Special equipment: four 18-ounce wide-mouthed jars
Wash the kumquats well under hot water. Slice each in half and remove any seeds with the tip of a paring knife.
In a medium saucepan, bring 3 cups water to a full boil, then add the kumquats and sugar. Stir well and simmer over medium-low heat until the kumquats begin to look semi-translucent and the mixture is the consistency of maple syrup, about 15 minutes.
Divide the candied kumquats among four 18-ounce wide-mouthed jars. Divide the grain alcohol among them to fill. Refrigerate for at least 2 weeks, but ideally 1 month. (The longer the liqueur ages, the better it will taste.)