ingredients. It comes in reusable packaging and is available in over 70 colors.
Premo Sculpey is a medium soft clay that bakes with excellent strength and flexibility. It can be stickier than other clays, but it's very easy to condition. It bakes with a slight sheen finish and can be found in more than 30 colors. Premo is my personal favorite when it comes to sculpting and jewelry making. Premo has an "artist's palette" of
colors, which are designed to mix together the same way that artist's
paints mix. Premo holds fine detail and is perfect for caning, mokume gane, mica shift, jewelry, home décor and functional item, as well as sculptural pieces.
Sculpey III was one of the first clays to come in individual colors. It's very soft and easy to condition and offers good open time. It bakes with a matte finish but isn't flexible and can be brittle after baking. Sculpey III is best suited for projects with density and bulk and can be found in more than 40 colors. This is a good clay to use for covering armatures, or as a base for other projects.
Studio Sculpey is softer than Premo and is easy to condition. It feels lighter and less dense than other brands and it bakes with a matte finish. It's available in more than 50 colors and has good strength and flexibility.
And that's a brief overview of some of the most popular bands of clay available in crafts stores and online.