Polymer clay is man-made, requiring heat to cure (harden). Each of the clay manufacturers prints the recommended curing temperature on the packaging. Following the recommended curing temperature is important, an oven thermometer is a good investment. If different brands of clay are mixed, cure at the one with the lower temperature, and cure for a longer period of time.
If the temperature gets too high, the clay will char, the fumes released are not good. If the temperature is too low, it will not fully cure and there will be lack of stability of the piece. Time for curing averages 20 minutes per 1/4 Inch of thickness, as a general rule.
A dedicated toaster or convection oven is a good investment. A home oven can be used, many recommend using a couple of liked size pans to create an oven in the oven. A pan can be used and covered with aluminum foil as well. Simply place your work in the bottom pan, invert one of the same size and place on top. Some metal office clips can be used to clamp the two pans together. That will contain any by products of curing clay confined. Once any object is used for polymer clay it should then be dedicated to clay only.
Because Polymer clay hardens in heat, do not leave it in your car during warm temperature months. Never put polymer clay in a microwave oven.