1. Rinse oranges under hot water. Quarter oranges lengthwise; slice crosswise as thinly as possible with a sharp knife.

2. Bring the orange slices, orange and lemon juices, and 6 cups water to a boil in a large nonreactive saucepan over medium-high heat. Reduce heat to medium-low. Simmer until orange peels are translucent and tender, about 1 1/2 hours.

3. Stir in sugar. Bring mixture to a boil over medium-high heat, stirring constantly, until sugar is dissolved. Continue cooking, stirring often and skimming foam from surface with a slotted spoon, until mixture is set and registers 220 degrees on a candy thermometer, about 15 minutes. (To test if the marmalade has set, place a small amount on a well-chilled plate. Press gently with your finger; marmalade should wrinkle. If it doesn't, continue to simmer, 5 to 10 minutes more. Test again; repeat as needed.)

4. Let cool, then transfer to an airtight container. (Alternatively, transfer marmalade to sterilized canning jars and process according to the jar manufacturer's instructions.)