1 3/4 pounds blood oranges (about 4 to 5 medium oranges)
1 lemon zest, finely grated and juiced
6 cups water
3 pounds plus 12 ounces sugar
Special Equipment: 10 eight-ounce canning jars with rings and lids, funnel, tongs, ladle, and 12-quart pot
Wash the blood oranges and lemon thoroughly. Cut the oranges into 1/8-inch slices using a mandolin, removing the seeds as you go. Stack the orange slices and cut them into quarters. Place the oranges into an 8-quart stainless steel pot. Add the lemon zest and juice and water to the pot, set over high heat and bring to a boil, approximately 10 minutes. Once boiling, reduce the heat to maintain a rapid simmer and cook, stirring frequently, for 40 minutes or until the fruit is very soft.
While the fruit is cooking, fill a large pot (at least 12 quarts) 3/4 full with water, set over high heat, and bring to a boil. Place 10 (8-ounce) jars and rings, canning funnel, ladle, and tongs into the boiling water and make sure the water covers the jars by at least an inch. Boil for 10 minutes. Turn off the heat, add the lids, and leave everything in the pot until the marmalade is ready.
Meanwhile, place a small plate in the freezer. Increase the heat under the orange mixture to return to full boil. Add the sugar and stir the mixture continually, until it reaches 222 to 223 degrees on a deep-fry or candy thermometer and darkens in color, approximately 15 to 20 minutes. You may need to adjust the heat in order to prevent boil over. Test the readiness of the marmalade by placing a teaspoon of the mixture onto the chilled plate and allowing it to sit for 30 seconds. Tilt the plate. The mixture should be a soft gel that moves slightly. If mixture is thin and runs easily, it is not ready.
Remove jars from the water and drain on a clean towel. Place a canning funnel onto the top of 1 of the jars and ladle in the marmalade just to below the bottom of the threads of the jar. Repeat until all of the mixture has been used. The amount of marmalade may vary by 1 to 2 jars. Wipe the rims and threads of the jars with a moist paper towel and top each with a lid. Place a ring on each jar and tighten.
Return the jars to the pot with boiling water, being certain that they don’t touch the bottom of the pot or each other. (If you don’t have a jar rack, try a round cake rack, or metal mesh basket. Even a folded kitchen towel on the pot bottom will do in a pinch.) Add additional water if necessary to cover the jars by at least an inch. Boil for 10 minutes. Using canning tongs, carefully remove the jars from the water, place in a cool dry place, and allow to sit at room temperature for at least 24 hours before opening. Once open, store in the refrigerator. Unopened marmalade will last for up to 6 months.