A double boiler is a specialized piece of kitchen equipment consisting of two fitted saucepans. The larger saucepan is partially filled with water brought to a simmer or boil. The inner saucepan uses this indirect heat to melt chocolate, cook custards and sauces, or even melt wax for candlemaking. One can also be improvised with a large saucepan and a bowl, or two saucepans separated by a trivet or other heat-resistant spacer.
Conventional stovetop cooking calls for direct heat under a metal or glass food container. This works well for heartier foods, such as meats or vegetables, but delicate sauces and chocolates often break down under direct heat. A double boiler allows eggs and other heat-sensitive sauce ingredients to heat slowly and evenly, eliminating the possibility of scorching or overcooking. Chocolate must almost always be melted this way, because direct heat will cause the temperature to rise too quickly and the consistency to be ruined.
Hobbyists working with wax or soap may also use a double boiler for melting their raw materials. The boiling water rarely rises above 212°F (100°C), which will prevent the wax or soap from scorching or solidifying between projects. Candy makers also use this type of pan to keep their syrups viable until molding.
If a recipe calls for a double boiler, there is rarely an alternative method. Cooks without this pan can improvise one with two sauce pans or a large pot of boiling water and a heat-proof bowl. Most models have a single lid, but many sauce recipes require constant stirring. Chocolate and water are not a good combination, so a lid should never be placed on melting chocolate. If a cook is using an improvised pan, he or she needs to be careful around the steam. The two elements should be kept separated — if the inner pan touches the bottom of the outer pan, the result could be a counterproductive direct heat.
There is a baking equivalent called a bain-marie. Some foods such as baked custards and souffles are placed inside a larger pan partially filled with water. The bain-marie provides the same sort of indirect heat, but inside the oven.