1. To slowly bring up the temperature of a cold or room temperature ingredient by adding small amounts of a hot or boiling liquid. Adding the hot liquid gradually prevents the cool ingredient, such as eggs, from cooking or setting. The tempered mixture can then be added back to hot liquid for further cooking. This process is used most in making pastry cream and the like.
2. To bring chocolate to a state in which it has snap, shine and no streaks. Commercially available chocolate is already tempered but this condition changes when it is melted. Tempering is often done when the chocolate will be used for candymaking or decorations. Chocolate must be tempered because it contains cocoa butter, a fat that forms crystals after chocolate is melted and cooled. Dull grey streaks form and are called bloom. The classic tempering method is to melt chocolate until it is totally without lumps (semisweet chocolate melts at a temperature of 104 degrees F.) One third of the chocolate is then poured onto a marble slab then spread and worked back and forth with a metal spatula until it becomes thick and reaches a temperature of about 80 degrees F. The thickened chocolate is then added back to the remaining 2/3 melted chocolate and stirred. The process is repeated until the entire mixture reaches 88-92 degrees for semisweet chocolate, 84-87 degrees for milk or white chocolate.