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DAY 1: Place the lemon eighths in a nonreactive saucepan where they will fit snugly in a single layer. Add enough cold water for the fruit to bob freely. Cover tightly and let rest overnight at room temperature.
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DAY 1: In a separate nonreactive saucepan, place the sliced oranges with water to reach 1 inch above the tops. Cover tightly and let rest overnight at room temperature.
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DAY 2: Prepare the cooked lemon juice: Bring the pan with the lemon eighths to a boil over high heat, then reduce the heat to medium. Cook the fruit at a lively simmer, covered, for 2 to 3 hours, or until the lemons are very soft and the liquid has become slightly syrupy. As the lemons cook, press down on them gently with a spoon every 30 minutes or so, adding a little more water if necessary. The water level should stay consistently high enough for the fruit to remain well-submerged as it cooks.
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DAY 2: When the lemons are finished cooking, strain their juice by pouring the hot fruit and liquid into a medium strainer or colander suspended over a heatproof storage container or nonreactive saucepan. Cover the entire setup well with plastic wrap and let drip overnight at room temperature.
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DAY 2: Meanwhile, prepare the orange slices: Bring the pan with them to a boil over high heat, then decrease the heat to medium and cook, covered, at a lively simmer for 30 to 40 minutes, or until the fruit is very tender. If necessary, add a little more water during the cooking; the fruit should remain submerged throughout the cooking process. When the oranges have finished cooking, remove the pan from heat, cover tightly, and let rest overnight at room temperature.
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DAY 3: Place a saucer with five metal teaspoons in a flat place in your freezer for testing the marmalade later.
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DAY 3: Remove the plastic wrap from the lemon eighths and their juice and discard the lemons. Strain the juice through a very fine mesh strainer to remove any lingering solids.
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DAY 3: In a large mixing bowl, combine the sugar, cooked lemon juice, fresh lemon juice, and orange slices and their liquid, stirring well. Transfer the mixture to an 11-quart copper preserving pan or a wide nonreactive kettle.
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DAY 3: Bring the mixture to a boil over high heat. Cook at a rapid boil until the setting point is reached; this will take a minimum of 30 minutes, but may take longer, depending on your individual stove and pan. Initially, the mixture will bubble gently for several minutes; then, as more moisture cooks out of it and its sugar concentration increases, it will begin foaming. Do not stir it at all during the initial bubbling; then, once it starts to foam, stir it gently every few minutes with a heatproof rubber spatula. As it gets close to being done, stir it slowly every minute or two to prevent burning, decreasing the heat a tiny bit if necessary. The marmalade is ready for testing when its color darkens slightly and its bubbles become very small.
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DAY 3: To test the marmalade for doneness, remove it from the heat and carefully transfer a small representative half-spoonful of marmalade onto one of your frozen spoons. It should look shiny, with tiny bubbles throughout. Replace the spoon in the freezer for 3 to 4 minutes, then remove and carefully feel the underside of the spoon. It should be neither warm nor cold; if still warm, return it to the freezer for a moment. Tilt the spoon vertically to see whether the marmalade runs; if it does not run, and if its top layer has thickened to a jelly consistency, it is done. If it runs, cook it for another few minutes, stirring, and test again, repeating more times if necessary.
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DAY 3: When the marmalade has finished cooking, turn off the heat but do not stir. Using a stainless steel spoon, skim off any surface foam and discard. Pour the marmalade into sterilized jars, processing according to the manufacturer's instructions or another method of your choice.
