Thoroughly wash the kumquats, and remove the stems. Add sugar to taste. I used a blend of 1/2 white caster sugar and 1/2 cane sugar.
Cut the kumquats in half, remove the seeds, cut in half again, then slice into thin wedges.
Put all the ingredients into a pot, then set on high heat. Reduce to low heat after it comes to a boil, and remove scum while it simmers.
Simmer over low heat for about 20 minutes, while removing the scum, and stirring from time to time to prevent burning.
Remove from heat while it's still watery, transfer to sterilized jars, close the lids, then cool.
Store in the refrigerator once it cools.
Story Behind this Recipe
When making marmalade with oranges or iyokan (Citrus iyo), you have to constantly remove the scum, and so it takes a lot of time and effort. So I thought, why not try making it with kumquats, since you can eat the skins? It turned out surprisingly easy and delicious!
The key is to remove it from heat while it's still a little runny, since jam hardens after it cools. Fill the jar while the jam is still hot, cap the lid, and turn it upside down to cool. For methods to sterilize jars and seal to store for long periods, refer to Recipe ID: 1007208.