Cut the oranges and lemons in half crosswise, then into very thin half-moon slices. (If you have a mandoline, this will be quite fast.) Discard any seeds. Place the sliced fruit and their juices into a stainless-steel pot. Add 8 cups water and bring the mixture to a boil, stirring often. Remove from the heat and stir in the sugar until it dissolves. Cover and allow to stand overnight at room temperature.
The next day, bring the mixture back to a boil. Reduce the heat to low and simmer uncovered for about 2 hours. Turn the heat up to medium and boil gently, stirring often, for another 30 minutes. Skim off any foam that forms on the top. Cook the marmalade until it reaches 220 degrees F on a candy thermometer. If you want to be doubly sure it's ready, place a small amount on a plate and refrigerate it until it's cool but not cold. If it's firm -- neither runny nor too hard -- it's done. It will be a golden orange color. (If the marmalade is runny, continue cooking it and if it's too hard, add more water.)
Pour the marmalade into clean, hot Mason jars; wipe the rims thoroughly with a clean damp paper towel, and seal with the lids. Store in the pantry for up to a year.
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