Wash and dry the fruit. Using a vegetable peeler, remove the zest from the fruit. Cut the zest strips into a fine confetti. Combine the zest in a pot with 6 cups of cold water. Bring to a boil, reduce temperature to medium high and simmer for half an hour.
While the zest cooks, cut the white pith away from the fruit and separate the fruit from the membranes (see instructions above for greater detail). Collect the interior fruit in a large measuring cup and set the membranes and any seeds aside.
When all the fruit has been broken down, bundle the reserved pith and seeds into a length of cheesecloth, tying the cloth well so that no seeds can escape.
Drain the zest, reserving the cooking liquid.
In a large stainless steel or enameled cast iron pot, combine zest, citrus fruit, 4 cups of zest cooking liquid, 6 cups of sugar and the cheesecloth bundle.
Bring to a boil and cook vigorously until the mixture reaches 220 degrees (this takes between 30-40 minutes).
When the marmalade reaches 220 degrees and sustains it for one minute, remove the pot from the heat. Stir for about a minute off the heat, to help the zest bits become evenly spread throughout the preserve.
Fill prepared jars (see above for jar preparation instructions), wipe rims, apply lids and screw rings. Lower into a prepared boiling water bath and process for five minutes at a gentle boil (do not start counting time until the pot has achieved a boil).
When time is up, remove jars from the pot and let them cool completely. When they are cool to the touch, check the seals by pushing down on the top of the lid. Lack of movement means a good seal.
Recipe adapted from several marmalade recipes in “So Easy to Preserve“