The following is a compilation of information from various members of the Polymer Clay Smooshers.
It is very important to cure polymer clay properly using the manufacturers directions. Time and temperature may vary depending on what brand you use. The single most important tool to have in your arsenal before anything else is an oven thermometer. I am fortunate enough to be married to someone who knows how to and keeps my oven calibrated. I still use an oven thermometer. One of our members uses a candy thermometer in her oven.
No matter what type of oven you use we cannot stress enough the importance of keeping an eye on that temp. If it swings too much during the curing process you could end up with either scorched clay or clay that is not completely cured. Polymer clay that is not cured properly can crumble months and even years down the road. Properly cured clay does not.
With most clays you can bake longer than the recommended time. Some think that this cures the piece harder. Many of us do this. I don't think any of us Smooshers use any Sculpey's but if you do be aware that it can darken that brand if you cook longer even at a lower temperature.
Stay tuned for part two of our post, baking surfaces.