A sugar pot or sugar boiler is an unlined copper pan that is used, primarily by pastry chefs, to make sugar syrups. Because copper is so responsive to the heat, the pan gives the chef instant control as he or she boils the syrup to just the right temperature.
The acidity of the unlined copper causes some of the sugar to "invert," or split into glucose and fructose, which helps resist the sugar's tendency to recrystallize. The pans are also unlined because a traditional tin lining would come too close to melting temperatures in some cases, and no one likes Sugar Syrup with Melted Tin. All well and good, you say, but isn't copper toxic? Yes, we answer. But Burt Wolf, author of The New Cook's Catalog (Canada, UK), rides to the rescue and says not to worry. The pans' use is limited to sugar syrups, which do not react adversely with the copper.
The sugar pot, which is called a poêlon in French (just something fun to share at parties), has a relatively large, flat bottom to allow the sugar to melt quickly. It generally has a lip on one or both sides to facilitate pouring. Some have a ring around the bottom outside of the pan (a little like the rings around Saturn) to catch drips. Sugar pots range in size from 3 cups to 4-1/2 quarts. They are hard to find.
To make a sugar syrup, add a quantity of sugar to a heavy-bottomed pan (the sugar pot is optional). Add some water to cover it. The exact amount is unimportant, but the more water you add, the longer your syrup will have to boil to evaporate the extra water. What you're doing in making a sugar syrup is dissolving the sugar in the water, then creating a supersaturated solution as the water evaporates. At it evaporates the temperature rises, and the character of the finished syrup is established. The higher the temperature to which you cook the syrup, the harder it will set when it cools. Depending on whom you ask, there are between seven and eleven stages to which you can take the syrup, and these stages are separated only by a few degrees. At the low end, the “thread stage” starts around 215°F (102°C) and caramel is reached at 320°F (160°C), just before burning.
Some trained chefs are adept at wetting their fingers, plucking a bit of the syrup from the pan, and dipping it in a bowl of water to determine how far along from Stage 1 to Stage 7/8/9/10/11 the syrup is. Most people use an accurate candy thermometer.
The great challenge of making a sugar syrup is that all the abundant sugar molecules are just waiting for an excuse to glom together into crystals, so you often add a bit of acid (lemon juice) or sweetener (corn syrup, honey or liquid glucose) to retard this tendency. You also do not stir the syrup once it begins to boil, and many people take the step of carefully washing down the inside of the pan with a damp pastry brush to dissolve any crystals that have formed at the edge.
Once the syrup reaches the desired temperature, the bottom of the pan is plunged into a bowl of cold water or the syrup is poured into a heat-proof measuring cup or other cool receptacle to stop the cooking.
For more on the naughty tendency of a sugar syrup to crystallize, click here.