The kumquat peel produces an intense and vibrant orange color; regular orange sauce looks quite pale by comparison. Adjust the amount of sugar to taste depending on the sweetness of the citrus.
Slice the kumquats without peeling them. Place the fruit and the water in a saucepan and bring to a boil. Cook until the kumquat slices have softened, about 5 minutes. Remove from the heat.
Dissolve the cornstarch in a small amount of the orange juice. Mix the solution into the remaining orange juice. Pour the mixture into a noncorrosive saucepan.