Cut the calamondins in half and flick out seeds with the tip of a knife. Cut fruit (peel and all) in thin slices and place in a medium saucepan. Stir in 4 cups water and bring to a boil. Add 2 cups sugar, stirring until dissolved. Taste and add more if you like--the amount depends on how sour the fruit is and your preferences.
Let the fruit mixture stand for several hours or up to 1 day at room temperature.
Bring the calamondin mixture to a boil again. Reduce the heat and simmer briskly until it thickens a little (this will take an hour or more). From this point on, keep a close eye on the marmalade, stirring often as it thickens and darkens in color. Cook to 220°F if you have a candy thermometer, or just go by looks (my method). To see if it will set up, spoon a bit onto a plate and refrigerate for a few minutes.
Stir in the candied ginger, if using. Cool until warm and spoon into clean jars. Refrigerate.
Calamondin or Kumquat Marmalade https://tavolatalk.com/calamondin-marmalade-huh/