Cut the kumquats lengthwise in half. Remove all the seeds and the small stems. Put the kumquats in a large bowl, cover by 21" with cold water and refrigerate for least 18 hours or up to 24 hours, to remove some of the bitterness from the rind.

Drain the kumquats and return to the bowl. The halved kumquats are the right size for serving the marmalade as an accompaniment to meat or cheese. If you will be using the marmalade to spread on toast, cut the kumquats into smaller pieces.

Quarter the tangerines, reserving the juices. Cut the flesh of the tangerine wedges away from the pith and the skin and remove and discard any seeds. Squeeze the juice from the tangerines into a measuring cup and add the juice from the cutting board. Set aside 1 cup of the juice for this recipe and reserve any remaining juice for another use.

Chop the remaining tangerine flesh and add to the bowl of kumquats. Transfer the kumquats and tangerines to a pot, add the juice and sugar, and attach a candy thermometer to the pot. Bring to a simmer, skimming off any foam that rises to the top, until the marmalade registers 215° to 220°. Remove from the heat. (See note)

Spoon the marmalade into a jar or other storage container. Cover and let cool to room temperature, then refrigerate for at least 2 hours or preferably, for 24. The marmalade can be refrigerated for up to 1 month. Makes about 2 1/2 cups.