(This text was copied from Scharfen-Berger's web page without permission in case a mere link to it disappeared or moved.)
You have probably seen the word temper used in reference to steel. Tempered steel is harder and more durable than untempered steel. Exactly the same is true of chocolate. More specifically, well-tempered chocolate is hard, snaps crisply when broken, feels dry to the touch and smooth in the mouth, will not bloom when kept away from heat, has an attractive sheen, and melts at a specific temperature (1-2 degrees below normal body temperature). It has these properties as ic cesult of the structure, size, and number of cocoa butter crystals present within the chocolate, all of which are controlled by the way in which it is tempered.
Unlike water, which is made up of identical molecules of H20 and can freeze or crystallize in only one way, cocoa butter, which contains several different types of fat molecules, can crystallize in a number of different ways. In practice, only one type of crystal provides the stability over time, as well as the properties described above, that are desirable in chocolate. This type of crystal begins to form at about 82 or 83 degrees and begins to melt at about 95 degrees. At the lower end of this range chocolate thickens; at the upper end it thins out. It is crucial, therefore, when tempering chocolate or working with already tempered chocolate to keep it within these temperatures limits. To be perfectly safe, it is better to remain 2 degrees above the lower limit and 2 degrees below the upper limit.
When you start with commercially produced chocolate (as opposed to chocolate you yourself have tempered), you should first melt all the chocolate to 115 degrees. This is necessary because it is not uncommon for some unstable crystals-the ones that crystallize and melt at lower temperatures-to hide in the center of large chocolate blocks. By melting chocolate to 115 degrees, you dissolve all crystals and start with a virgin product. It is perfectly safe to melt both dark and milk chocolate to this temperature.
Once you understand the basic principles behind the behavior of cocoa butter crystals, there are several tempering methods to choose from. The one we prefer needs no previously tempered chocolate.
1. Break up chocolate and melt completely over double boiler to 115 degrees.
2. Bring entire mass of liquid chocolate to approximately 95 degrees while stirring over a bath of cool water.
3. Remove 1/4 to 1/3 of the choocolate and cool this portion further to about 85 degrees while stirring to make certain the temperature is uniform. This step can also be carried out on a marble slab.
4. Maintain remaining chocolate at approximately 95 degrees, stirring occasionally. You can either leave a thermometer in this bowl, or if you periodically touch the chocolate to your lips, no temperature difference should be perceptible.
5. When the cooler portion of the chocolate just begins to thicken, add it to the 95 degrees portion, remove from the warm bath, and stir gently to make sure the two portions are fully mixed. The chocolate should now be around 91-92 degrees and fully tempered.