Professional confectioners use a copper kettle connected with an apparatus that makes a partial vacuum in the kettle and allows the sugar to boil at a lower temperature than the ordinary boiling point, thus lessening the danger of scorching the sugar. For the home candy-maker, aluminum is the best substitute for copper as a conductor of heat. Sirup boiled in an aluminum kettle rarely scorches, and the smooth surface makes it easy to keep the sides wiped free from sugar crystals as they form.