Lampworking

The basics of lampwork beadmaking.:
Lampworkers use a torch to melt the tips of glass rods, then wind the molten glass around a mandrel, a narrow stainless steel rod.
Later, when the bead is removed, the space occupied by the mandrel becomes the hole used to string the bead.
Glass cools from the outside in and the outer layers shrink as cooling takes place. Bringing a bead out of the flame and leaving it in the open air allows the outside of the bead to cool rapidly around its molten interior. A stress point develops between the cool, shrinking glass and the hot center. The stress can cause a bead to crack, either immediately or at a later time.
Beads Should Be Kiln AnnealedTo prevent cracks, beads must be annealed then slowly cooled. The best way to do this is in a kiln, where temperatures can be closely regulated. The beadmaker anneals, or "soaks" the beads to make sure that all glass within them is the same temperature.
The soaking temperature is high enough for glass to flow on some molecular level, but not so high that the bead ends up in a puddle on the kiln floor. After annealing, the artist begins to reduce the heat in the kiln, taking several hours to bring the beads to room temperature.
This process produces glass beads with less stress, so they're less likely to crack. Very small glass beads are sometimes slowly cooled between layers of insulation. It's not the same as annealing, but the process is usually successful because the small amount of glass in tiny beads cools at a more even rate.

Source:http://jewelry.about.com/cs/glassbeads/a/buying_beads.htm