Firing is the process of heating the clay to make a finished piece. During firing, the temperature causes chemical changes in the clay, making it hard and strong. Polymer clay fires at low temperatures (below 300° Fahrenheit), so you don't need a special kiln - you can fire it in a kitchen oven.

You can lay flat pieces in a metal or glass baking sheet or pan (line the pan with baker's parchment or ordinary white paper to avoid getting shiny spots where the clay touches the hot surface).

Beads sometimes develop flat spots when laid on a baking sheet; these can be suspended on lengths of stiff wire (such as florist's wire) laid across the edges of the pan. You can also fold a piece of heavy paper accordian-style and lay the beads in the folds.

Irregular pieces can be laid on a bed of polyester fiberfill. The fill doesn't melt or burn at normal firing temperatures, but be sure it's not sticking up enough to touch the oven's heating element.

You don't need to fire the prepared pieces immediately, but if you're going to wait longer than a few hours, lay a sheet of waxed paper over them to keep dust and lint from getting on them.

Recommendations on this vary, and you'll notice that different brands of clay call for different firing temperatures. It's a good idea to get an oven thermometer and use it to determine the actual temperature your oven gives. The oven temperature must reach at least 210° Fahrenheit or so for the polymer to fuse properly; if the temperature goes above 300° Fahrenheit, the clay may burn, giving off dangerous fumes.

Within these limits, any temperature will work, although you should avoid firing Sculpey translucent, Fimo art translucent, and possibly other translucent clays at high temperatures, because it may change their color. Manufacturers recommend temperatures between 250 and 275° Fahrenheit for all except transparent clays.

The firing time depends mainly on the thickness of your pieces. For clay a quarter of an inch thick, 20 minutes is plenty; for thicker pieces you may need to go up to an hour to get maximum strength.

You can put your clay pieces into either a cold or a hot oven, although putting them into a preheated oven may make it easier to gauge the firing time.

If you're covering a surface (such as glass or metal) with clay, however, it may be better to put the piece into a cold oven, heat it to firing temperature, then let the piece cool down in the oven after firing. This gives the underlying material more time to expand and contract with the changes in temperature, making cracking of the clay less likely.

Yes. You can use any oven - convection, toaster, etc - as long as the temperature is reliable. It's a good idea to use an oven thermometer to make sure you have the correct temperature, since ovens often have a lot of variation in temperature. If your oven is small and the clay is only a few inches away from the heating element, place a tent of aluminum foil over the clay to diffuse the heat and avoid burning.

Some people prefer to use a toaster oven and put it outside (on a deck or balcony) during firing, for an extra margin of safety.

In general, the color of polymer clay stays the same after being fired. There are a few exceptions to this. Some reds turn darker after firing. Sculpey translucent tends to turn yellowish if it's fired above 220° or for more than 15 or 20 minutes (although this can actually be used to advantage for techniques such as faux ivory). Fimo art translucent may also darken if fired at the higher temperatures.

If you're in doubt, you can fire small samples of the colors you're using to see what the end result will look like.

You can fire a clay piece for up to several hours, as long as the temperature does not get above 300° Fahrenheit. You can fire a piece, then add more clay and fire again (for example, if you want to embed small clay pieces in larger pieces). The new clay will stick to the pre-fired clay, making a strong bond.

You can also fire a piece most of the way (10-15 minutes), take it out and carve or drill it while it's still a little soft, then finish firing it.