from P.J. Gartin a Clemson Extension Master Gardener, Charleston, SC
4 cups juice, pulp, and fruit of calamondins
3 cups water
4 cups sugar*
Note: If you have more (or less) of the orange-water mixture than 4 cups, no problem; just adjust the amount of sugar you use in a 1-to-1 ratio (e.g., for 3 cups orange mixture, use 3 cups sugar; for 4½ cups oranges, use 4½ cups sugar.
Rinse the calamondins and cut them in half crosswise. Working over a bowl or large measuring cup to collect the juice, use your fingers or a paring knife to dig out and dispose of the seeds. You'll need 4 cups of juice, flesh, and rinds. Put the fruit (including the rinds) in a food processor and, using the slicing blade, pulse a few times; the mixture should be somewhat chunky, not pureed. You can also do this with a knife if you don’t have a food processor, but be sure to catch the juices that come out as you slice.
Put the oranges in a heavy non-corrosive saucepan and add the water. Bring to a rolling boil. Continue to boil for 15 minutes, then remove the pot from the heat and let the mixture stand overnight (refrigeration isn't necessary).
The next day, return the orange/water mixture to the stove and bring to a boil. Add the sugar to the pot very slowly, a little at a time (it can take as long as 10 minutes to add it all). Keep stirring as you add the sugar until it's completely dissolved. Attach a candy thermometer to the pot and cook until the mixture reaches 220 degrees.
Remove the pot from the heat, ladle the marmalade into jars, and seal. Because of the high acid level of the marmalade, you don't need to process the jars in a hot-water bath.