Step 1Combine the Campari, sweet vermouth and gin in a large non-reactive saucepan. Add the sugar and gently warm the liquid over low heat, stirring frequently. When the sugar is dissolved, remove from heat. Divide the orange slices between 1 to 2 large (gallon-size) resealable bags or place in a large glass bowl, and pour over the Negroni mixture. Seal the bags (removing any excess air), or cover the bowl, and refrigerate the slices for at least 4 hours, up to overnight. Drain the oranges (save the infused Negroni mixture for cocktails) and fit each slice with a popsicle stick. Freeze the slices on a parchment-lined baking sheet until firm, 3 to 4 hours. Wrap the frozen popsicles in plastic wrap if not serving immediately. The frozen popsicles will keep 1 to 2 weeks.