In the same saucepan, combine 4 cups of the sugar with the corn syrup and 2 cups of fresh water, the orange extract (optional), and bring to a boil over moderately high heat; stir constantly until the sugar is dissolved. Add the oranges and cook over low heat, stirring once or twice, until they begin to look glassy, about 1 1/2 hours.Raise the heat to high and boil vigorously for 7 minutes; the rinds will begin to look translucent. Remove from the heat and let the oranges cool in the syrup. Using a slotted spoon, transfer the oranges to a baking sheet lined with parchment paper. Let the oranges stand uncovered overnight or until tacky to the touch. Reserve the cooking syrup.Bring the syrup to a boil over high heat. Add the oranges and boil for 10 minutes. Let cool completely in the syrup. Using a slotted spoon, transfer the oranges, skin side down, to a rack. Let stand until tacky.