Step 1

Fill a large pot with water and add the oranges and lemons. Bring to a boil, cover and simmer over moderate heat for 1 hour, turning the fruit a few times. Set a colander over a bowl and gently drain the fruit.

Step 2

When the fruit is cool enough to handle, halve the oranges and lemons and scrape the insides onto a large piece of dampened cheesecloth. Tie the cloth into a bundle. Cut the peels into very thin strips.

Step 3

In a wide, heavy pot, combine the citrus peels with the sugar and 7 cups of water and bring to a boil, stirring to dissolve the sugar. Add the cheesecloth bundle and boil over moderately high heat for 20 minutes, stirring occasionally. Using tongs or a slotted spoon, transfer the cheesecloth bundle to a bowl; when cool enough to handle, squeeze out as much liquid as possible. Add the liquid to the pot and discard the bundle. Continue cooking the marmalade until the syrup is very thick and glossy and the temperature registers 220° on a candy thermometer, about 20 minutes longer. Pour the marmalade into sterile jars, let cool and refrigerate.