Opalescent Glass, both antique and contemporary, contains a mixture of heat-sensitive chemicals that when reheated turn an opaque white (opalesce). After a piece of glassware was removed from the mold and shaped, it was returned to the "glory hole" or the furnace for reheating. Those areas nearest the heat opalesced. It was the skill of the glass maker that determined where and to what degree the glass was turned opalescent. Many patterns found with Carnival treatment were first produced in Opalescent Glass.