3 pounds Florida tangerines (unpeeled, washed, cut crosswise into thin slices, seeds discarded)
4 cups sugar
Place a small plate in the freezer. In a large pot, bring tangerines and 6 cups of water to a boil over high heat. Reduce heat to medium and cook tangerines at a rapid simmer until peels are tender, about 20 minutes.
Add sugar, increase heat to medium-high, and stir until sugar dissolves. Return to a boil and cook, stirring often, until mixture is thick and darkens slightly, 40 to 45 minutes. To test for doneness, drop a spoonful on the frozen plate and freeze for 2 minutes (it’s done if it has a slight film that wrinkles when pushed with your finger; if it spreads out and thins immediately, continue cooking). Transfer marmalade to airtight containers, cover, and let cool completely.
Store marmalade in the refrigerator for up to 1 month, or freeze it for up to 6 months.