• 2 lb (1 kg) blood oranges
• About 6 cups (3 lb/1.5 kg) sugar, or as needed
• 2 cups (16 fl oz/500 ml) fresh blood orange juice
• ½ cup (4 fl oz/125 ml) fresh lemon juice

Have ready hot, sterilized jars and their lids. Place 2 or 3 small plates in the freezer.

Cut off the stem ends of the oranges. Slice each orange as thinly as possible, preferably on a mandoline. Place in a large nonreactive saucepan and add 8 cups (64 fl oz/2 l) water. Bring to a boil over medium-high heat and cook, uncovered, stirring frequently, for 15 minutes. Remove from the heat and let cool slightly.

Measure the orange slices and their liquid and return to the pan. For each 1 cup (8 fl oz/250 ml), add ¾ cup (6 oz/185 g) sugar. Stir in the orange and lemon juices. Bring to a boil over medium-high heat and boil rapidly for 10 minutes. Reduce the heat to medium and cook, stirring frequently, until slightly thickened and gelatinous, 7–10 minutes longer. Remove from the heat. Use 1 tsp marmalade and a chilled plate to test if the marmalade is ready.

Ladle the hot marmalade into the jars, leaving ¼ inch (6 mm) of headspace. Remove any air bubbles and adjust the headspace, if necessary. Wipe the rims clean and seal tightly with the lids.

Process the jars for 10 minutes in a boiling-water bath (for detailed instructions, including cooling and testing seals, Canning Step by Step). The sealed jars can be stored in a cool, dark place for up to 1 year. If a seal has failed, store the jar in the refrigerator for up to 1 month.