When melting chocolate, it is best to use a bain-marie, or a double boiler. This is a French invention, and it is simply a pan of hot or simmering water with a bowl suspended over it. It is used for making delicate sauces, custards and for melting chocolate, which direct heat would damage or bum. Only use a bain-marie if the bowl makes a proper seal with the pan of water underneath. If a small bowl is left to stand in the middle of a large pan of simmering water, steam may contaminate the chocolate, and even a small amount of water or steam ruins good chocolate. If you do not have a proper bain-marie, a useful addition to any kitchen is a large stainless steel bowl, available from any good cook ware shop or catering supply store, which works just as well. Placed over a pan of hot water, it is cheaper and more versatile than a bain-marie. Just make sure the bowl overlaps the sides of the pan, creating a good seal. In fact, all the chocolate chefs I have seen doing demonstrations favour this method.