Chocolate that hasn't been tempered can develop a mottled look on its surface, known as fat bloom. This is caused by crystals of cocoa butter rising to the surface of the melted chocolate and setting as a film. It doesn't affect the chocolate's taste, but it looks ugly and melts quickly, making it a messy eating experience.
To temper chocolate, you heat it and cool it, changing its structure. When chocolate is melted beyond 36C - even if it was an already-tempered block to begin with - the cocoa butter can form into six types of crystals. All these crystals have different melting points and variable hardness, and only one boasts all the attributes for tempering. You temper chocolate to achieve a result where only the desirable crystals form, giving you a luscious finished product. While the physics of tempering is complex, the process is quite simple. There are three basic stages: heating, cooling and crystallising, and reheating.
The first stage is to gently melt the chocolate until all cocoa butter crystals are completely melted; this happens at 48C. You can melt chocolate using a water bath or bain-marie, but you must be vigilant in ensuring absolutely no moisture gets in the bowl. The best method is to bring three centimetres of water to a gentle simmer in a medium saucepan. Place the chocolate in a heatproof bowl that fits snugly onto the saucepan but ensure the bowl doesn't touch the surface of the water. Direct contact is too hot for the chocolate. Now stir the chocolate frequently until it melts and monitor the temperature with a thermometer. You want to warm it only until it just reaches 48C, then remove the bowl from the saucepan. You can also use a microwave to melt smaller amounts of chocolate in 10- to 15-second increments, stirring well after each burst of heat. Either way, be careful not to overheat the chocolate because once it's heated over 60C it's ruined.
The next step is to cool and crystallise the chocolate. Tabling is the traditional method and a good way to temper large quantities of chocolate. It involves cooling and agitating some of the melted chocolate on a cool surface (like marble) until the desired crystals form, then introducing it back into the remaining chocolate.
An easier way to cool and crystallise chocolate is a process called seeding. You add some perfectly tempered chocolate into your melted chocolate - a quarter of the total chocolate is needed, either finely chopped or grated first. All Lindt chocolate, for instance, is tempered, so you can grate it straight from the block or you can keep some of your own already tempered chocolate on hand for the next time you temper.
After cooling and crystallising, you need to carefully reheat the chocolate to bring it to working temperature. This will melt away some of the unstable cocoa butter crystals that make chocolate set without a gloss. To reheat the chocolate, you have to be exact with the temperature. A hot-air gun - available from hardware stores - is a good way to reheat chocolate, but believe it or not, a hair dryer does the trick just as well.
Warm the chocolate with your hot-air gun or hair dryer, stirring continuously, monitoring with a digital thermometer until it reaches the desired temperature. For dark chocolate, the perfect working temperature is 32C, for milk chocolate it's 30C, and for white it's 29C.
It's a good idea to check a small sample to see if the chocolate is tempered correctly as you go. The easiest way to do this is to dip a small piece of baking paper into the chocolate. If it sets quickly and without streaks, you have perfectly tempered chocolate.