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1
Rinse the oranges under hot water.
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2
Quarter the oranges lengthwise; slice crosswise as thinly as possible with a sharp knife.
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3
Bring the orange slices, orange and lemon juices, and 6 cups water to a boil in a large nonreactive saucepan over medium-high heat.
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4
Reduce heat to medium-low.
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5
Simmer until the orange peels are translucent and tender, about 1 1/2 hours.
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6
Stir in the sugar.
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7
Bring the mixture to a boil over medium-high heat, stirring constantly, until the sugar is dissolved.
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8
Continue cooking, stirring often and skimming foam from the surface with a slotted spoon, until the mixture is set and registers 220F on a candy thermometer, about 15 minutes.
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9
(To test if the marmalade has set, place a small amount on a well-chilled plate.
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10
Press gently with your finger; the marmalade should wrinkle.
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11
If it doesnt, continue to simmer, 5 to 10 minutes more.
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12
Test again; repeat as needed.)
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13
Let cool, then transfer to an airtight container.
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14
Marmalade can be refrigerated up to 1 month.
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15
(Alternatively, transfer the marmalade to sterilized canning jars and process according to the jar manufacturers instructions.)
