Preheat the oven to 300°. In a heavy medium saucepan, combine 1 cup of the sugar with the lemon juice. Stir with a wooden spoon over moderate heat until the sugar dissolves, then cook, stirring occasionally, until the sugar turns deep brown, about 10 minutes. Pour the hot caramel into an 8-cup metal ring mold; holding the mold carefully by the rim, tilt the mold to coat the interior with the caramel. If needed, use a wooden spoon to push the caramel up the side.